Editing Gall

{{short description|Abnormal growths especially on plants induced by parasitic insects and other organisms}}
{{about|the abnormal growths in plants}}
[[File:Andricus lignicola - Cola-nut Gall.JPG|thumb|upright=1.35|[[Cola-nut gall]]s (''[[Andricus lignicola]]'') on [[pedunculate oak]], caused by a cynipid [[gall wasp]] ]]

'''Galls''' (from the [[Latin]] {{lang|la|galla}}, 'oak-apple') or '''''cecidia''''' (from the Greek {{transliteration|grc|kēkidion}}, anything gushing out) are a kind of swelling growth on the external [[Tissue (biology)|tissues]] of plants. Plant galls are abnormal outgrowths<ref>{{cite web|url=https://www.merriam-webster.com/dictionary/gall |title=gall(4) |website=Merriam-Webster Online Dictionary |accessdate=November 16, 2007 |quote=an abnormal outgrowth of plant tissue usually due to insect or mite parasites or fungi and sometimes forming an important source of tannin}}</ref> of [[plant]] tissues, similar to [[benign tumor]]s or [[wart]]s in animals. They can be caused by various [[parasite]]s, from [[virus]]es, [[fungi]] and [[bacteria]], to other [[plant]]s, [[insect]]s and [[mite]]s. Plant galls can be such highly organized structures that their cause can be determined without the actual agent being identified. This applies particularly to insect and mite plant galls. The study of plant galls is known as cecidology.

== Anatomy ==

=== Shape and size ===

Galls develop on various plant organs, providing nutrition and shelter to inducing insects.  Galls display vast variation in [[Morphology (biology)|morphology]], size, and wall composition. The size of insect galls can range significantly, from approximately two inches in diameter to less than one-sixteenth of an inch. Some galls are so small that they are merely slightly thickened patches on leaves.<ref name="IPWD">{{cite web |title=TPWD: Plant Galls -- Young Naturalist |url=https://tpwd.texas.gov/publications/nonpwdpubs/young_naturalist/plants/plant_galls/ |access-date=2024-05-09 |website=tpwd.texas.gov}}</ref> Their shape can range from spherical to bursiform, bullet-shaped, flower-shaped, cylindrical, or diamond-like. Factors influencing gall morphology include plant species, tissue type, gall-inducing agent, and environmental conditions.<ref name="Krikorian 1988">{{Cite journal |last=Krikorian |first=A. D. |date=June 1988 |title=''Plant Galls and Gall Inducers''. Jean Meyer , S. Cheskin |url=http://dx.doi.org/10.1086/415876 |journal=The Quarterly Review of Biology |volume=63 |issue=2 |pages=225–226 |doi=10.1086/415876 |issn=0033-5770|url-access=subscription }}</ref><ref name="Barnes 1993">{{Cite journal |last=Barnes |first=Jeffrey K. |date=1993-01-01 |title=Biology of Insect-Induced Galls |url=http://dx.doi.org/10.1093/aesa/86.1.122 |journal=Annals of the Entomological Society of America |volume=86 |issue=1 |pages=122–123 |doi=10.1093/aesa/86.1.122|url-access=subscription }}</ref><ref>{{Cite journal |last1=Crespi |first1=Bernard |last2=Worobey |first2=Michael |date=December 1998 |title=Comparative Analysis of Gall Morphology in Australian Gall Thrips: The Evolution of Extended Phenotypes |url=http://dx.doi.org/10.1111/j.1558-5646.1998.tb02248.x |journal=Evolution |volume=52 |issue=6 |pages=1686–1696 |doi=10.1111/j.1558-5646.1998.tb02248.x |pmid=28565317}}</ref><ref>{{Cite journal |last1=Heard |first1=Stephen B. |last2=Buchanan |first2=Corinne K. |date=October 1998 |title=Larval Performance and Association Within and Between Two Species of Hackberry Nipple Gall Insects, Pachypsylla spp. (Homoptera: Psyllidae) |url=http://dx.doi.org/10.1674/0003-0031(1998)140[0351:lpaawa]2.0.co;2 |journal=The American Midland Naturalist |volume=140 |issue=2 |pages=351–357 |doi=10.1674/0003-0031(1998)140[0351:lpaawa]2.0.co;2|url-access=subscription }}</ref><ref>{{Cite journal |last1=Florentine |first1=S. K. |last2=Raman |first2=A. |last3=Dhileepan |first3=K. |date=October 2005 |title=Effects of Gall Induction by Epiblema Strenuana on Gas Exchange, Nutrients, and Energetics in Parthenium Hysterophorus |url=http://dx.doi.org/10.1007/s10526-004-5525-3 |journal=Biocontrol |volume=50 |issue=5 |pages=787–801 |doi=10.1007/s10526-004-5525-3 |hdl=1959.17/64564}}</ref> They typically exhibit symmetrical forms, although their end shapes vary due to differences in the physical actions and chemical stimuli of different insects. Around 90% of galls occur on the leaves of [[Dicotyledon|dicotyledons]].<ref name="Raman 2011">{{Cite journal |last=Raman |first=Anantanarayanan |date=2011-06-01 |title=Morphogenesis of insect-induced plant galls: facts and questions |url=https://www.sciencedirect.com/science/article/pii/S036725301100003X |journal=Flora - Morphology, Distribution, Functional Ecology of Plants |volume=206 |issue=6 |pages=517–533 |doi=10.1016/j.flora.2010.08.004|url-access=subscription }}</ref> Galls can develop on various parts of the host plant, such as roots, leaf bases, branches, or leaflets. Internally, galls also exhibit diverse structures. Some are simple, comprising only outgrown and curved leaf tissues, while others feature complex, hierarchical arrangements with multiple chambers containing different types of tissues, including [[Ground tissue|collenchyma]], [[Ground tissue|parenchyma]], physalides-parenchyma, and a nutritive cellular layer.<ref>{{Cite journal |last1=Arduin |first1=M. |last2=Kraus |first2=J.E. |date=1995-06-25 |title=Anatomia e Ontogenia de Galhas Foliares de Piptadenia gonoacantha (Fabales, Mimosaceae) |journal=Boletim de Botânica |volume=14 |pages=109 |doi=10.11606/issn.2316-9052.v14i0p109-130 |doi-access=free }}</ref><ref>{{Cite journal |last1=Kraus |first1=Jane E. |last2=Arduin |first2=Marcos |last3=Venturelli |first3=Margarida |date=December 2002 |title=Anatomy and ontogenesis of hymenopteran leaf galls of Struthanthus vulgaris Mart. (Loranthaceae) |url=http://dx.doi.org/10.1590/s0100-84042002012000009 |journal=Revista Brasileira de Botânica |volume=25 |issue=4 |pages=449–458 |doi=10.1590/s0100-84042002012000009}}</ref><ref>{{Citation |last1=Maresquelle |first1=H. J. |title=Physiologie et morphogenèse des galles d'origine animale (zoocécidies) |date=1965 |work=Differenzierung und Entwicklung / Differentiation and Development |pages=1927–1976 |url=http://dx.doi.org/10.1007/978-3-642-50088-6_49 |access-date=2024-05-08 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |isbn=978-3-642-50090-9 |last2=Meyer |first2=J. |doi=10.1007/978-3-642-50088-6_49 |url-access=subscription }}</ref>

