Arabidopsis cytosolic glutamine synthetase AtGLN1;1 is a potential substrate of AtCRK3 involved in leaf senescence

While considerable progress has been achieved in plant CDPK studies in the past decade, there is relatively no information about the potential substrates of CRKs. In this report, a yeast two-hybrid screen was performed with truncated form of AtCRK3 as bait to identify its interacting proteins in an effort to dissect its physiological roles. One gene encoding cytosolic glutamine synthetase AtGLN1;1 was isolated. Further analyses indicated that AtGLN1;1 could interact specifically with AtCRK3 and the kinase domain of AtCRK3 and the catalytic domain of AtGLN1;1 were responsible for such interaction, respectively. Furthermore, in vitro and in vivo co-immunoprecipitation results strongly supported that they could physically interact with each other. Phosphorylation assays revealed that AtGLN1;1 could be specifically phosphorylated by AtCRK3 in vitro. All the results demonstrate that AtGLN1;1 may be a substrate of AtCRK3. In addition, both AtGLN1;1 and AtCRK3 could be induced by natural or artificially induced leaf senescence, implying that such interaction may be involved in the regulation of nitrogen remobilization during leaf senescence.