Gall

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. The severity of injury to the plant from the feeding activities of the insect varies depending on the insect. The Osmosis 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. like 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 . Gall growth involves both cell enlargement and division, but the specific factors triggering cell enlargement remain unclear.

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 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.

The 'zigzag' model introduced by Jones & Dangl (2006) 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) 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.

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 into the plant cells, where it becomes integrated into the . 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.

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. Recent studies on gall induced by gall wasps (Hymenoptera: Cynipidae) Dryocosmus quercuspalustris on Quercus rubra ( Quercus rubra L.) leaves demonstrate the complexity of genetic mechanisms underlying galls by quantifying the tissue-specific gene expression. 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 transcriptional changes associated with gall development. According to the 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. 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.

Insect galls are usually induced by chemicals injected by the 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 , 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, , , , and even and . Gall-inducing insects are usually species-specific and sometimes tissue-specific on the plants they gall.

Gall-inducing insects include , , Tephritidae, Agromyzidae, , , , thrips, gall moths, and Curculionidae.Volovnik, S. V. (2010). "Weevils Lixinae (Coleoptera, Curculionidae) as Gall Formers", Entomological Review, 90(5): 585–590. .

Many gall insects remain to be described. Estimates range up to more than 210,000 species, not counting of gall-forming insects.