=== Structure ===

In a general [[gall wasp]] gall, the outermost layer is the epidermis followed by outer cortex and then inner cortex. In some galls these two cortex layers are separated by a lignified layer. The innermost part of a gall is the larval chamber. The nutritive layer is situated between the larval chamber and the inner cortex. There is a nutritional gradient (high to low) from inside to outside of the gall while defense gradient to the opposite direction.<ref name="Schultz 2022">{{Cite journal |last1=Schultz |first1=Jack C. |last2=Stone |first2=Graham N. |date=June 2022 |title=A tale of two tissues: Probing gene expression in a complex insect-induced gall |journal=Molecular Ecology |language=en |volume=31 |issue=11 |pages=3031–3034 |doi=10.1111/mec.16482 |pmc=9321127 |pmid=35466464}}</ref><ref name="Williams 1994">{{Cite book |editor-last=Williams |editor-first=Michele A. J. |url=https://academic.oup.com/book/53725 |title=Plant Galls: Organisms, Interactions, Populations |date=1994-10-06 |publisher=Oxford University PressOxford |isbn=978-0-19-857769-0 |doi=10.1093/oso/9780198577690.001.0001}}</ref>

=== Morphogenesis ===

Gall [[morphogenesis]] involves the regulation of the organ on which the gall occurs while maintaining differentiation freedom. Gall development begins from a single or group of metaplasied cells and progresses through promoter-mediated cell expansion, cell multiplication, programmed differentiation, and control of symmetry.<ref name="Raman 2011" />

Plant response involves the establishment of metaplasied cells and localized metabolic changes to repair the wound and neutralize stress. [[Osmosis|Osmotic]] stress leads to the development of metaplasied cells, characterized by increased quantities of osmotically active material. The rejection response by the plant triggers the synthesis of defense compounds and [[enzyme]]s.<ref>{{Citation |last=Carmen |first=Cogălniceanu Gina |title=Electrical Control of Plant Morphogenesis |work=Plant Tissue Culture Engineering |pages=397–415 |url=http://dx.doi.org/10.1007/1-4020-3694-9_21 |access-date=2024-05-09 |place=Berlin/Heidelberg |publisher=Springer-Verlag |doi=10.1007/1-4020-3694-9_21 |doi-broken-date=1 November 2024 |isbn=1-4020-3594-2|url-access=subscription }}</ref><ref>{{cite book |last=Sinnott |first=Edmund W. |url=http://dx.doi.org/10.5962/bhl.title.4649 |title=Plant morphogenesis |date=1960 |publisher=McGraw-Hill |location=New York|doi=10.5962/bhl.title.4649 }}</ref>

