Arabidopsis MKK10-MPK6 mediates red-light-regulated opening of seedling cotyledons through phosphorylation of PIF3

Photomorphogenesis is an important process in which seedlings emerge from soil and begin autotrophic growth. Photomorphogenesis mechanisms include light signal perception, signal transduction, and the modulation of light-responsive genes expression, ultimately leading to cellular and developmental changes. Phytochrome-interacting factors (PIFs) play pivotal roles in negatively regulating photomorphogenesis. Light-induced activation of phytochromes triggers the rapid phosphorylation and degradation of PIFs, but the kinases responsible for the phosphorylation of PIFs are largely unknown. Here, we show that Arabidopsis MPK6 is a kinase involved in phosphorylating PIF3 and regulating red (R) light-induced cotyledon opening, a crucial process during seedling photomorphogenesis. MPK6 was activated by R light, and cotyledon opening angle in R light was reduced in mpk6 seedlings. MKK10, a MAPKK whose function is currently unclear, appears to act as a kinase upstream of MPK6 in regulating cotyle...