Overexpression of OsHox32 Results in Pleiotropic Effects on Plant Type Architecture and Leaf Development in Rice

BACKGROUND: The Class III homeodomain Leu zipper (HD-Zip III) gene family plays important roles in plant growth and development. Here, we analyze the function of OsHox32, an HD-Zip III family member, and show that it exhibits pleiotropic effects on regulating plant type architecture and leaf development in rice. RESULTS: Transgenic lines overexpressing OsHox32 (OsHox32-OV) produce narrow leaves that roll towards the adaxial side. Histological analysis revealed a decreased number of bulliform cells in OsHox32-OV lines. In addition, the angle between the leaf and culm was reduced, resulting in an erect plant phenotype. The height of the plants was reduced, resulting in a semi-dwarf phenotype. In addition, the chlorophyll level was reduced, resulting in a decrease in the photosynthetic rate, but water use efficiency was significantly improved, presumably due to the rolled leaf phenotype. OsHox32 exhibited constitutive expression in different organs, with higher mRNA levels in the stem, leaf sheath, shoot apical meristems and young roots, suggesting a role in plant-type and leaf development. Moreover, OsHox32 mRNA levels were higher in light and lower in the dark under both long-day and short-day conditions, indicating that OsHox32 may be associated with light regulation. Photosynthesis-associated and chlorophyll biosynthesis-associated genes were down-regulated to result in the reduction of photosynthetic capacity in OsHox32-OV lines. mRNA level of six rice YABBY genes is up-regulated or down-regulated by OsHox32, suggesting that OsHox32 may regulate the architecture of plant type and leaf development by controlling the expression of YABBY genes in rice. In addition, OsHox32 mRNA level was induced by the phytohormones, indicating that OsHox32 may be involved in phytohormones regulatory pathways. CONCLUSIONS: OsHox32, an HD-Zip III family member, plays pleiotropic effects on plant type architecture and leaf development in rice.