=== Differentiation ===

* Development of novel cell types: Galls exhibit unique cell types such as abnormally thick-walled dead cells (e.g., xylary elements and sclereids) and thin-walled living cells. These cells differentiate in specific patterns, contributing to the structure of the gall.<ref name="Gasson 2000">{{cite journal |last=Gasson |first=P |date=September 2000 |title=Fink S. 1999.Pathological and regenerative plant anatomy. Encyclopedia of plant anatomy XIV. 1095 pp. Berlin, Stuttgart: Gebrüder Borntraeger. |url=http://dx.doi.org/10.1006/anbo.2000.1242 |journal=Annals of Botany |volume=86 |issue=3 |pages=707–708 |doi=10.1006/anbo.2000.1242 }}</ref><ref>{{cite book |last1=Maresquelle |first1=H. J. |title=Physiologie et morphogenèse des galles d'origine animale (zoocécidies) |date=1965 |work=Differenzierung und Entwicklung / Differentiation and Development |pages=1927–1976 |url=http://dx.doi.org/10.1007/978-3-642-50088-6_49 |access-date=2024-05-09 |place=Berlin, Heidelberg |publisher=Springer Berlin Heidelberg |isbn=978-3-642-50090-9 |last2=Meyer |first2=J.|doi=10.1007/978-3-642-50088-6_49 }}</ref><ref name=":Rohfritsch 1982">{{cite journal |last1=Rohfritsch |first1=O. |title=Insect Galls |date=1982 |journal=Molecular Biology of Plant Tumors |pages=131–152 |url=http://dx.doi.org/10.1016/b978-0-12-394380-4.50011-6 |access-date=2024-05-09 |publisher=Elsevier |isbn=978-0-12-394380-4 |last2=Shorthouse |first2=J.D.|doi=10.1016/b978-0-12-394380-4.50011-6 |url-access=subscription }}</ref>
* Nutritive tissue: Most galls contain specialized nutritive tissue that provides nutrition to the inducing arthropod and sometimes to their progeny. The structure of this tissue varies depending on the insect species inducing the gall and their feeding behaviors. Nutritive tissue differentiation is influenced by the length and nature of the insect's mouthparts.<ref>{{cite journal |last1=Bronner |first1=R. |last2=Westphal |first2=E. |last3=Dreger |first3=F. |date=February 1989 |title=Chitosan, a component of the compatible interaction between Solanum dulcamara L. and the gall mite Eriophyes cladophthirus Nal |url=http://dx.doi.org/10.1016/0885-5765(89)90020-9 |journal=Physiological and Molecular Plant Pathology |volume=34 |issue=2 |pages=117–130 |doi=10.1016/0885-5765(89)90020-9|url-access=subscription }}</ref>
* Characteristics of nutritive cells: Nutritive cells exhibit dynamic features such as enriched cytoplasm, fragmented vacuoles, hypertrophied nucleus and nucleolus, and abundant cell [[organelle]]s. They contain elevated levels of carbohydrates, lipids, soluble sugars, and proteins, along with intense phosphatase activity.<ref name="Gasson 2000"/>
* Changes in nutritive tissue: The activity of the nutritive tissue is maintained as long as the inhabiting larva continues to feed. However, when feeding ceases, the dynamic profile of the tissue gradually diminishes, and it is eventually replaced by inactive parenchyma. Removal or death of the [[larva]] leads to rapid changes in the distribution of carbohydrates and lipids within the tissue.<ref>{{cite journal |last=Schwartz |first=W. |date=1966 |title=M. S. Mani, Ecology of Plant Galls (Monogr. Biol. Vol. XII). 434 u. XII S., 164 Abb., 9 Taf. The Hague 1964: Dr. W. Junk Publishers. 40.-hfl |url=http://dx.doi.org/10.1002/jobm.3630060116 |journal=Zeitschrift für allgemeine Mikrobiologie |volume=6 |issue=1 |pages=91 |doi=10.1002/jobm.3630060116 |doi-broken-date=2 December 2024|url-access=subscription }}</ref>
* Accumulation of [[Phenolic acid|phenolic]] substances: Cells lining the larval chamber in mature-old galls accumulate phenolic substances, indicating changes in gall tissue composition over time.<ref>{{cite journal |last=Jensen |first=P. Boysen |date=January 1948 |title=Formation of Galls by Mikiola fagi |url=http://dx.doi.org/10.1111/j.1399-3054.1948.tb07113.x |journal=Physiologia Plantarum |volume=1 |issue=1 |pages=95–108 |doi=10.1111/j.1399-3054.1948.tb07113.x |issn=0031-9317|url-access=subscription }}</ref>
* Mineral content: Gall tissues contain elevated levels of various minerals, which may play a role in gall development and function.<ref name=":7">{{cite encyclopedia |last=Mapes |first=Carol C. |title=Gall Formation |date=2005 |encyclopedia=Encyclopedia of Entomology |pages=942–944 |url=https://doi.org/10.1007/0-306-48380-7_1732 |access-date=2024-05-09 |place=Dordrecht |publisher=Springer Netherlands |doi=10.1007/0-306-48380-7_1732 |isbn=978-0-306-48380-6|url-access=subscription }}</ref><ref name="Barnes 1993" />

=== Types ===

There are two primary categories of galls: closed and open.<ref name="IPWD" /> Insects such as wasps, moths, and flies, possessing chewing mouthparts during their adult or larval stages, typically inhabit completely enclosed galls. Upon reaching maturity, the adult exits either by chewing its way out or utilizing an opening created by the larval stage. Conversely, insects with sucking mouthparts rely on partially open galls or those that naturally open to facilitate emergence. An example of the latter type is the aphid, which forms marble-sized galls on the leaf stems of cottonwood trees. While these galls have thin walls, they harbor entire colonies of aphids within. When the time is right, a slit appears on one side of the gall, allowing the aphids to escape as the slit's lips unfold.<ref name="Krikorian 1988" /><ref name=":Rohfritsch 1982"/>

== Physiology ==

Insects induce the formation of galls on plants from which they receive various services, such as a source of nutrition and a place to lay eggs, develop, and be provided protection from the environment and enemies. The gall producers are specific to specific plants, thus inducing galls with unique appearances (such as balls, knobs, lumps, and warts) and a range of colors (red, green, yellow, and black). Different taxonomic groups of gall inducers vary in the complexity and diversity of gall formation and organization, with insect induced galls generally being more complex and diverse.<ref name="Gatjens-Boniche 1359–1382">{{Cite journal |last=Gatjens-Boniche |first=Omar |date=2019-12-01 |title=The mechanism of plant gall induction by insects: revealing clues, facts, and consequences in a cross-kingdom complex interaction |url=https://revistas.ucr.ac.cr/index.php/rbt/article/view/33984 |journal=Revista de Biología Tropical  |volume=67 |issue=6 |pages=1359–1382 |doi=10.15517/rbt.v67i6.33984 |doi-access=free}}</ref> Additionally, gall frequency varies based on factors such as weather, plant susceptibility, and pest populations.

=== Gall formation ===

There are four stages of gall development: initiation, growth and differentiation, maturation, and dehiscence. Gall tissues are nutritive and present high concentrations of lipids, proteins, nitrogen, and other nutrients.

The formation of galls, called induction, begins with insect saliva. Insect saliva contains various chemicals that induce shock and osmotic changes in the host plant cell.<ref name="Raman 2011"/> The severity of injury to the plant from the feeding activities of the insect varies depending on the insect. The [[Osmosis|osmotic]] changes that occur as a result are characterized by increased quantities of osmotically active material, leading to cell [[metaplasia]] and formation of a gall.

