An SPX-RLI1 Module Regulates Leaf Inclination in Response to Phosphate Availability in Rice

Abstract

Leaf erectness is one of the key traits of plant architecture; in grains, plants with upright leaves can be planted close together, thus benefiting yield/unit area. Many factors, such as hormones, affect leaf inclination; however, how nutrition status, in particular phosphate (Pi) availability, affects leaf inclination remains largely unexplained. Here, we show that in rice (<i>Oryza sativa</i>), Pi deficiency stress inhibits lamina joint cell elongation, thus restricting lamina joint size and inducing leaf erectness in rice. The Pi starvation-induced proteins SPX1 (for Syg1/Pho81/XPR1) and SPX2 play a negative role in the regulation of leaf inclination. We further identified an SPX1-interacting protein, REGULATOR OF LEAF INCLINATION1 (RLI1), which positively regulates leaf inclination by affecting lamina joint cell elongation in rice. The <i>rli1</i> mutants showed reduced leaf inclination and the <i>RLI1</i> overexpressors showed increased leaf inclination. RLI1 directly activates the downstream genes <i>BRASSINOSTEROID UPREGULATED1</i> (<i>BU1</i>) and <i>BU1-LIKE 1 COMPLEX1</i> to control elongation of the lamina joint cells, therefore enhancing leaf inclination. We also found that Pi deficiency repressed the expression of <i>RLI1</i> SPX1 protein interacts directly with RLI1, which could prevent RLI1 binding to the promoters of downstream genes. Therefore, SPX and RLI1 form a module to regulate leaf inclination in response to external Pi availability in rice.