Plant developmental processes are controlled by sequential expressed genes, the spatially and temporally expression of those genes can also be affected by environmental effects. Studies on the phenotypically related mutants have revealed that the responsible genes generally interact in a way of additively, epistatically or synergistically. In rice, several genes related to root elongation have been identified (Kitano and Futsuhara 1989, Liang and Ichii 1996, Ichii and Ishikawa 1997, Inukai et al. 2001, Yao et al. 2002, 2003). However, genetic interaction between these genes remains largely unclear, except that opposite effects have been observed with rrl1, rrl2 and crl2 seedlings based on a seminal root elongation assay (Inukai et al. 2001), yet the interaction between those genes during later root developmental stages remains unknown.

We previously reported two short-root mutants, srt5 and srt6, in rice (Yao et al. 2002, 2003). Based on our observation, srt5 and srt6 inhibit root elongation mainly at early growth stages. The root elongation in srt5 can be partially rescued by exogenous application of abscisic acid (ABA) and metabolizable sugars, and ABA regulates root elongation in a sugar-mediated manner (Yao et al. 2004). While root elongation in srt6 seedlings is ABA-insensitive.

To elucidate the interaction between srt5 and srt6, we constructed a double mutant (i.e., srt5; srt6) by crossing srt5 to srt6. srt5; srt6 seedlings shown an intermediate root elongation compared to monogenic parents (Fig. 1a), indicating that srt6 partially suppressed srt5 at the seedling stage. However, root elongation in srt5; srt6 mutant is faster than that in srt6 mutant, in adult plants, the suppressing effect of srt6 on srt5 is invisible (Fig. 1b). Aerial parts in adult plants of the double mutant, including seed fertility (Fig. 1c), resembled that of srt5 plants (about 10% of that of the wild type, Yao et al. 2002). These results indicate that the srt5 gene acts epistatically over srt6 at later plant growth stages. Exogenously applied ABA could rescue root elongation in the double mutant to the levels of that in srt5 (Fig. 2a), indicating that srt5 is epistatic to srt6 in ABA response. In contrast, exogenous sugar could only partially rescue

the root elongation in double mutant compared to that in srt5 (Fig. 2b), suggesting that srt6 might partially counteract the phenotype of srt5 in sugar-mediated root elongation.

Taken together, these results indicate that srt5 and srt6, depending on their temporal expression, interact in a way of independently and/or epistatically during rice root elongation, while sugar and ABA play a critical role in srt5 and srt6 controlled root elongation.

References

Ichii, M. and M. Ishikawa, 1997. Genetic analysis of newly induced short-root mutants in rice (Oryza sativa L.). Breed. Sci. 47: 121-125.

Inukai, Y., M. Miwa, Y. Nagato, H. Kitano and A. Yamauchi, 2001. RRL1, RRL2 and CRL2 loci regulating root elongation in rice. Breed. Sci. 51: 231-239.

Kitano, H. and Y. Fustuhara, 1989. Inheritance and gene expression of a root-growth inhibiting mutant in rice (Oryza sativa L.). Proc. of the 6th Internatl. Congr., SABRAO, 349-352.

Liang, Z.W. and M. Ichii, 1996. Genetic analysis of a short-root mutant LM10 induced from rice (Oryza sativa L.) cutivar IR8. Breed. Sci. 46: 373-377. (in Japanese with English summary).

Yao, S.-G., S. Taketa and M. Ichii, 2002. A novel short-root gene that affects specifically early root development in rice (Oryza sativa L.). Plant Sci. 163: 207-215.

Yao, S.-G., S. Taketa and M. Ichii, 2003. Isolation and characterization of an abscisic acid-insensitive mutation that affects specifically primary root elongation in rice (Oryza sativa L.). Plant Sci. 164: 971-978.

Yao, S.-G., J. Mushika, S. Taketa and M. Ichii, 2004. The short-root mutation srt5 defines a sugar-mediated root growth in rice (Oryza sativa L.). Plant Sci. 167: 49-54.