Gall growth occurs gradually over time, with the length, breadth, and height of galls increasing proportionally. The growth rate is greatest during the insect's early developmental stages and slows as it approaches adulthood. [[Plant hormone]]s like [[auxin]]s play a crucial role in gall growth. The presence of stress and insect secretions stimulates the synthesis of growth-promoting substances, possibly involving a combination thereof, such as auxins and [[kinin]]s. Gall growth involves both cell enlargement and division, but the specific factors triggering cell enlargement remain unclear.<ref name="Stone 2003"/><ref name="Larson 1991"/>

The earliest impact from the insect leads to metaplasia in the affected cells, during which they undergo changes in structure and function. However, when the chemical shock is of high intensity, metaplasia does not occur. Instead, the plant cells surrounding the shock site die, thereby rejecting the insect and defending the plant tissue. Enzymes like [[invertase]]s are involved in gall growth, with greater activity correlating with stronger gall development. Gall-inducing insect performance is influenced by plant vigor and module size, with larger, fast-growing plant modules resulting in larger galls. Conversely, galls are easily induced on smaller plant modules.<ref name="Barnes 1993"/><ref name="Raman 2011"/><ref name="Williams 1994"/><ref name=":7"/>

== Genetics ==

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Galls are unique growths on plants, and how the plant's genetic instructions could produce these structures in response to external factors is still a fresh field of science. [[Genetics|Genetic]] mechanisms of gall formation is a unique interplay between the parasite and the host plant in shaping the developmental trajectory of the gall organ.

The 'zigzag' model introduced by Jones & Dangl (2006)<ref>{{Cite journal |last1=Jones |first1=Jonathan D. G. |last2=Dangl |first2=Jeffery L. |date=November 2006 |title=The plant immune system |url=http://dx.doi.org/10.1038/nature05286 |journal=Nature |volume=444 |issue=7117 |pages=323–329 |doi=10.1038/nature05286 |pmid=17108957|url-access=subscription }}</ref> demonstrates the molecular interactions underlying gall induction. This model, refined over time and subject to ongoing enhancements, illustrates the intricate dynamics between antagonistic molecular players. Pattern-triggered immunity (PTI), constitutes the initial defense layer of plant cells, activated upon detection of "danger signals." These signals, termed damage-associated-molecular-patterns (DAMPs) if originating from the plant or microbe/pathogen-associated-molecular-patterns (MAMPs, PAMPs, or HAMPs)<ref>{{Cite journal |last1=Zipfel |first1=Cyril |last2=Felix |first2=Georg |date=August 2005 |title=Plants and animals: a different taste for microbes? |url=http://dx.doi.org/10.1016/j.pbi.2005.05.004 |journal=Current Opinion in Plant Biology |volume=8 |issue=4 |pages=353–360 |doi=10.1016/j.pbi.2005.05.004 |pmid=15922649|url-access=subscription }}</ref> if from the parasite, engage pattern-recognition receptors (PRRs) triggering signaling cascades. PRRs, classified as receptor-like kinases (RLKs), mediate intercellular communication by bridging external stimuli with intracellular defense mechanisms. Antagonists, employing effector-triggered susceptibility (ETS) manipulate host-cell functions through effector molecules encoded by effector genes, aiming primarily at suppressing plant defenses. Some effectors exploit plant traits, known as "plant susceptibility traits," diverting the plant's resources in favor of the parasite. Effectoromics, involving high-throughput expression screens, aids in identifying effector candidates crucial for colonization. Conversely, Effector-Triggered Immunity (ETI) responsible for plant's counterattack, leveraging effectors as "danger signals" to render the parasite avirulent. During ETI, nucleotide-binding domain leucine-rich repeat (NLR)-containing receptors detect perturbations induced by effectors, leading to downstream signaling events that promote defense responses. However, parasites can counteract ETI by modifying ETS, undermining the efficacy of resistance genes deployed in agriculture. The evolutionary arms race between plants and parasites, underscored by the expansion of gene families involved in biotic interactions, shapes their genomic landscape, influencing their adaptive strategies and diversification.<ref>{{Cite journal |last=Voinnet |first=Olivier |date=2005-02-10 |title=Induction and suppression of RNA silencing: insights from viral infections |url=http://dx.doi.org/10.1038/nrg1555 |journal=Nature Reviews Genetics |volume=6 |issue=3 |pages=206–220 |doi=10.1038/nrg1555 |pmid=15703763 |issn=1471-0056|url-access=subscription }}</ref><ref>{{Cite journal |last1=Kessler |first1=André |last2=Baldwin |first2=Ian T. |date=June 2002 |title=PLANT RESPONSES TO INSECT HERBIVORY: The Emerging Molecular Analysis |url=http://dx.doi.org/10.1146/annurev.arplant.53.100301.135207 |journal=Annual Review of Plant Biology |volume=53 |issue=1 |pages=299–328 |doi=10.1146/annurev.arplant.53.100301.135207 |pmid=12221978|url-access=subscription }}</ref>

Crown galls formed under the influence of the bacterium ''Agrobacterium tumefaciens'' exhibit several distinctive characteristics when compared to other types of galls. This bacterium transfers genetic material known as [[Transfer DNA|T-DNA]] into the plant cells, where it becomes integrated into the [[chromosome]]s. The T-DNA contains genes that encode for production of auxin, cytokinin and opines. As a result, the infected plant cells undergo rapid multiplication, essentially transforming into "bacterial factories" that produce more bacterial bodies.<ref name="IPWD"/>

Certain bacteria, like ''Rhodococcus fascians'', induce the formation of leafy galls on plants, affecting their growth. These galls act as permanent sinks, diverting nutrients away from other parts of the plant and causing growth suppression elsewhere. The bacteria possess virulence genes that control their ability to colonize plants and produce cytokinins, which influence plant growth. While parasitic gall-inducers are typically harmful to plants, researchers are exploring ways to harness their growth-promoting abilities for agricultural benefit. Some derivatives of ''R. fascians'' are being investigated for their potential to promote balanced plant growth, and scientists are also studying plant interactions with these bacteria to discover traits that could enhance crop yields.

