The culm length of two semidwarf rice mutants (PKOS1, HyKOS1) obtained from low-energy N-ion beam
bombardments of dehusked Thai jasmine rice (Oryza sativa L. cv. KDML 105) seeds showed 25.7% and
21.5% height reductions and one spindly rice mutant (TKOS4) showed 21.4% increase in comparison with
that of the KDML 105 control. A cDNA-RAPD analysis identified differential gene expression in internode
tissues of the rice mutants. Two genes identified from the cDNA-RAPD were OsSPY and 14-3-3, possibly
associated with stem height variations of the semidwarf and spindly mutants, respectively. The OsSPY
gene encoded the SPY protein which is considered to be a negative regulator of gibberellin (GA). On the
other hand, the 14-3-3 encoded a signaling protein which can bind and prevent the RSG (repression of
shoot growth) protein function as a transcriptional repressor of the kaurene oxidase (KO) gene in the
GA biosynthetic pathway. Expression analysis of OsSPY, 14-3-3, RSG, KO, and SLR1 was confirmed in rice
internode tissues during the reproductive stage of the plants by semi-quantitative RT-PCR technique. The
expression analysis showed a clear increase ofthe levels of OsSPY transcripts in PKOS1 and HyKOS1 tissue
samples compared to that of the KDML 105 and TKOS4 samples at the age of 50–60 days which were at
the ages of internode elongation. The 14-3-3 expression had the highest increase in the TKOS4 samples
compared to those in KDML 105, PKOS1 and HyKOS1 samples. The expression analysis of RSG and KO
showed an increase in TKOS4 samples compared to that of the KDML 105 and that of the two semidwarf
mutants. These results indicate that changes of OsSPY and 14-3-3 expression could affect internode
elongation and cause the phenotypic changes of semidwarf and spindly rice mutants, respectively