Differential accumulation of the S-adenosylmethionine decarboxylase transcript in rice seedlings in response to salt and drought stresses

Differences in gene expression between salinity-stressed and normally grown rice seedlings were compared by using the differential display (DD) technique. One DD-derived cDNA clone was characterized as a partial sequence of the rice S-adenosylmethionine decarboxylase (SAMDC) gene by sequence analysis and a homology search of GenBank databases. The full-length cDNA for the rice SAMDC gene, designated SAMDC1, was further isolated by the RT-PCR approach and was found to be different from another rice SAMDC gene released in GenBank. Comparison of the deduced polypeptide of SAMDC1 with SAMDC proteins from other plant species revealed several homologous regions, in particular the conserved proenzyme cleavage site and the putative PEST domain. Southern blot analysis indicated that the SAMDC1 gene was present as a single-copy sequence in the rice genome. Northern hybridization showed that the transcript of SAMDC1 was differentially accumulated in rice seedlings in response to salinity, drought and exogenous abscisic acid (ABA) stresses. Furthermore, levels of the SAMDC1 transcript under saline conditions were compared between a salt-tolerant japonica rice variety, Lansheng, and a salt-sensitive japonica rice variety, 77-170. It was observed that elevation in the level of the SAMDC1 transcript occurred earlier in Lansheng than in 77-170 when both were affected by salinity stress. In addition, relative to the control, higher levels of the SAMDC1 transcript were detected in Lansheng under low salt conditions or salt-stressed for shorter times, and also in 77-170 under high salt conditions or salt-stressed for prolonged times. The results suggest that expression of the SAMDC1 gene in seedlings is positively correlated with the salt tolerance of rice.