Most of the transcriptomic studies on plant galls used entire gall samples resulting both gall and non-gall cells leading to thousands of gene expressions during gall development.<ref>{{Citation |last1=Betancourt |first1=Eunice Kariñho |title=Ecological Genomics of Plant-Insect Interactions: The Case of Wasp-Induced Galls |date=2020 |work=Evolutionary Ecology of Plant-Herbivore Interaction |pages=315–341 |url=http://dx.doi.org/10.1007/978-3-030-46012-9_17 |access-date=2024-05-09 |place=Cham |publisher=Springer International |isbn=978-3-030-46011-2 |last2=Soto |first2=Paulina Hernández |last3=Cortés |first3=Nancy Calderón |last4=Anaya |first4=Martha Rendón |last5=Estrella |first5=Alfredo Herrera |last6=Oyama |first6=Ken|doi=10.1007/978-3-030-46012-9_17 |url-access=subscription }}</ref><ref>{{Cite journal |last1=Hearn |first1=Jack |last2=Blaxter |first2=Mark |last3=Schönrogge |first3=Karsten |last4=Nieves-Aldrey |first4=José-Luis |last5=Pujade-Villar |first5=Juli |last6=Huguet |first6=Elisabeth |last7=Drezen |first7=Jean-Michel |last8=Shorthouse |first8=Joseph D. |last9=Stone |first9=Graham N. |date=2019-11-04 |title=Genomic dissection of an extended phenotype: Oak galling by a cynipid gall wasp |journal=PLOS Genetics |volume=15 |issue=11 |pages=e1008398 |doi=10.1371/journal.pgen.1008398 |doi-access=free |pmc=6855507 |pmid=31682601}}</ref> Recent studies on gall induced by gall wasps (Hymenoptera: Cynipidae)<ref>{{Citation |last1=Sunnucks |first1=Paul |title=The biogeography and population genetics of the invading gall wasp ''Andricus quercuscalicis ''(Hymenoptera: Cynipidae) |date=1994-10-06 |work=Plant Galls |pages=351–368 |url=https://doi.org/10.1093/oso/9780198577690.003.0021 |access-date=2024-05-09 |publisher=Oxford University PressOxford |doi=10.1093/oso/9780198577690.003.0021 |isbn=978-0-19-857769-0 |last2=Stonet |first2=G N |last3=Schonrogget |first3=K |last4=Csokat |first4=G|url-access=subscription }}</ref> ''Dryocosmus quercuspalustris'' on [[Quercus rubra|northern red oak]] (''Quercus rubra L.'') leaves demonstrate the complexity of genetic mechanisms underlying galls by quantifying the tissue-specific gene expression.<ref name=":10">{{Cite journal |last1=Martinson |first1=Ellen O. |last2=Werren |first2=John H. |last3=Egan |first3=Scott P. |date=June 2022 |title=Tissue-specific gene expression shows a cynipid wasp repurposes oak host gene networks to create a complex and novel parasite-specific organ |url=https://onlinelibrary.wiley.com/doi/10.1111/mec.16159 |journal=Molecular Ecology |language=en |volume=31 |issue=11 |pages=3228–3240 |doi=10.1111/mec.16159 |pmid=34510608|url-access=subscription }}</ref> There are substantial differences in [[gene expression]] between inner and outer gall tissues compared to adjacent leaf tissues. Approximately 28% of oak genes display differential expression in the gall compared to leaves, indicating significant [[Transcription (biology)|transcriptional]] changes associated with gall development.<ref name=":10" /> According to the [[Transcriptomics technologies|transcriptome analysis]], while the outer gall transcriptome resembles that of twigs, leaf buds, and reproductive structures, the inner gall transcriptome is distinct from normal oak tissues, underscoring the complexity of gall formation.<ref name="Schultz 2022" /> Furthermore, there is an upregulation of genes related to sugar and amino acid metabolism in both outer and inner gall tissues, suggesting a role in transporting plant metabolites to support the nutritional needs of the developing gall wasp larva. The defense-related genes are found to be suppressed in inner gall tissues as a strategy to accommodate the feeding activity of the parasite.<ref>{{Cite journal |last1=Weis |first1=Arthur E. |last2=Abrahamson |first2=Warren G. |date=1986 |title=Evolution of Host-Plant Manipulation by Gall Makers: Ecological and Genetic Factors in the Solidago-eurosta System |url=https://www.jstor.org/stable/2461318 |journal=The American Naturalist |volume=127 |issue=5 |pages=681–695 |doi=10.1086/284513 |jstor=2461318|url-access=subscription }}</ref>

== Taxonomic range ==

Plant galls are caused by a wide range of organisms, including animals such as insects, mites, and nematodes; fungi; bacteria; viruses; and other plants.

=== Insects ===

{{further|List of insect galls}}

[[Gall-inducing insect|Insect galls]] are the highly distinctive plant structures formed by some [[Herbivore|herbivorous]] insects as their own microhabitats. They are plant tissue which is controlled by the insect.  Galls act as both the habitat and food source for the maker of the gall. The interior of a gall can contain edible nutritious starch and other tissues. Some galls act as "physiologic sinks", concentrating resources in the gall from the surrounding plant parts.<ref name="Larson 1991">Larson, K. C.; Whitham, T. G. (1991).  "Manipulation of food resources by a gall-forming aphid: the physiology of sink-source interactions", ''[[Oecologia]]'' '''88'''(1): 15–21. {{doi|10.1007/BF00328398}}.</ref> Galls may also provide the insect with physical protection from predators.<ref>Weis, A. E.; Kapelinski, A. (1994).  "Variable selection on ''Eurosta''{{'}}s gall size. II. A path analysis of the ecological factors behind selection", ''Evolution'' '''48'''(3): 734–745. {{doi|10.1111/j.1558-5646.1994.tb01357.x}}.</ref><ref name="Stone 2003">Stone, G. N.; Schonrogge, K. (2003) "The adaptive significance of insect gall morphology", ''Trends in Ecology & Evolution'' '''18'''(10): 512–522. {{doi|10.1016/S0169-5347(03)00247-7}}.</ref>

Insect galls are usually induced by chemicals injected by the [[larva]]e of the insects into the plants and possibly mechanical damage. After the galls are formed, the larvae develop inside until fully grown, when they leave. To form galls, the insects must take advantage of the time when plant cell division occurs quickly: the growing season, usually spring in temperate climates, but which is extended in the tropics.

