Abstract
Lamina inclination is a key agronomical character that determines plant architecture and is sensitive to auxin and brassinosteroids (BRs). Loose Plant Architecture1 (LPA1) in rice (Oryza sativa) and its Arabidopsis homologues (SGR5/AtIDD15) have been reported to control plant architecture and auxin homeostasis. This study explores the role of LPA1 in determining lamina inclination in rice. LPA1 acts as a positive regulator to suppress lamina bending. Genetic and biochemical data indicate that LPA1 suppresses the auxin signalling that interacts with C-22-hydroxylated and 6-deoxo BRs, which regulates lamina inclination independently of OsBRI1. Mutant lpa1 plants are hypersensitive to indole-3-acetic acid (IAA) during the lamina inclination response, which is suppressed by the brassinazole (Brz) inhibitor of C-22 hydroxylase involved in BR synthesis. A strong synergic effect is detected between lpa1 and d2 (the defective mutant for catalysis of C-23-hydroxylated BRs) during IAA-mediated lamina inclination. No significant interaction between LPA1 and OsBRI1 was identified. The lpa1 mutant is sensitive to C-22-hydroxylated and 6-deoxo BRs in the d61-1 (rice BRI1 mutant) background. We present evidence verifying that two independent pathways function via either BRs or BRI1 to determine IAA-mediated lamina inclination in rice. RNA sequencing analysis and qRT-PCR indicate that LPA1 influences the expression of three OsPIN genes (OsPIN1a, OsPIN1c and OsPIN3a), which suggests that auxin flux might be an important factor in LPA1-mediated lamina inclination in rice.
Highlights
Rice (Oryza sativa) is a globally important cereal crop that provides carbohydrates for more than half of the world’s population
We demonstrate that a main function of Loose Plant Architecture1 (LPA1) in lamina inclination is to suppress the indole-3-acetic acid (IAA) signalling pathway that interacts with C-22-hydroxylated BRs, which is independent of OsBRI1
To verify that the IAA-mediated lamina inclination of lpa1 was enhanced in the presence of C-22-hydroxylated BRs, we examined the sensitivity of lamina joints to C-22-hydroxylated and 6-deoxo BRs in lpa1 and lpa1;d61-1 (Fig. 5A; Supplementary Fig. S3A)
Summary
Rice (Oryza sativa) is a globally important cereal crop that provides carbohydrates for more than half of the world’s population. Rice leaf blades naturally bend downwards as plants become mature. Many genes have been reported to regulate leaf angle in rice and other plants. Rice Leaf Inclination (OsLC2) is involved in leaf bending by influencing cell division and hormone-responsive genes (Zhao et al, 2010). Rice Leaf and Tiller Angle Increased Controller (OsLIC) is an antagonistic factor of BRASSINAZOLE-RESISTANT1 (BZR1) and has an epistatic relationship with BR synthesis and BR signalling mutants (Wang et al, 2008; Zhang et al, 2012). Rice Increased Leaf Inclination (OsILI1) and its binding protein ILI1 Binding bHLH Protein (OsIBH1) function downstream of BZR1 in BR-mediated regulation of cell elongation (Zhang et al, 2009). Increased Leaf Angle (ILA1) affects rice leaf angle by regulating mechanical tissue formation at the leaf lamina joint (Ning et al, 2011). The Arabidopsis OsLPA1 homologues (AtIDD14, AtIDD15 and AtIDD16) control branch angles by regulating the expression of auxin biosynthetic and transport genes (Cui et al, 2013)
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