Wheat <i>TaSPL8</i> Modulates Leaf Angle Through Auxin and Brassinosteroid Signaling

Abstract

In grass crops, leaf angle is determined by development of the lamina joint, the tissue connecting the leaf blade and sheath, and is closely related to crop architecture and yield. In this study, we identified a mutant generated by fast neutron radiation that exhibited an erect leaf phenotype caused by defects in lamina joint development. Map-based cloning revealed that the gene <i>TaSPL8</i>, encoding a SQUAMOSA PROMOTER BINDING-LIKE (SPL) protein, is deleted in this mutant. <i>TaSPL8</i> knock-out mutants exhibit erect leaves due to loss of the lamina joint, compact architecture, and increased spike number especially in high planting density, suggesting similarity with its <i>LIGULESS1</i> homologs in maize (<i>Zea mays</i>) and rice (<i>Oryza sativa</i>). Hence, <i>LG1</i> could be a robust target for plant architecture improvement in grass species. Common wheat (<i>Triticum aestivum</i>, 2<i>n</i> = 6× = 42; BBAADD) is an allohexaploid containing A, B, and D subgenomes and the homeologous gene of <i>TaSPL8</i> from the D subgenome contributes to the length of the lamina joint to a greater extent than that from the A and B subgenomes. Comparison of the transcriptome between the <i>Taspl8</i> mutant and the wild type revealed that <i>TaSPL8</i> is involved in the activation of genes related to auxin and brassinosteroid pathways and cell elongation. TaSPL8 binds to the promoters of the <i>AUXIN RESPONSE FACTOR</i> gene and of the brassinosteroid biogenesis gene <i>CYP90D2</i> and activates their expression. These results indicate that <i>TaSPL8</i> might regulate lamina joint development through auxin signaling and the brassinosteroid biosynthesis pathway.