The [[meristem]]s, where plant cell division occurs, are the usual sites of galls, though insect galls can be found on other parts of the plant, such as the leaves, [[Plant stem|stalks]], [[branch]]es, [[bud]]s, [[root]]s, and even [[flower]]s and [[fruit]]s.  Gall-inducing insects are usually species-specific and sometimes tissue-specific on the plants they gall.

[[Gall-inducing insect]]s include [[gall wasp]]s, [[gall midge]]s, [[Tephritidae|gall flies]], [[Agromyzidae|leaf-miner flies]], [[aphid]]s, [[scale insect]]s, [[psyllid]]s, [[thrips]], gall moths, and [[Curculionidae|weevils]].<ref>Volovnik, S. V. (2010). "Weevils Lixinae (Coleoptera, Curculionidae) as Gall Formers", ''Entomological Review'', '''90'''(5): 585–590. {{doi|10.1134/S0013873810050052}}.</ref>

Many gall insects remain to be described. Estimates range up to more than 210,000 species, not counting [[parasitoid]]s of gall-forming insects.<ref name="Brandão‐Dias Zhang Pirro Vinson 2022">{{cite journal | last1=Brandão-Dias | first1=Pedro F. P. | last2=Zhang | first2=Yuanmeng Miles | last3=Pirro | first3=Stacy | last4=Vinson | first4=Camila C. | last5=Weinersmith | first5=Kelly L. | last6=Ward | first6=Anna K. G. | last7=Forbes | first7=Andrew A. | last8=Egan | first8=Scott P. |display-authors=4 | title=Describing biodiversity in the genomics era: A new species of Nearctic Cynipidae gall wasp and its genome | journal=Systematic Entomology | publisher=Wiley | volume=47 | issue=1 | year=2022 | issn=0307-6970 | doi=10.1111/syen.12521 | pages=94–112}}</ref>

==== Cynipid wasps ====

{{main|Gall wasp}}

More than 1400 species of cynipid wasps cause galls. Some 1000 of these are in the tribe [[Cynipini]], their hosts mostly being [[oak]] trees and other members of the [[Fagaceae]] (the beech tree family).<ref name="Brandão‐Dias Zhang Pirro Vinson 2022"/> These are often restricted taxonomically to a single host species or a group of related species.

<gallery class=center mode=nolines caption="Cynipid wasp galls">
File:Andricus foecundatrix Quercus01.jpg|[[Oak artichoke gall]] caused by ''[[Andricus foecundatrix]]''
File:Andricus foecundatrix larva on Quercus robur. vrouwelijke gal met larve op zomereik.jpg|Artichoke gall cut open to reveal wasp larva
File:Gallwespe bedient sich Eichel2.jpg|[[Knopper gall]] caused by ''[[Andricus quercuscalicis]]''
File:Eikengallen op mannelijke bloeiwijze.jpg|Oak gall caused by ''[[Neuroterus albipes]]'' forma ''laeviusculus''
File:Oak Gall.jpg|Marble gall of oak caused by ''[[Andricus kollari]]''
File:Andricus kollari - Capanne di Marcarolo.JPG|Cherry oak gall caused by ''[[Cynips quercusfolii]]''
File:Larve Eikengalwesp (Cynips quercusfolii).jpg|Cherry oak gall cut open to reveal wasp larva
File:Cynips quercusfolii.jpg|Cherry oak gall wasp adult
File:Red-Pea gall Cynips divisa on Oak.JPG|[[Red-pea gall]] (''Cynips divisa'') on [[Pedunculate Oak|pedunculate oak]]
File:Kokkocynips rileyi oak gall crop.jpg|''[[Kokkocynips rileyi]]'' oak gall
File:Phylloteras poculum oak gall crop.jpg|''[[Phylloteras poculum]]'' oak galls
</gallery>

==== Non-cynipid wasps ====

Some wasps from other groups, such as the [[Diplolepididae]] and the [[Chalcidoidea]], also cause plant galls.

<gallery class=center mode=nolines caption="Non-cynipid wasp galls">
File:Diplolepis-rosae.jpg|Rose bedeguar gall on a [[List of Rosa species|wild rose]], caused by ''[[Diplolepis rosae]]'', a [[Diplolepididae|diplolepid]] wasp<!--moved from cynipid-->
File:Diplolepis rosae 12 ies.jpg|Section through young bedeguar gall showing wasp larvae and cells
File:Gall attack.JPG|Gall on [[Eucalyptus]] due to [[chalcid wasp]] ''[[Leptocybe invasa]]'', India
</gallery>

==== Hemipteran bugs ====

Among the [[hemiptera]]n bugs that cause galls are the [[Psyllidae|psyllid]] bug ''[[Pachypsylla celtidisumbilicus]]'', and the [[woolly aphid]] ''[[Adelges abietis]]'', which parasitises [[conifer]]ous trees such as the Sitka spruce and the Norway spruce.

<gallery class=center mode=nolines caption="Hemipteran galls">
File:Developing Pineapple Gall.JPG|Developing pineapple pseudocone galls on [[Norway spruce]], caused by woolly aphid ''[[Adelges abietis]]''
File:Sacchiphantes abietis-e.jpg|Pineapple gall cut open to show the woolly aphid larvae inside
File:Pineapple gall.JPG|[[Pineapple gall]] on [[Sitka spruce]] caused by ''Adelges abietis''
File:Pachypsylla celtidisumbilicus gall.jpg|''[[Pachypsylla celtidisumbilicus]]'' hackberry gall
</gallery>

==== Dipteran flies ====

Some [[diptera]]n flies such as the [[Cecidomyiidae|cecidomyiid]] gall midges ''[[Dasineura investita]]'' and  ''[[Neolasioptera boehmeriae]]'', and some [[Agromyzidae]] leaf-miner flies cause galls.

