An AGC kinase cascade determines the correct timing of pollen germination

In flowering plants, the correct timing of pollen germination is crucial for successful fertilization and, therefore, an important trait for seed and fruit crops. Environmental factors tightly regulate pollen germination, with germination-suppressing conditions in the pollen-producing anther and germination-promoting conditions on the receiving stigma. Here we show that the differential environmental sensitivity of pollen germination in Arabidopsis thaliana is regulated by an AGC kinase cascade involving 3-PHOSPHOINOSITIDE-DEPENDENT KINASE 1 (PDK1) as upstream regulator, the AGC1.5 and AGC1.7 kinases as intermediate transducers and pollen germination promoting GUANINE NUCLEOTIDE EXCHANGE FACTORS (GEFs) as their likely phosphorylation targets. Loss of AGC kinase signaling resulted in precocious pollen germination in the anther, whereas AGC1.5/7 kinase overexpression or expression of a phosphomimic ROPGEF version inhibited pollen germination. Our data reveals that pollen germination requires the fine-tuned activity of an PDK1-AGC1 kinase cascade, preventing humidity-dependent precocious pollen germination on the anther.