Mol Plant. 2018 Jul 25. pii: S1674-2052(18)30222-3. [Epub ahead of print]
Double fertilization of angiosperms requires targeted delivery of immotile sperm to the proximity of eggs through pollen tubes, tip-growing cells whose polarity is maintained through dynamic association of active Rho GTPases of plants (ROP-GTP) at the apical plasma membrane (PM). Guanine nucleotide exchange factors (RopGEFs) catalyze the activation of ROPs and thereby affect spatiotemporal ROP signaling. RopGEFs interact with receptor-like kinases (RLKs) and can be phosphorylated in vitro. However, the biological relevance of their phosphorylation was unclear. We report here that the Arabidopsis AGC1.5 subfamily of cytoplasmic kinases is critical for the restricted localization of ROP-GTP during pollen tube growth. Functional loss of AGC1.5 and AGC1.7 resulted in the mis-targeting of active ROPs and defects in downstream events of ROP signaling in pollen tubes. AGC1.5 interacts with RopGEFs through their catalytic PRONE domains and phosphorylates PRONE domains at a conserved Ser residue. Functional loss of AGC1.5 and AGC1.7 resulted in the mis-targeting of RopGEFs in pollen tubes, similar to that by a mutation that renders RopGEFs non-phosphorylatable by AGC1.5. Results presented here provide mechanistic insights into the spatiotemporal activation of ROPs during pollen tube polar growth.
Keywords: AGC kinase; phosphorylation; polarity; pollen tube; tip growth