All organs of plants are originated from two meristems, shoot apical meristem (SAM) and root apical meristem (RAM). Among these two meristems, SAM provides cells to aerial organs, stems, leaves and flowers. Therefore, understanding the molecular mechanism of SAM maintenance and differentiation helps understanding mechanisms determining plant morphology. In Arabidopsis, CLAVATA (CLV ) and WUSCHEL (WUS) are known as key regulators controlling SAM maintenance and differentiation. CLV and WUS work in the same pathway making a feedback loop, and SAM regulation mechanism is well described (Clark.2001). In rice, however, little is known about the molecular mechanism controlling SAM.

We screened EMS treated rice population (cv. Taichung 65) for mutants that potentially have defects in the maintenance of SAM. A mutant with reduced number of floral organs was obtained and it was named lonely guy (log). log has smaller panicles with reduced number of spikelets. The number of floral organs in a spikelet was also reduced. A wild type rice spikelet has one lemma, one palea, two lodicules, six stamens and one pistil (Fig. 1.A). We observed a range of severity in the phenotype of log spikelets. In weak phenotype, the pistil was absent while one or a few stamens were observed. The lemma was normal, but the palea was narrowed (Fig. 1.B). In intermediate phenotype, neither the pistil nor the stamens were observed. The palea was absent and the lemma was narrowed (Fig. 1.C). In strong phenotype, in addition to the lack of the pistil and the stamens, the palea and the lemma were also absent (Fig. 1.D). The average number of each organ was 0.94 for lemma, 0.76 for palea, 1.52 for lodicules, 0.51 for stamens and 0.0 for pistil (Fig. 2). The reduction rate was more remarkable in inner organs. These phenotype suggested LOG might play an important role to maintain SAM in reproductive phase.

To isolate LOG gene, we have started positional cloning. We used F₂ population ob-

tained from a cross between Taichung 65 (log/log) and Kasalath (LOG /LOG). From the rough mapping using 22 F₂ plants, LOG locus was mapped between RM23 and RM237 on chromosome1 (Fig. 3.A). Using inner SSR markers between these two markers, we further reduced the distance to about 9.7cM between RM5638 and RM6716 (Fig. 3.B). We expected that LOG encodes a homeobox gene as WUS, but no ORFs encoding homeobox genes were predicted in this region. Therefore, LOG might be a new regulator of meristem maintenance.

Reference

Clark, S. E., 2001. Cell signalling at the shoot meristem. Nature Review:molecular cell biology 2: 276-284.