In the endosperm of japonica rice, du-1 and du-2 mutations cause the reduct
ion of amylose contents. It was previously shown that the Wx(b) allele of r
ice, which is predominantly distributed in japonica rice, has a mutation in
the 5' splice site of intron 1 resulting in the creation of two weak 5' sp
lice sites within exon 1. In du-1 and du-2 mutants, spliced Wx(b) transcrip
ts were highly reduced, whereas the processing of transcripts derived from
three other genes highly expressed in endosperm was not apparently influenc
ed. Results of competitive RT-PCR analysis indicate that transcripts splice
d at the two newly created 5' splice sites were equally affected in these t
wo mutants. Genetic and molecular analyses of the effects of du-1 and du-2
on Wx(a) pre-mRNA with normal splice sites indicate that these two mutation
s do not affect the processing of Wx(a) pre-mRNA after splicing, suggesting
that du-1 and du-2 are mutations of genes required for the efficient splic
ing of mutated Wx(b) pre-mRNA. Furthermore, du-1 and du-2 showed differenti
al effects in endosperm and pollen. Although both mutations caused similar
effects on the splicing of Wx(b) transcripts in endosperm, du-1 caused high
er reduction of Wx(b) mRNA in pollen than in endosperm, while du-2 had a le
sser effect in pollen than in endosperm. Based on these results, we propose
that the du-1 and du-2 loci of rice encode tissue-specifically regulated s
plicing factors that are involved in alternative splicing of pre mRNA in ri
ce.