The Glycine- and Proline-Rich Protein AtGPRP3 Negatively Regulates Plant Growth in Arabidopsis

Glycine- and proline-rich proteins (GPRPs) comprise a small conserved family that is widely distributed in the plant kingdom. GPRPs are relatively short peptides (<200 amino acids) that contain three typical domains, including an N-terminal XYPP-repeat domain, a middle hydrophobic domain rich in alanine, and a C-terminal HGK-repeat domain. These proteins have been proposed to play fundamental roles in plant growth and environmental adaptation, but their functions remain unknown. In this study, we selected an Arabidopsis GPRP (AtGPRP3) to profile the physiological role of GPRPs. Transcripts of AtGPRP3 could be detected in the whole Arabidopsis plant, but greater amounts were found in the rosette, followed by the cauline. The AtGPRP3::GFP fusion protein was mainly localized in the nucleus. The overexpression and knockout of AtGPRP3, respectively, retarded and accelerated the growth of Arabidopsis seedlings, while the increase in the growth rate of atgprp3 plants was offset by the complementary expression of AtGPRP3. CAT2 and CAT3, but not CAT1, interacted with AtGPRP3 in the nuclei of Arabidopsis protoplasts. The knockout of CAT2 by CRISPR-Cas9 retarded the growth of the Arabidopsis seedlings. Together, our data suggest that AtGPRP3 negatively regulates plant growth, potentially through CAT2 and CAT3.

Glycine- and Proline-Rich Protein OsGPRP3 Regulates Grain Size and Quality in Rice

The grain size and shape of rice are limited by the growth of the spikelet hulls and are important selective target during domestication and breeding. In this study, we identified a glycine- and proline-rich protein (OsGPRP3), which belongs to a conserved family rarely studied. We found that OsGPRP3 was highly expressed in the seed at 10 days after pollination (DAP) using qRT-PCR, pOsGPRP3::GUS and in situ hybridization. Knockout and knockdown of OsGPRP3 led to significant decrease of 1000-grain weight, grain width, and grain thickness. We further found that the content of storage protein and total lipid were decreased in osgprp3 lines. In particular, the contents of C14:0 (myristic acid), C16:0 (palmitic acid), C18:1 (oleic acid), and C18:2 (linoleic acid) were reduced in osgprp3 lines. Cytological experiments revealed that the cell width of spikelet hull in osgprp3 lines was significantly reduced than that in WT. Taken together, our results reveal that OsGPRP3 regulates the grain size and shape of rice by influencing the cell width of spikelet hulls and the accumulation of storage protein and lipids.