The rice transcription factors OsHOX22 and OsHOX24 oppositely modulate the lamina joint inclination

Abstract

OsHOX22 and OsHOX24 are members of the homeodomain-leucine zipper I (HD-Zip I) subfamily of transcription factors (TFs) involved in ABA-mediated salt and drought tolerance. Phylogenetic analyses resolve these TFs into the same subgroup (γ-clade). We used CRISPR/Cas9 technology to obtain two Japonica CVs. Kitaake edited rice lines (edOsHOX22 and edOsHOX24) by a non-homologous end-joining mechanism. These plants displayed opposite phenotypes in the lamina joint inclination of the second leaf of 9-day-old seedlings. The edOsHOX22 genotype presented a larger leaf angle than the wild-type (WT), resembling plants with increased endogenous brassinosteroid (BR) levels. In addition, the BR treatment repressed OsHOX22 expression in WT plants. Otherwise, the edOsHOX24 plants exhibited the opposite phenotype, and BRs induced OsHOX24 expression. This evidence suggested a mutual negative and positive regulation between OsHOX22 and OsHOX24 and the phytohormone, respectively. Besides, BR biosynthesis and signaling genes exhibited altered levels in edOsHOX22 and edOsHOX24 plants compared with controls. Notably, under abiotic stress, they agonistically modified their expression, and the ABA signaling cascade masked the BR response, closing the leaf angle in both genotypes. Our findings indicated that OsHOX22 and OsHOX24 play differential roles in the lamina joint inclination event. This study contributes two new players to the complex mechanisms involved in the leaf angle opening under different environmental cues in rice.