<gallery class=center mode=nolines caption="Midge galls">
File:Gall of Japanagromyza inferna in Centrosema virginianum L. - ZooKeys-374-045-g006.jpg|Gall of ''[[Japanagromyza]] inferna'' ([[Agromyzidae]]) in ''[[Centrosema virginianum]]''
File:Dasineura investita gall.jpg|Nettle gall caused by ''[[Dasineura investita]]''  ([[Cecidomyiidae]])
File:Neolasioptera boehmeriae crop.jpg|False nettle stem gall caused by gall midge ''[[Neolasioptera boehmeriae]]''  ([[Cecidomyiidae]])
File:Schizomyia impatientis galls.jpg|''[[Schizomyia impatientis]]'' ([[Cecidomyiidae]]) jewelweed flower gall
</gallery>

=== Mites ===

[[Mites]] in the family [[Eriophyidae]] often cause galls to form on their hosts. The family contains more than 3,000 described species which attack a wide variety of plants.

<gallery class=center mode=nolines caption="Mite galls">
File:Eriophyes tilae tilae close up.JPG|Lime nail galls caused by the mite ''[[Eriophyes tiliae]]''
File:Gall mites on purple coneflower.jpg|Galls on [[purple coneflower]] caused by an undescribed mite species
File:Poison Ivy Leaf Mite imported from iNaturalist photo 125991874 on 9 November 2023.jpg|Poison ivy leaf galls caused by ''[[Aculops rhois]]''
</gallery>

=== Nematodes ===

[[Nematode]]s are microscopic worms that live in soil. Some nematodes (''Meloidogyne'' species or [[root-knot nematode]]s) cause galls on the roots of susceptible plants. The galls are often small.<ref name="Krusberg 1963">{{cite journal |last=Krusberg |first=L. R. |title=Host Response to Nematode Infection |journal=Annual Review of Phytopathology |publisher=Annual Reviews |volume=1 |issue=1 |year=1963 |issn=0066-4286 |doi=10.1146/annurev.py.01.090163.001251 |pages=219–240}}</ref><ref name="Williamson Hussey 1996">{{cite journal |last1=Williamson |first1=V. M. |last2=Hussey |first2=R. S. |title=Nematode pathogenesis and resistance in plants. |journal=The Plant Cell |publisher=Oxford University Press |volume=8 |issue=10 |year=1996 |issn=1040-4651 |doi=10.1105/tpc.8.10.1735 |pages=1735–1745|doi-access=free |pmid=8914324 |pmc=161311 }}</ref>

<gallery class=center mode=nolines caption="Nematode galls">
File:A juvenile root-knot nematode (Meloidogyne incognita) penetrates a tomato root - USDA-ARS.jpg|Juvenile ''[[Meloidogyne]]'' penetrating a host plant
File:Nematode nodules.jpg|Root-knot galls caused by the nematode ''[[Meloidogyne]]''
</gallery>

=== Fungi ===

Many [[rust fungi]] induce gall formation, including [[western gall rust]], which infects a variety of [[pine]] trees, and [[Gymnosporangium|cedar-apple rust]]. Galls are often seen in ''[[Millettia pinnata]]'' leaves and fruits. Leaf galls appear like tiny clubs; however, flower galls are globose. ''[[Exobasidium]]'' often induces spectacular galls on its hosts.

The fungus ''[[Ustilago esculenta]]'' associated with ''Zizania latifolia'', a wild rice, produces an edible gall highly valued as a food source in the [[Zhejiang]] and [[Jiangsu]] provinces of China.<ref>Terrell, E. E.; Batra, L. R. "''Zizania latifolia'' and ''Ustilago esculenta'', a grass-fungus association", ''Economic Botany'' '''36'''(3): 274–285. {{doi|10.1007/BF02858549}}.</ref>

<gallery class=center mode=nolines caption="Fungal galls">
File:Gymnosporangium juniperii telial form.jpg|Gall on conifer ''[[Juniperus virginiana]]'' caused by [[Gymnosporangium]] ([[Pucciniales]])
File:Rhododendron ferrugineum b.JPG|Leaf galls on ''[[Rhododendron ferrugineum]]'' caused by fungus ''[[Exobasidium]] rhododendri''
</gallery>

=== Bacteria and viruses ===

Gall-causing bacteria include ''[[Agrobacterium tumefaciens]]'' and ''[[Pseudomonas savastanoi]]''.

Gall forming virus was found on rice plants in central Thailand in 1979 and named rice gall dwarf. Symptoms consisted of gall formation along leaf blades and sheaths, dark green discoloration, twisted leaf tips, and reduced numbers of tillers. Some plants died in the glasshouse in the later stages of infection. The causal agent was transmitted by the hemipteran bug ''[[Nephotettix nigropictus]]'' after an incubation of two weeks. Polyhedral particles of 65&nbsp;nm diameter in the cytoplasm of phloem cells were always associated with the disease. No serologic relationship was found between this virus and that of rice dwarf.

