The SHOOT ORGANIZATION2 Gene Coordinates Leaf Domain Development Along the Central-Marginal Axis in Rice

We describe how rice leaves are regionalized and regulated along the central-marginal axis. The shoot organization2 (sho2) mutant, a weak allele of SHOOTLESS4 that is a ZIPPY/ARGONAUTE7 homolog in rice, shows a variety of leaf abnormalities; filamentous leaves, bi- or trifurcated leaves, separation of the filamentous structure from the leaf blade or deletion of the margin. All of these phenotypes can be interpreted as combinatorial defects in the growth of the central, lateral and marginal domains along the central-marginal axis, on the condition that the growth of the central domain is predominant. The leaf founder cells for the lateral and marginal domains are recruited normally in sho2, indicating that sho2 is defective in the growth of leaf domains after the founder cells are recruited. The expression pattern of SHO2 in the outer layer of the shoot apical meristem and the adaxial surface of the leaf, as well as the altered expression of HD-ZIP III and ETTIN homologs in the central domain of sho2 leaves, suggest that normal development of the central domain is a prerequisite for the synchronous growth of the three domains. This synchrony is thought to be mediated by a small interfering RNA-dependent process.