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
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.
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.