Rockefeller University, 1230 York Avenue, New York, NY 10021-6399, USA
We have identified a rice gene responsive to ABA and water-stress. It encodes a
basic, glycine-rich, cytosolic protein of unknown function. RAB 21 mRNA
accumulates in rice embryos, leaves, roots and suspension cells upon treatment
with NAC1 and/or ABA. The effects of NaC1 and ABA are not cumulative,
suggesting these inducers share a common response pathway. Induction of RAB 21
mRNA accumulation by ABA is rapid in cultured cells and does not require
protein synthesis, indicating preformed factors mediate this response. We have
sequenced four RAB 21 genes (mapped to a locus<30 kB in length). One of them
(Gene 1) corresponds to the cDNA. Gene-specific probes show that the four genes
are coordinately expressed. Their promoter regions share a single conserved
decanucleotide sequence, which overlaps a GC-rice sequence repeated in the Gene
1 promoter. Functional analysis of Gene 1 promoter-CAT constructs in
transformed rice cells indicates that 1) negative regulatory sequences lie
upstream of -440, and 2) strong positive elements lie between -440 and -55
which contains the GC-rice repeats. Initial gel-shift and footprinting
experiments indicate that these repeats specifically bind a nuclear factor(s).
To determine whether RAB 21 gene expression is correlated with salt tolerance,
we are measuring RAB 21 mRNA and protein levels in a collection of salt-
tolerant, anther culture-derived lines in co-operation with Dr. J. Zapata,
IRRI.