<i>Arabidopsis thaliana</i> homeobox 12 (<scp>ATHB</scp>12), a homeodomain‐leucine zipper protein, regulates leaf growth by promoting cell expansion and endoreduplication

Summary Arabidopsis thaliana homeobox 12 ( ATHB 12 ), a homeodomain‐leucine zipper class I ( HD ‐ Z ip I) gene, is highly expressed in leaves and stems, and induced by abiotic stresses, but its role in development remains obscure. To understand its function during plant development, we studied the effects of loss and gain of function. Expression of ATHB 12 fused to the EAR‐motif repression domain (SRDX) – P 35 S :: ATHB 12 SRDX ( A 12 SRDX ) and P ATHB 12 :: ATHB 12 SRDX – slowed both leaf and root growth, while the growth of ATHB 12 ‐overexpressing seedlings ( A 12 OX ) was accelerated. Microscopic examination revealed changes in the size and number of leaf cells. Ploidy was reduced in A 12 SRDX plants, accompanied by decreased cell expansion and increased cell numbers. By contrast, cell size was increased in A 12 OX plants, along with increased ploidy and elevated expression of cell cycle switch 52s ( CCS 52s), which are positive regulators of endoreduplication, indicating that ATHB 12 promotes leaf cell expansion and endoreduplication. Overexpression of ATHB 12 led to decreased phosphorylation of Arabidopsis thaliana ribosomal protein S6 ( A t RPS 6), a regulator of cell growth. In addition, induction of ATHB 12 in the presence of cycloheximide increased the expression of several genes related to cell expansion, such as EXPANSIN A10 ( EXPA 10 ) and DWARF4 ( DWF 4 ). Our findings strongly suggest that ATHB 12 acts as a positive regulator of endoreduplication and cell growth during leaf development.