<i>WUSCHEL-related homeobox</i> family genes in rice control lateral root primordium size

The development of a plastic root system is essential for stable crop production under variable environments. Rice plants have two types of lateral roots (LRs): S-type (short and thin) and L-type (long, thick, and capable of further branching). LR types are determined at the primordium stage, with a larger primordium size in L-types than S-types. Despite the importance of LR types for rice adaptability to variable water conditions, molecular mechanisms underlying the primordium size control of LRs are unknown. Here, we show that two <i>WUSCHEL-related homeobox</i> (<i>WOX</i>) genes have opposing roles in controlling LR primordium (LRP) size in rice. Root tip excision on seminal roots induced L-type LR formation with wider primordia formed from an early developmental stage. <i>QHB/OsWOX5</i> was isolated as a causative gene of a mutant that is defective in S-type LR formation but produces more L-type LRs than wild-type (WT) plants following root tip excision. A transcriptome analysis revealed that <i>OsWOX10</i> is highly up-regulated in L-type LRPs. <i>OsWOX10</i> overexpression in LRPs increased the LR diameter in an expression-dependent manner. Conversely, the mutation in <i>OsWOX10</i> decreased the L-type LR diameter under mild drought conditions. The <i>qhb</i> mutants had higher <i>OsWOX10</i> expression than WT after root tip excision. A yeast one-hybrid assay revealed that the transcriptional repressive activity of QHB was lost in <i>qhb</i> mutants. An electrophoresis mobility shift assay revealed that <i>OsWOX10</i> is a potential target of QHB. These data suggest that QHB represses LR diameter increase, repressing <i>OsWOX10</i> Our findings could help improve root system plasticity under variable environments.