WUSCHEL-related homeobox 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 deter-mined 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 WUSCHEL-related homeobox (WOX) 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. QHB/OsWOX5 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 transcrip-tome analysis revealed that OsWOX70 is highly up-regulated in L-type LRPs. OsWOX70 overexpression in LRPs increased the LR diameter in an expression-dependent manner. Conversely, the mutation in OsWOX70 decreased the L-type LR diameter under mild drought conditions. The qhb mutants had higher OsWOX70 expression than WT after root tip excision. A yeast one-hybrid assay revealed that the transcriptional repressive activity of QHB was lost in qhb mutants. An electrophoresis mobility shift assay revealed that OsWOX70 is a potential target of QHB. These data suggest that QHB represses LR diameter increase, repressing OsWOX70. Our findings could help improve root system plasticity under variable environments.

Automated summary

The development of a plastic root system is crucial for stable crop production. In this study, the molecular mechanisms underlying the control of lateral root primordium size in rice were investigated. Two WOX genes were found to have opposing roles in controlling primordium size.