A phospholipid-dependent PDK1-AGC kinase cascade regulates pollen tube growth

ABSTRACT 3-PHOSPHOINOSITIDE-DEPENDENT PROTEIN KINASE1 (PDK1), a conserved master regulator of AGC kinases, is encoded by two redundant genes in Arabidopsis thaliana, PDK1 and PDK2. pdk1 pdk2 mutants exhibit a broad range of defects, including apolar or arrested pollen tube growth, a phenotype also observed in agc1 . 5 agc1 . 7 mutants. Pollen-specific expression of constitutively active AGC1.5 in pdk1 pdk2 restores polar pollen tube growth, indicating that PDK1 functions upstream of redundant AGC1.5/AGC1.7 signaling in this process. In contrast, the PDK1 splice variant PDK1S0, lacking the phospholipid-binding PH domain, cannot restore polar pollen tube growth. Our results indicate a key role for the phospholipid PI(4,5)P 2 in recruiting PDK1 through its PH domain to establish polar pollen tube growth, as PI(4,5)P 2 marks the pollen germination initiation site together with PDK1, it forms a dome at the plasma-membrane of the pollen tube tip beneath which PDK1 remains largely cytosolic and exhibits reciprocal feedback regulation with the PDK1-AGC1.5/1.7 kinases. Defects in endocytosis and actin organization further support that phospholipid-dependent PDK1–AGC signaling maintains pollen tube growth polarity.