Genetic manipulation of stress-induced mitogen-activated protein kinase modulates early stages of the nodulation process in Medicago sativa

Leguminous plants have established a mutualistic endosymbiotic interaction with nitrogen-fixing rhizobia to secure nitrogen sources in new specialized organs called root nodules. Before nodule formation, the development of early symbiotic structures is essential for rhizobia docking, internalization, targeted delivery, and intracellular accommodation. We have recently reported that overexpression of stress-induced mitogen-activated protein kinase (SIMK) in alfalfa affects root hair, nodule and shoot formation. However, detailed subcellular spatial distribution, activation, and developmental relocation of SIMK during the early stages of alfalfa nodulation remain unclear. Here, we qualitatively and quantitatively characterized SIMK distribution patterns in rhizobium-infected root hairs using live-cell imaging and immunolocalization, employing alfalfa stable transgenic lines with genetically manipulated SIMK abundance and kinase activity. In the SIMKK-RNAi line, showing downregulation of SIMKK and SIMK, we found a considerably decreased accumulation of phosphorylated SIMK around infection pockets and infection threads, which was strongly increased in the GFP-SIMK line, constitutively overexpressing GFP-tagged SIMK. Thus, genetically manipulated SIMK modulates root hair capacity to form infection pockets and infection threads. These results shed new light on SIMK spatio-temporal participation in the early interactions between alfalfa and rhizobia, and its internalization into root hairs, showing that local accumulation of active SIMK indeed modulates nodulation in alfalfa. One sentence summaryGenetic manipulation of SIMK in alfalfa revealed that SIMK modulates root hair capacity to form infection pockets and infection threads during the early interactions between alfalfa and rhizobia.