International Journal of Molecular Sciences Review Recent Progress Regarding the Molecular Aspects of Insect Gall Formation Seiji Takeda 1,2,3,† , Tomoko Hirano 1,3,†, Issei Ohshima 1,3 and Masa H. Sato 1,3,* 1 Graduate School of Life and Environmental Sciences, Kyoto Prefectural University, Shimogamo-Hangi-cho, Sakyo-ku, Kyoto 606-8522, Japan; [email protected] (S.T.); [email protected] (T.H.); [email protected] (I.O.) 2 Biotechnology Research Department, Kyoto Prefectural Agriculture Forestry and Fisheries Technology Center, Kitainayazuma Oji 74, Seika, Kyoto 619-0244, Japan 3 Center for Frontier Natural History, Kyoto Prefectural University, Shimogamo-Hangi-cho, Sakyo-ku, Kyoto 606-8522, Japan * Correspondence: [email protected] † These authors contributed equally to this work. Abstract: Galls are characteristic plant structures formed by cell size enlargement and/or cell proliferation induced by parasitic or pathogenic organisms. Insects are a major inducer of galls, and insect galls can occur on plant leaves, stems, floral buds, flowers, fruits, or roots. Many of these exhibit unique shapes, providing shelter and nutrients to insects. To form unique gall structures, gall-inducing insects are believed to secrete certain effector molecules and hijack host developmental programs. However, the molecular mechanisms of insect gall induction and development remain largely unknown due to the difficulties associated with the study of non-model plants in the wild. Recent advances in next-generation sequencing have allowed us to determine the biological processes in non-model organisms, including gall-inducing insects and their host plants. In this review, we first summarize the adaptive significance of galls for insects and plants. Thereafter, we summarize Citation: Takeda, S.; Hirano, T.; recent progress regarding the molecular aspects of insect gall formation. Ohshima, I.; Sato, M.H. Recent Progress Regarding the Molecular Keywords: adaptive significance of insect galls; gall-inducing insects; gall formation mechanism; Aspects of Insect Gall Formation. Int. insect effectors J. Mol. Sci. 2021, 22, 9424. https:// doi.org/10.3390/ijms22179424 Academic Editor: Tomotsugu 1. Introduction Koyama Galls are induced on plants by viruses, mycoplasma, bacteria, fungi, nematodes, Received: 12 June 2021 insects, mites, and other plants. They are defined by an abnormal plant organ devel- Accepted: 27 August 2021 opment with ectopic cell proliferation and expansion, generating a wide range of gall Published: 30 August 2021 morphologies [1,2]. Among them, insect-induced galls have attracted the attention of many researchers because of their unique shapes and wide range of variation. The estimated Publisher’s Note: MDPI stays neutral number of gall-inducing insects ranges from 21,000 to 211,000 [3,4]. Furthermore, host with regard to jurisdictional claims in plant species span numerous phylogenetic lineages, suggesting that gall-inducing systems published maps and institutional affil- have evolved independently during the insects evolution [3–6]. iations. Insect galls can be induced on plant leaves, stems, floral buds, flowers, fruits, or roots, and exhibit unique shapes (Figure1). Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Int. J. Mol. Sci. 2021, 22, 9424. https://doi.org/10.3390/ijms22179424 https://www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2021, 22, 9424 2 of 11 Int. J. Mol. Sci. 2021, 22, 9424 2 of 11 FigureFigure 1.1.Examples Examples of insect of galls insect generated galls on leaves.generated (A) Glochidion on leaves. obovatum (gallA) byGlochidion the micromoth obovatumCaloptilia cecidophoragall by. the (B) Eurya japonica gall by the micromoth Borboryctis euryae.(C) Distylium racemosum gall by the aphid Neothoracaphis yanonis. micromoth Caloptilia cecidophora. (B) Eurya japonica gall by the micromoth Borboryctis euryae. (C) (D) Rhus javanica gall by the aphid Schlechtendalia chinensis.(E) Artemisia montana gall by the gall midge Rhopalomyia Distyliumyomogicola.