PEP-ASSOCIATED PROTEIN 3 regulates rice tiller formation and grain yield by controlling chloroplast biogenesis.

Plastid-encoded RNA polymerase (PEP) plays a pivotal role in chloroplast development by governing the transcription of chloroplast genes, and PEP-associated proteins (PAPs) modulate PEP transcriptional activity. Therefore, PAPs provide an intriguing target for those efforts to improve yield, by enhancing chloroplast development. In this study, we identified the rice (Oryza sativa) OsPAP3 gene and characterized its function in chloroplast development. OsPAP3 expression was light-dependent and leaf-specific, similar to the PEP-dependent chloroplast gene RUBISCO LARGE SUBUNIT (OsRbcL), and OsPAP3 protein localized to chloroplast nucleoids where PEP functions. Analysis of loss-of-function and gain-of-function mutants showed that the expression of OsPAP3 is tightly linked to chloroplast gene expression and chloroplast biogenesis in rice. Homozygous knockout mutants of OsPAP3 had fewer chloroplasts than wild type, whereas plants overexpressing OsPAP3 had more chloroplasts. Also, OsPAP3 knockout suppressed the PEP-dependent expression of chloroplast genes, but OsPAP3 overexpression increased their expression. These findings indicate that OsPAP3 regulates chloroplast biogenesis in rice by controlling the PEP-dependent expression of chloroplast genes. More importantly, data from 3 seasons of field cultivation revealed that the overexpression of OsPAP3 improves rice grain yield by approximately 25%, largely due to increased tiller formation. Collectively, these observations suggest that OsPAP3 regulates rice growth and productivity by promoting chloroplast development.

Auxin controls the division of root endodermal cells.

The root endodermis forms a selective barrier that prevents the free diffusion of solutes into the vasculature; to make this barrier, endodermal cells deposit hydrophobic compounds in their cell walls, forming the Casparian strip. Here, we showed that, in contrast to vascular and epidermal root cells, endodermal root cells do not divide alongside the root apical meristem in Arabidopsis thaliana. Auxin treatment induced division of endodermal cells in wild-type plants, but not in the auxin signaling mutant auxin resistant3-1. Endodermis-specific activation of auxin responses by expression of truncated AUXIN-RESPONSIVE FACTOR5 (ΔARF5) in root endodermal cells under the control of the ENDODERMIS7 promoter (EN7::ΔARF5) also induced endodermal cell division. We used an auxin transport inhibitor to cause accumulation of auxin in endodermal cells, which induced endodermal cell division. In addition, knockout of P-GLYCOPROTEIN1 (PGP1) and PGP19, which mediate centripetal auxin flow, promoted the division of endodermal cells. Together, these findings reveal a tight link between the endodermal auxin response and endodermal cell division, suggesting that auxin is a key regulator controlling the division of root endodermal cells, and that PGP1 and PGP19 are involved in regulating endodermal cell division.

Root Development and Stress Tolerance in rice: The Key to Improving Stress Tolerance without Yield Penalties.

Roots anchor plants and take up water and nutrients from the soil; therefore, root development strongly affects plant growth and productivity. Moreover, increasing evidence indicates that root development is deeply involved in plant tolerance to abiotic stresses such as drought and salinity. These findings suggest that modulating root growth and development provides a potentially useful approach to improve plant abiotic stress tolerance. Such targeted approaches may avoid the yield penalties that result from growth-defense trade-offs produced by global induction of defenses against abiotic stresses. This review summarizes the developmental mechanisms underlying root development and discusses recent studies about modulation of root growth and stress tolerance in rice.