Global changes in gene expression during compatible and incompatible interactions of cowpea (Vigna unguiculata L.) with the root parasitic angiosperm Striga gesnerioides. Vigna unguiculata

Background. Cowpea, Vigna unguiculata L. Walp., is one of the most important food and forage legumes in the semi-arid tropics. While most cowpea accessions are susceptible to the root parasitic weed Striga gesnerioides, several cultivars have been identified that show race-specific resistance. Cowpea cultivar B301 contains the RSG3-301 gene for resistance to S. gesnerioides race SG3, but is susceptible to race SG4z. When challenged by SG3, roots of cultivar B301 develop a strong resistance response characterized by a hypersensitive reaction and cell death at the site of parasite attachment. In contrast, no visible response occurs in B301 roots parasitized by SG4z. Results. Gene expression in the roots of the cowpea cultivar B301 during compatible (susceptible) and incompatible (resistant) interactions with S. gesnerioides races SG4z and SG3, respectively, were investigated at 6 and 13 days post-inoculation (dpi), in the early and late stages of the resistance response using a Nimblegen custom design cowpea microarray. A total of 111 genes were differentially expressed in B301 roots at 6 dpi; this number increased to 2102 genes at 13 dpi. At 13 dpi, a total of 1944 genes were differentially expressed during compatible (susceptible) interactions of B301 with SG4z . Genes and pathways involved in signal transduction, programmed cell death and apoptosis, and defense response to biotic and abiotic stress were differentially expressed in the early resistance response; at the later time point, enrichment was primarily for defense-related gene expression, and genes encoding components of lignifications and secondary wall formation. In compatible interactions (B301 – SG4z), multiple defense pathways were repressed, including those involved in lignin biosynthesis and secondary cell wall modifications, while cellular transport processes for nitrogen and sulfur were increased. Conclusion. Distinct changes in global gene expression profiles occur in host roots following successful and unsuccessful attempted parasitism by Striga. Induction of specific defense related genes and pathways defines components of a unique resistance mechanism. Some genes and pathways up-regulated in the host resistance response to SG3 are repressed in the susceptible interactions, suggesting that the parasite is targeting specific components of the host’s defense. These results add to our understanding of plant-parasite interactions and the evolution of resistance to parasitic weeds. Overall design: A Nimblegen custom design cowpea microarray investigating gene expression in the roots of the cowpea cultivar B301 during compatible (susceptible) and incompatible (resistant) interactions with S. gesnerioides races SG4z and SG3, respectively, at 6 and 13 days post-inoculation (dpi), in the early and late stages of the resistance response.

背景。豇豆（Vigna unguiculata L. Walp.）是半干旱热带地区最重要的食用与饲用豆科作物之一。多数豇豆种质资源对根寄生杂草独脚金（Striga gesnerioides）敏感，但已鉴定出多个呈现小种专化抗性的栽培品种。豇豆栽培品种B301携带抗SG3小种的RSG3-301基因，但对SG4z小种敏感。当受SG3小种侵染时，B301的根系会产生强烈的抗性响应，以过敏反应和寄生位点细胞死亡为特征；与之相反，被SG4z小种寄生的B301根系则无可见响应。
结果。本研究采用Nimblegen定制豇豆基因芯片，在接种后天数（days post-inoculation, dpi）6天和13天——即抗性响应的早期与晚期阶段，分别检测了豇豆栽培品种B301在与独脚金SG3小种产生不亲和（抗性）互作、与SG4z小种产生亲和（敏感）互作时的根系基因表达情况。在接种后6天，B301根系共有111个基因呈现差异表达；该数量在接种后13天增至2102个。接种后13天时，B301与SG4z小种亲和（敏感）互作过程中共有1944个基因差异表达。早期抗性响应中，信号转导、程序性细胞死亡与细胞凋亡、以及生物和非生物胁迫防御响应相关的基因与通路出现差异表达；在晚期时间点，富集的通路主要为防御相关基因表达，以及木质化和次生细胞壁形成相关的编码基因。在亲和互作（B301–SG4z）中，多个防御通路受到抑制，包括木质素生物合成与次生细胞壁修饰相关通路，而氮和硫的细胞转运过程则被增强。
结论。在独脚金成功或失败的寄生侵染后，宿主根系的全局基因表达谱会出现显著差异。特定防御相关基因与通路的诱导，构成了独特抗性机制的组成部分。宿主对SG3小种的抗性响应中上调的部分基因与通路，在敏感互作中受到抑制，这表明寄生杂草靶向宿主防御系统的特定组分。本研究结果增进了我们对植物-寄生生物互作以及寄生杂草抗性演化的理解。
整体实验设计：采用Nimblegen定制豇豆基因芯片，检测豇豆栽培品种B301在与独脚金SG3小种产生不亲和（抗性）互作、与SG4z小种产生亲和（敏感）互作时的根系基因表达情况，检测时间点为接种后天数（days post-inoculation, dpi）6天和13天，覆盖抗性响应的早期与晚期阶段。