Regulatory mechanisms of plastic development of heterorhizic lateral roots in rice

Abstract

Rice produces two distinct types of lateral roots, S-type and L-type, which differ in morphology, anatomy, and function. Hardpans in rainfed lowlands or compacted soils restrict deep rooting and reduce crop productivity, whereas plastic development of lateral roots helps maintain shoot growth when the rooting zone is limited. However, the genetic basis of such root plasticity has remained unclear due to the difficulty of observing underground traits. Here, we investigated the regulatory mechanisms controlling plastic lateral root formation in rice using a seminal-root tip excision method. Mutant analysis revealed that the WUSCHEL-RELATED HOMEOBOX (WOX) family transcription factor QHB/OsWOX5 acts as a brake that limits the size of lateral root primordia. Transcriptome comparison between S-type and L-type primordia further identified OsWOX10 as an accelerator that promotes primordium enlargement. The expression of OsWOX10 is directly repressed by QHB/OsWOX5. We also found that auxin distribution contributes to primordium size control: in L-type primordia, auxin accumulates at the basal region, leading to OsWOX10 activation. Using a semi-hydroponic phenotyping system, we demonstrated that L-type lateral roots induced after root-tip excision compensate for the loss of further main root elongation and help maintain overall root system development and shoot growth. These findings reveal the molecular framework underlying plastic lateral root development in rice and highlight its importance for plant growth under restricted rooting-zone conditions.