( rFa)cemosumUlmus parvifolia gallgall by bytheTetraneura aphid akinire Neothoracaphis. Panels (A,D) areyanonis from a. previous(D) Rhus study javanica [7]. gall by the aphid Schlechtendalia chinensis. (E) Artemisia montana gall by the gall midge Rhopalomyia yomogicola. (F) Ulmus parvifolia gall by TetraneuraAlthough akinire galls often. Panels resemble (A,D fruits) are or from floral a buds,previous their morphologiesstudy [7]. are generally considerably different from those of plant structures. Therefore, gall-inducing insects appear to hijack the plant developmental system to generate a novel structure in plants. Although galls oftenGall-inducing resemb insectsle fruits produce or stimulifloral thatbuds, initiate their the morphologies development and are maintenance generally of considerably differentgall from tissue. Interestingly,those of plant phytohormones, structures. such Therefore, as indole acetic gall acid-inducing and cytokinins, insects are detected at higher concentrations in gall-inducing insects than in galls generated on host appear to hijack the plantsplant [ 8developmental–12]. There is also evidencesystem thatto generate certain amino a novel acids and str proteinsucture arein possibleplants. Gall-inducing insectsignalss produce for gall stimuli induction that [6]. These initiate results the suggest development that substances and transferred maintenance to plants viaof gall tissue. Interestingly,an insect phytohormones ovipositor, secreted, fromsuch larval as indole saliva and/or acetic excrement acid and from cytokinins larvae, induce, are the detected at higher concentrationsreprogramming of in plant gall cells-inducing [5,6,13]. insects than in galls generated on host The unique shapes and wide range of variation in galls have attracted the attention plants [8–12]. There ofis researchers also evidence in entomology, that certain botany, andamino ecology, acids as well and as amateurproteins nature are enthusiasts.possible signals for gall inductionEach specific [6]. These insect speciesresults generates suggest more that or lesssubstances a fixed gall transferred shape on their to respective plants via an insect ovipositor,host plants,secreted indicating from that larval gall-inducing saliva and/or insects activate excrement specific from developmental larvae, pathways induce of each host plant and tightly control these pathways. However, the molecular and cellular the reprogramming ofmechanisms plant cells of gall [5,6,13] development. remain poorly understood, and further studies are needed The unique shapesthat focusand onwide a variety range of insect of variation and plant taxa. in galls have attracted the attention of researchers in entomology,In this review,botany, we assessand ecology, insect galls as from well the as perspective amateur of nature plant molecular enthusiasts. biology, with the hope of promoting future molecular plant studies on insect galls. First, we review Each specific insect speciesthe evolutionary generates aspects more of the or gall-inducing less a fixed life historiesgall shape of insects on andtheir plants respective and their host plants, indicatingadaptive that significance. gall-inducing We then briefly insects review activate the recent advancesspecific made developmental in the molecular pathways of each hostaspects plant of insectand galltightly formation. control these pathways. However, the molecular and cellular mechanisms of gall development remain poorly understood, and further studies are needed that focus on a variety of insect and plant taxa. In this review, we assess insect galls from the perspective of plant molecular biology, with the hope of promoting future molecular plant studies on insect galls. First, we review the evolutionary aspects of the gall-inducing life histories of insects and plants and their adaptive significance. We then briefly review the recent advances made in the molecular aspects of insect gall formation. 2. Significance of Morphology and Function of Galls for Insects The shape and cell status of galls have raised three major hypotheses for their adaptive significances of gall-inducing insects: nutrient hypothesis, microenvironment hypothesis, and enemy hypothesis [3,6]. These are profits of galls for insects, rather than host plants. The nutrient hypothesis explains that galls play a role as a nutrient source for Int. J. Mol. Sci. 2021, 22, 9424 3 of 11 2. Significance of Morphology and Function of Galls for Insects The shape and cell status of galls have raised three major hypotheses for their adaptive significances of gall-inducing insects: nutrient hypothesis, microenvironment hypothesis, and enemy hypothesis [3,6]. These are profits of galls for insects, rather than host plants. The nutrient hypothesis explains that galls play a role as a nutrient source for insects, and in turn, insects somehow keep regulating plant cell differentiation to provide nutrients, since gall-inducing insects can live within galls for several months. For instance, aphid– induced galls on leaves accumulate a much higher amount of amino acids than intact leaves, suggesting that the galls provide a nitrogen source to gall-inducing insects [14,15]. Photoassimilates accumulate in some aphid-induced galls, suggesting that galls act as a sink organ [16,17]. Structurally, leaf galls of Glochidion obovatum, induced