Silencing of the Rac1 GTPaseMtROP9inMedicago truncatulaStimulates Early Mycorrhizal and Oomycete Root Colonizations But Negatively Affects Rhizobial Infection    

Abstract

RAC/ROP proteins (r-related GTPases of plants) are plant-specific small G proteins that function as molecular switches within elementary signal transduction pathways, including the regulation of reactive oxygen species (ROS) generation during early microbial infection via the activation of NADPH oxidase homologs of plants termed RBOH (for respiratory burst oxidase homolog). We investigated the role of Medicago truncatula Jemalong A17 small GTPase MtROP9, orthologous to Medicago sativa Rac1, via an RNA interference silencing approach. Composite M. truncatula plants (MtROP9i) whose roots have been transformed by Agrobacterium rhizogenes carrying the RNA interference vector were generated and infected with the symbiotic arbuscular mycorrhiza fungus Glomus intraradices and the rhizobial bacterium Sinorhizobium meliloti as well as with the pathogenic oomycete Aphanomyces euteiches. MtROP9i transgenic lines showed a clear growth-reduced phenotype and revealed neither ROS generation nor MtROP9 and MtRBOH gene expression after microbial infection. Coincidently, antioxidative compounds were not induced in infected MtROP9i roots, as documented by differential proteomics (two-dimensional differential gel electrophoresis). Furthermore, MtROP9 knockdown clearly promoted mycorrhizal and A. euteiches early hyphal root colonization, while rhizobial infection was clearly impaired. Infected MtROP9i roots showed, in part, extremely swollen noninfected root hairs and reduced numbers of deformed nodules. S. meliloti nodulation factor treatments of MtROP9i led to deformed root hairs showing progressed swelling of its upper regions or even of the entire root hair and spontaneous constrictions but reduced branching effects occurring only at swollen root hairs. These results suggest a key role of Rac1 GTPase MtROP9 in ROS-mediated early infection signaling.

MtNF-YA1, A Central Transcriptional Regulator of Symbiotic Nodule Development, Is Also a Determinant of Medicago truncatula Susceptibility toward a Root Pathogen

“…Among the 529 probes induced in non-inoculated mutant control and infected A17, 36 genes are unambiguously related to stress responses already known to contribute to resistance toward A. euteiches (Supplementary Tables 2 and 4) (Colditz et al, 2004, 2005, 2007; Djébali et al, 2009, 2011; Schenkluhn et al, 2010; Kiirika et al, 2012; Rey et al, 2013). The 1436 probes showing repression spanned more diverse processes (Supplementary Tables 3 and 4).…”

NFP, a LysM protein controlling Nod factor perception, also intervenes in Medicago truncatula resistance to pathogens

“…Interestingly, a syntaxin gene is found among the genes significantly more highly expressed in the WT than in the nfp mutant after A. euteiches inoculation (Table S6). Identification of several genes encoding G‐signalling proteins more highly expressed in the WT reinforced a putative role of NFP in controlling processes associated with secretion at the membrane level and are coherent with the recent finding that a member of this protein family is involved in resistance signalling to A. euteiches (Kiirika et al ., ).…”

Gigaspora margarita with and without its endobacterium shows adaptive responses to oxidative stress

“…concentrations could change the outcome of the symbiosis by being perceived as a signaling molecule (Kiirika et al 2012;Arthikala et al 2014;Belmondo et al 2016). …”