Gibberellins (GAs) are growth factors with tetracyclic diterpenoid structure that are essential regulators of diverse growth and developmental processes in plant (Davis 1995). To date a series of genes encoding the enzymes involved in GA biosynthetic pathway has been cloned from a variety of species; a nearly complete picture of the GA biosynthetic pathway has been revealed (Hedden and Phillips 2000). In contrast to the rapid progress in GA biosynthesis, there is less information on how plants perceive GA and how GA signal is transmitted, for plant growth.

GAMYB has been identified as a positive regulator for a-amylase expression in barley aleurone cells (Gubler et al. 1995). The protein can induce the expression of Alpha-amylase gene and other GA-inducible genes by interacting with GA-response cis-acting elements of these genes in the aleurone tissue (Gubler et al. 1999). Recent studies showed that GAMYB is not only involved in GA-signaling in cereal aleurone cells but also in other GA-regulated events, such as floral initiation and development (King et al. 2001). However, there is no genetic approach to characterize the molecular function of GAMYB in this context.

GAMYB in rice (OsGAMYB) has been isolated and briefly characterized by Gubler et al. (1997). Using the reported sequence of OsGAMYB, we examined the expression of OsGAMYB in various rice organs by RNA gel blot analysis, to confirm that GAMYB functions not only in germinating seeds but also in other tissues and organs after germination (Fig. 1). The result showed that OsGAMYB was expressed at a high level in the endosperm, embryo and other organs and tissues, such as shoot apices and node. This indicates that OsGAMYB may function as a transducer of GA-signal not only in aleurone cells but also in shoot apices and node.

To determine the map position of OsGAMYB, we looked for nucleotide polymorphism between japonica rice Nipponbare and indica rice Kasalath. We found a single nucleotide polymorphism (SNP) between these cultivars and performed linkage analysis using 98 backcross inbred lines (BILs) derived from Nipponbare and Kasalath (seeds given by Dr. Yano, NIAR). Results showed that OsGAMYB locus was mapped between R2417 and C86, on the middle of the long arm of chromosome 1 and tightly linked with C813 (Fig. 2). We searched for GA-related mutations already mapped around this region, but could not find one. This indicates that the loss-of-function mutant of OsGAMYB has not been described yet, or it may show no phenotype. We suspected that rice has another gene homologous to OsGAMYB. The second gene may function as a redundant manner of OsGAMYB. To evaluate this, we searched

for a gene homologous to GAMYB in the available data base but could not find any genes encoding the conserved regions, from various plant species, except OsGAMYB. This strongly suggests that OsGAMYB is the only GA-related MYB gene in rice. We are currently producing transgenic plants carrying antisense or RNAi construct of OsGAMYB, to examine its biological function in shoot apices. These plants will provide information on the function of OsGAMYB.

References

Davies P.J, Physiology, Biochemistry and Molecular Biology. Dordrecht, The Netherlands: Kluwer, 1995. 833 pp.

Gubler F., R. Kalla, J. K. Roberts and J. V. Jacobsen, 1995. Gibberellin-regulated expression of a myb gene in barley aleurone cells: Evidence of myb transactivation of a high-pl alpha-amylase gene promoter. Plant Cell. 7: 1879-1891.

Gubler F., D. Raventos, M. Keys, R. M. Watts and J. V. Jacobsen, 1999. Target genes and regulatory domains of the GAMYB transcriptional activator in cereal aleurone. Plant Journal. 17: 1-9.

Gubler F., R. J. Watts, R. Kalla, P. Matthews, M. Keys and J. V. Jacobsen, 1997. Cloning of a rice cDNA encoding a transcription factor homologous to barley GAMyb. Plant & Cell Physiology. 38: 362-365.

King R. W., T. Moritz, L. T. Evans, O. Junttila and A. J. Herlt, 2001. Long-Day Induction of Flowering in Lolium temulentum Involves Sequential Increases in Specific Gibberellins at the Shoot Apex. Plant Physiol. 127: 624-632.

Hedden P. and A.L. Phillips, 2000. Gibberellin metabolism: new insights revealed by the genes. Trends Plant Sci. 5: 523-530.