Role of an Arabidopsis mitogen-activated protein kinase kinase kinase in ROS-mediated leaf senescence.

Mitogen-activated protein kinase (MAPK) cascades regulate growth, development, stress responses, and immunity in plants by transmitting signals from upstream regulators to downstream components. In this study, we identify a MAPK cascade composed of MAPK kinase kinase 19 (MKKK19), three MAPK kinases (MKK3/MKK5/MKK9), and MAPK 6 (or MPK6) that is involved in Arabidopsis leaf senescence. The kinase oxidative signal-inducible 1 (OXI1) functions upstream of the MKKK19-MKK3/MKK5/MKK9-MPK6 cascade to promote leaf senescence, whereas two of RESPIRATORY BURST OXIDASE HOMOLOGS (RBOHs), RBOHD and RBOHF, act downstream to mediate the accumulation of reactive oxygen species (ROS). Loss-of-function mutation of OXI1, MKKK19, MKK3/MKK5/MKK9, or MPK6 resulted in delayed leaf senescence associated with reduced ROS levels, whereas transgenic lines overexpressing OXI1, MKKK19, MKK3/MKK5/MKK9, or MPK6 displayed the opposite phenotypes. Epistatic analyses supported the involvement of OXI1, MKKK19, MKK3/MKK5/MKK9, MPK6, and RBOHD/RBOHF in the same signaling pathway for leaf senescence. In conclusion, genetic and biochemical analysis of the Arabidopsis MKKK19-MKK3/MKK5/MKK9-MPK6 cascade through OXI1 and RBOHD/RBOHF revealed a vital role for the MAPK cascade and ROS in natural leaf senescence.