Functional Analysis Of AHK3 And AXR2 In SSPP-mediated Delayed Leaf Senescence Signaling

Senescence represents the final stage of leaf development and it is strictly regulated by genetic programming.Leaf senescence occurs in an age-dependent manner but is also tightly controlled by both environmental factors and endogenous signals such as plant hormones.Among all plant hormone,cytokinin has long been widely acknowledged as a negative regulator of leaf senescence.Auxin was also taken as a negative regulator of leaf senescence,but recent emerging evidence has suggested that it positively regulates leaf senescence.Our group previously reported that overexpression of the protein phosphatase-encoding gene SSPP led to delayed leaf senescence with significantly enhanced cytokinin responses and reduced auxin responses.In this study,the function of the cytokinin receptor histidine kinase gene AHK3 and the auxin negative regulator AXR2was investigated in the signal transduction pathway of SSPP-mediated delayed leaf senescence.AHK3,among the three cytokinin receptors,plays a major role in the regulation of leaf senescence.Here,the cytokinin receptor AHK3 knockout mutant ahk3-3 and 35S:SSPP transgenic arabidopsis were crossed.The phenotypic analysis revealed that knockout of AHK3 rescued the delayed leaf senescence induced by SSPP overexpression.In addition,it was found that loss of AHK3 reversed the hypersensitivity of 35S:SSPP to exogenous cytokinin treatment.Collectively,our results demonstrated that AHK3-mediated cytokinin signaling is required for the delayed leaf senescence caused by SSPP overexpression.The phenotypic analysis of axr2-1/35S:SSPP hybrid Arabidopsis exhibited a superimposed phenotype of axr2-1and 35S:SSPP.In addition,it was found that overexpression of SSPP did not affect the localization pattern of AXR2 in the nucleus.Besides,We obtained three independent lines of GmSARK^?LRR overexpressing Arabidopsis lacking the LRR domain.The subsequent phenotypic analysis and molecular assays demonstrated that loss of LRR domain totally inhibited GmSARK-induced precocious leaf senescence.The results implied that the LRR domain may be import for the ligand binding and the signal transduction pathway of GmSARK-induced precocious leaf senescence.The findings of this thesis laid foundation for further understanding towards the molecular mechanism underlying the functional roles of auxin and cytokinin in SSPP-mediated delayed leaf senescence signaling in higher plants.