Auxin plays important roles in various aspects of plant development. Auxin is actively and directionally transported from the place of synthesis by efflux-dependent cell-to-cell movement. The direction of auxin flow was determined by asymmetric cellular localization of efflux carriers represented by PIN protein (see Friml 2003, for review). Combined function of differentially expressed PIN proteins results in the net auxin flux throughout a plant body. This efflux mediated auxin gradient is used as a dictation in the formation of plant organs. So far, 8 PIN genes have been described in Arabidopsis while their sequences suggested that two of them, namely PIN5 and PIN8, are members of divergent family that may play
different roles (Friml et al. 2003).

Large number of physiological studies has shown that auxin is involved in various aspects of rice development although little is known about the underlying mechanisms at molecular level. In order to elucidate how organ formation is controlled by auxin, we attempted to analyze auxin flow accompanying organ initiation in rice. As a first step, we isolated PIN

homologs from rice and performed a phylogenetic analysis. Six putative PIN homologs were identified from the search of KOME database (http://cdna01.dna.affrc.go.jp/cDNA/) and an additional clone was isolated from genome sequence.

Phylogenetic analysis showed that 4 out of 7 OsPINs were classified in a same clade with PIN1, one with PIN2 and the rest two with PIN6 (Figure 1). The multiplification of PIN1 like OsPINs was a notable characteristic of PIN homologs. On the other hand, our search failed to isolate OsPINs that were classified in same clades with PIN3, PIN4 and PIN7. Whether rice does not have cognate orthologs of these PIN genes, which play roles in embryogenesis in Arabidopsis, remains to be answered till the completion of rice genome sequencing.

Tissue specificity of OsPIN expression was examined by RT-PCR analysis and results were summarized in Table 1. Except for OsPIN2, which was expressed at very low level, moderate level of transcripts was observed for the rest 6 genes. Although the sequence of OsPIN6b was found only in the genome sequence, we confirmed its expression. Expression of OsPIN6b was observed only in SAMs, whereas all other OsPINs displayed expression in both SAMs and roots.

Precise determination of the polarity of cellular localization and tissue/cell type specificity of their expression is next essential step to determine how these osPINs are involved in the establishment of the auxin flow and auxin gradient. Furthermore, analysis of their loss of function phenotypes should provide valuable clues to understand how auxin flow controls various aspects in rice development.

References

Friml, J., 2003. Auxi transport - shaping the plant. Current Opinion in Plant Biology 6: 7-12.

Friml. J., A. Vieten, M. Sauer, D. Weijer, H. Schwarz, T. Hamann, R. Offringa and G. Jurgens, 2003. Efflux-dependent auxin gradients establish the apical-basal axis of Arabidopsis. Nature 426: 147-153.