An AGC kinase cascade regulates ROP signaling for the correct timing of pollen germination

In flowering plants, the timing of pollen germination is crucial for successful fertilization and an important trait for seed and fruit crops. Environmental factors tightly regulate pollen germination, with germination-suppressing conditions on the pollen-producing anther and germination-promoting conditions on the receiving stigma. Using genetic, biochemical, and cell biological methods, we show that this environmental sensitivity of pollen germination in Arabidopsis thaliana is regulated by a 3-PHOSPHOINOSITIDE-DEPENDENT KINASE 1 (PDK1)-AGC1.5/AGC1.7 kinase cascade. We show that PDK1 hyperphosphorylates downstream AGC1.5/7 kinases to enhance their activity. Pollen-specific phosphoproteomics identified multiple proteins involved in pollen growth whose phosphorylation is controlled by AGC1.5/7. Two germination-promoting RHO OF PLANTS GUANINE NUCLEOTIDE EXCHANGE FACTORS (ROPGEFs) are phosphorylated by AGC1.5/7 at a novel site, reducing ROPGEF plasma membrane polarization. Loss of PDK1-AGC kinase signaling resulted in precocious pollen germination on the anther, which was rescued by high AGC1.5 kinase activity. Conversely, AGC1.5/7 overexpression or a phosphomimic ROPGEF version inhibited precocious pollen germination. Distinct subcellular localization of PDK1 and AGC1 kinases upon pollen germination is likely to alleviate their inhibitory effect. Our data indicate that the correct timing of pollen germination requires the fine-tuned activity of a PDK1-AGC1 kinase cascade inhibiting ROPGEFs on the anther, preventing precocious pollen germination.