A conserved but plant-specific CDK-mediated regulation of DNA replication protein A2 in the precise control of stomatal terminal division.

Abstract

The R2R3-MYB transcription factor FOUR LIPS (FLP) controls the stomatal terminal division through transcriptional repression of the cell cycle genes <i>CYCLIN-DEPENDENT KINASE</i> (<i>CDK</i>) <i>B1s</i> (<i>CDKB1s</i>), <i>CDKA;1</i>, and <i>CYCLIN A2s</i> (<i>CYCA2s</i>). We mutagenized the weak mutant allele <i>flp-1</i> seeds with ethylmethane sulfonate and screened out a <i>flp-1</i> suppressor 1 (<i>fsp1</i>) that suppressed the <i>flp-1</i> stomatal cluster phenotype. <i>FSP1</i> encodes RPA2a subunit of Replication Protein A (RPA) complexes that play important roles in DNA replication, recombination, and repair. Here, we show that <i>FSP1/RPA2a</i> functions together with <i>CDKB1s</i> and <i>CYCA2s</i> in restricting stomatal precursor proliferation, ensuring the stomatal terminal division and maintaining a normal guard-cell size and DNA content. Furthermore, we provide direct evidence for the existence of an evolutionarily conserved, but plant-specific, CDK-mediated RPA regulatory pathway. Serine-11 and Serine-21 at the N terminus of RPA2a are CDK phosphorylation target residues. The expression of the phosphorylation-mimic variant <i>RPA2a</i>^{<i>S11,21/D</i>} partially complemented the defective cell division and DNA damage hypersensitivity in <i>cdkb1;1 1;2</i> mutants. Thus, our study provides a mechanistic understanding of the CDK-mediated phosphorylation of RPA in the precise control of cell cycle and DNA repair in plants.