Identification and functional characterization of a leucine-rich repeat receptor-like kinase gene that is involved in regulation of soybean leaf senescence

We report here the cloning and characterization of a soybean receptor-like kinase (RLK) gene, designated GmSARK (Glycine max senescence-associated receptor-like kinase), which is involved in regulating leaf senescence. The conceptual protein product of GmSARK contains typical domains of LRR receptor-like kinases: a cytoplasmic domain with all the 11 kinase subdomains, a transmembrane domain and an extracelullar domain containing 9 Leucine-Rich Repeat (LRR) units that may act as a receptor. The expression of GmSARK in soybean leaves was up-regulated in all the three tested senescence systems: senescing cotyledons, dark-induced primary leaf senescence and the natural leaf senescence process after florescence. Furthermore, the RNA interference (RNAi)-mediated knocking-down of GmSARK dramatically retarded soybean leaf senescence. A more complex thylakoid membrane system, higher foliar level of chlorophyll content and a very remarkable delay of senescence-induced disintegration of chloroplast structure were observed in GmSARK-RNAi transgenic leaves. A homolog of maize lethal leaf-spot 1 gene, which has been suggested to encode a key enzyme catalyzing chlorophyll breakdown, was isolated and nominated Gmlls1. The expression level of Gmgtr1 gene, which encodes a key enzyme of chlorophyll synthesis, was also analyzed. It was found that Gmlls1 was up-regulated and Gmgtr1 was down-regulated during senescence in wild-type soybean leaves. However, both of the up-regulation of Gmlls1 and down-regulation of Gmgtr1 were retarded during senescence of GmSARK-RNAi transgenic leaves. In addition, over-expression of the GmSARK gene greatly accelerated the senescence progression of CaMV 35S:GmSARK transgenic plants. Taken together, these results strongly suggested the involvement of this LRR-RLK in regulation of soybean leaf senescence, maybe via regulating chloroplast development and chlorophyll accumulation. Multiple functions of GmSARK besides its regulation of leaf senescence were also discussed.