Arabidopsis gene At5g09530 has been previously annotated as a cell wall protein of either the hydroxyproline-rich glycoprotein (HRGP), extensin-like, or proline-rich protein families (e.g. PRP10). However, At5g09530 shows important differences between its amino acid sequence and these other proteins. At5g09530 lacks any motifs typical of major groups of cell wall proteins, but contains 36 repeats of a unique pentapeptide (Pro-Glu-Leu|Ile|Val-Pro-Lys), which we have named the PELPK motif. This motif is repeated in only one other Arabidopsis protein (At5g09520), but proteins containing repeated PELPK motifs are found in many other angiosperms. At5g09530 is predicted to encode an intrinsically disordered protein. We characterized the phenotype of transgenic Arabidopsis with either reduced (RNAi) or increased constitutive (35S promoter) transcript expression of At5g09530. RNAi lines exhibited significantly slower germination and root growth, while overexpression lines had accelerated germination and root growth compared to wild type. Similarly, when grown on soil, RNAi lines had delayed growth and flowering, while overexpression lines had accelerated growth and flowering as compared to wild type. Based on amino acid composition, the presence of a distinct repeated pentapeptide motif and predicted intrinsically disordered structure, we conclude that At5g09530 is not an HRGP, PRP, or extensin-like protein. Because At5g09530 is a distinct and conserved protein, we propose to name it PELPK1, and to name its presumptive inparalog (At5g09520) PELPK2. PELPK1 is necessary for normal rates of germination and growth, while overexpression of PELPK1 is sufficient to accelerate germination and growth.