<gallery class=center mode=nolines caption="Microbial pathogen galls">
File:Crown-gall detail.jpg|Crown gall on ''[[Kalanchoe]]'' infected with ''[[Agrobacterium tumefaciens]]''
File:Citrus_vein_enation_(CVEV)_on_Fortunella_japonica.jpg|Citrus vein enation woody gall on ''[[Fortunella japonica]]'' caused by a pathogen with an aphid [[Vector (parasitology)|vector]]<!--https://iocv.ucr.edu/citrus-diseases/vein-enation-woody-gall-->
</gallery>

=== Plants ===

The [[hemiparasitic]] plant [[mistletoe]] forms woody structures sometimes called galls on its hosts.<ref name="The Royal Society 1958 pp. 188–206">{{cite journal | title=Modes of union and interaction between parasite and host in the Loranthaceae. III. Further observations on ''Viscum'' and ''Korthalsella'' | journal=Proceedings of the Royal Society of London. Series B - Biological Sciences | publisher=The Royal Society | volume=148 | issue=931 | date=18 February 1958 | issn=2053-9193 | doi=10.1098/rspb.1958.0013 | pages=188–206}}</ref> More complex interactions are possible; the parasitic plant ''[[Cassytha filiformis]]'' sometimes preferentially feeds on galls induced by the cynipid wasp ''[[Belonocnema treatae]]''.<ref>{{Cite journal |last1=Egan |first1=Scott P. |last2=Zhang |first2=Linyi |last3=Comerford |first3=Mattheau |last4=Hood |first4=Glen R. |date=August 2018 |title=Botanical parasitism of an insect by a parasitic plant |journal=Current Biology |volume=28 |issue=16 |pages=R863–R864 |doi=10.1016/j.cub.2018.06.024|doi-access=free |pmid=30130501 }}</ref>

== Physiology of insect-induced galls ==

Insects induce the formation of galls on plants from which they receive various services, such as a source of nutrition and a place to lay eggs, develop, and be provided protection from the environment and enemies. The gall producers are specific to specific plants, thus inducing galls with unique appearances (balls, knobs, lumps, warts, etc.) and a range of colors (red, green, yellow, and black). Different taxonomic groups of gall inducers vary in the complexity and diversity of gall formation and organization, with insect induced galls generally being more complex and diverse.<ref name="Gatjens-Boniche 1359–1382"/> 

There are four stages of gall development: initiation, growth and differentiation, maturation, and dehiscence. Gall tissues are nutritive and present high concentrations of lipids, proteins, nitrogen, and other nutrients. The formation of galls begins with insect saliva on plants inducing a chemical shock.<ref name="Raman 2011"/> A mild shock induces [[metaplasia]] and gall formation. An intense shock kills plant cells in the vicinity, blocking the insect's attack.<ref name="Raman 2011"/>

== Uses ==

Galls are rich in [[resin]]s and [[tannic acid]] and have been used widely in the manufacturing of permanent [[ink]]s (such as [[iron gall ink]]) and astringent ointments, in [[dyeing]], and in [[Tanning (leather)|leather tanning]]. The [[Talmud]]<ref>Bavli, tractate Gittin:19a</ref> records using [[gallnut]]s as part of the tanning process as well as a dye-base for ink.

Medieval [[Arabic language|Arabic]] literature records many uses for the gall, called {{transliteration|ar|ˁafṣ}} in Arabic. The [[Oak marble gall|Aleppo gall]], found on [[oak]] trees in northern [[Syria]], was among the most important exports from Syria during this period, with one merchant recording a shipment of galls from [[Port Saint Symeon|Suwaydiyya]] near [[Antioch]] fetching the high price of 4½ [[dinar]]s per 100 pounds. The primary use of the galls was as a [[mordant]] for black dyes; they were also used to make a high-quality [[ink]]. The gall was also used as a medication to treat [[fever]] and [[intestine|intestinal]] ailments.<ref name="Mediterranean Society">{{cite book |last1=Goitein |first1=Shelomo Dov |last2=Sanders |first2=Paula |title=A Mediterranean Society: Daily life |date=1967 |publisher=University of California Press |isbn=0520048695 |pages=405 |url=https://books.google.com/books?id=UzOvJHFTTeUC |access-date=22 June 2020}}</ref>

== See also ==

* [[British Plant Gall Society]]
* [[Forest pathology]]
* [[List of insect galls]]

Similar structures (not galls):
* [[Burl]]
* [[Witch's broom]]

== References ==

{{reflist}}

== Further reading ==
* {{cite book |last=Blanche |first=Rosalind |year=2012  |title=Life in a Gall: The Biology and Ecology of Insects that Live in Plant Galls |publisher=CSIRO Publishing |location=Collingwood, Vic. |isbn=978-0643106437 |ref=none}}
* {{cite book |last=Redfern |first=Margaret |title=Plant Galls |year=2011 |publisher=Collins |location=London |isbn=978-0002201445 |ref=none}}
* {{cite book |last=Russo |first=Ron |title=Field Guide to Plant Galls of California and Other Western States |year=2007 |publisher=University of California Press |location=Berkeley, California |isbn=978-0520248854 |ref=none}}

==External links==
{{Commons category|Galls}}
{{wiktionary|gall|gallnut}}
* [http://www.britishplantgallsociety.org/ British Plant Gall Society]
* [http://www.bio.brandeis.edu/fieldbio/Plant_Galls/samplepage.html A Field Guide to Plant Galls of the North East U.S.] {{Webarchive|url=https://web.archive.org/web/20201112013223/http://www.bio.brandeis.edu/fieldbio/Plant_Galls/samplepage.html |date=2020-11-12 }}
* [http://waynesword.palomar.edu/pljuly99.htm To Be or Not To Be a Gall: The Story of Strange Growths on Plants] {{Webarchive|url=https://web.archive.org/web/20140321085200/http://waynesword.palomar.edu/pljuly99.htm |date=2014-03-21 }}
* [http://www.bio.brandeis.edu/fieldbio/galls/galls.html Insect Galls] {{Webarchive|url=https://web.archive.org/web/20201109220152/http://www.bio.brandeis.edu/fieldbio/galls/galls.html |date=2020-11-09 }}. ''Brandeis University''
* {{cite web | title=Common oak galls | work=University of Kentucky Entomology | url=http://www.uky.edu/Ag/Entomology/entfacts/trees/ef408.htm | access-date=2006-09-11 | archive-date=2006-09-13 | archive-url=https://web.archive.org/web/20060913044856/http://www.uky.edu/Ag/Entomology/entfacts/trees/ef408.htm | url-status=dead }}

{{Authority control}}

[[Category:Galls| ]]