Molecular functions of class II Glycine-rich proteins (GRPs) in plant parasitism and plant immunity

菟丝子（菟丝子）是一种无根无叶的全寄生植物，它通过缠绕在茎上并穿透寄主组织来感染双子叶植物。具有多细胞感染器官，称为吸器，Cuscuta sp .连接到宿主的脉管系统，吸收水，溶质，营养物质和碳水化合物。通过这种方式，菟丝子不仅危害寄主植物，而且对许多作物构成额外的威胁，此外还有其他植物病原体和食草动物。栽培番茄是一个值得注意的例外，它表现出对库库塔反射的积极防御，包括诱导乙烯、活性氧以及一种超敏反应（HR）等免疫反应，最终导致番茄对库库塔反射的完全抗性。在之前的工作中，我们发现富含亮氨酸的重复受体蛋白Cuscuta receptor 1 （CuRe1）是番茄免疫系统识别C. reflexa并诱导上述防御反应的关键成分。作为寄生植物C. reflexa的病原体相关分子模式（PAMP），我们最近发现了一种定位于寄生虫细胞壁的C. reflexa甘氨酸丰富蛋白（CrGRP），该蛋白似乎存在于Cuscuta植物的所有部位和组织中。CrGRP与CuRe1物理相互作用，我们发现了一个21个氨基酸的长肽表位，crip21（富含半胱氨酸的肽21），它足以与CuRe1结合并诱导纳米摩尔范围内的番茄免疫反应。在blast分析之后，可以在所有其他生物（微生物、真菌、节肢动物和脊椎动物）和其他植物中发现CrGRPs的同源物，甚至在番茄中也是如此。然而，番茄GRP （SlGRP）或Slcrip21肽不能被番茄CuRe1识别，并且我们发现了一个单独的aa残基（Tyr11），该残基消除了其作为防御触发器的功能，也证明了CrGRP不作为潜在损伤相关分子模式（DAMP）。CrGRP和SlGRP的原始功能尚不清楚，经过文献研究发现，对这两种grp及其衍生肽在植物中的分子功能和生理作用知之甚少。因此，我们希望在拟议的工作中处理下列问题：i) CrGRP对菟丝子有哪些作用，与寄主的易感性或抗性有何关系？ii)总的来说，在菟丝子持续感染的过程中，grp对寄主植物有什么生理作用和分子功能？iii) grp是如何与其他蛋白质或（细胞壁）成分结合的，它是否与宿主相互作用有关？iv)与crip21高度相似的肽基序可以在其他寄生植物、昆虫或微生物植物病原体中发现，它们是否具有触发或抑制植物防御的功能？v)在茄科植物中，潜在的crip21受体是如何分布的？它们识别来自其他植物病原体的crip21样肽的效率如何？

Cuscuta spp. (dodder) are root- and leaf-less holoparasitic plants which infect dicotyledonous plants by winding around the shoots and penetrating host tissues. With multicellular infection organs, called haustoria, Cuscuta spp. connect to the host’s vasculature and withdraw water, solutes, nutrients and carbohydrates. In this manner, dodders harm host plants and pose an extra threat to many crops in addition to other phytopathogens and herbivores. Cultivated tomato as one notable exception displays an active defense against Cuscuta reflexa including immune responses like the induction of ethylene, reactive-oxygen species as well as a type of hypersensitive response (HR), in common finally leading to a full resistance of tomato against C. reflexa. In previous works we identified the leucine-rich repeat receptor protein Cuscuta Receptor 1 (CuRe1) as a critical component of tomato’s immune system to recognize C. reflexa and to induce the defense responses described. As a pathogen-associated molecular pattern (PAMP) of the parasitic plant C. reflexa, we recently identified a C. reflexa glycine-rich protein (CrGRP) that localizes to the parasite’s cell walls and that seems present in all parts and tissues of Cuscuta plants. CrGRP physically interacts with CuRe1 and we identified a 21 amino acid long peptide epitope, crip21 (cysteine-rich peptide 21), that is sufficient to bind to CuRe1 and to induce tomato immune responses in nanomolar ranges. After BLAST-analyses, homologs of CrGRPs can be found in all other organisms (microbes, fungi, arthropods and vertebrates) and other plants, even in tomato. The Solanum lycopersicum GRP (SlGRP) or Slcrip21 peptide, however, was not able to be recognized by tomato CuRe1 and we identified a single aa residue (Tyr11) that abolishes its function as defense-trigger and also demonstrates that CrGRP is not acting as potential damage-associated molecular pattern (DAMP). The original function of CrGRP and SlGRP are unknown and after literature research, it turned out that in general not much is known about the molecular function or physiological role of such GRPs and derived peptides in plants. In the proposed work, we therefore would like to address the following questions; i) What functions has CrGRP for Cuscuta spp. and how are those related to susceptibility or resistance of hosts? ii) what physiological roles and molecular functions do have GRPs for host plants in general and during an ongoing infection process by dodder? iii) How do GRPs associate with other proteins or (cell wall) components and does it matter for Cuscuta – host interaction? iv) Peptide motifs with high similarity to crip21 can be found in other parasitic plants, in insect or in microbial phytopathogens – do they have any function as trigger or suppressor of plant defense? and v) How are potential receptors for crip21 are distributed among the Solanaceae and how efficient do they recognize crip21-like peptides from other phytopathogens?