Differential expression during drought conditioning of a root‐specific <i>S</i>‐adenosylmethionine synthetase from jack pine (<i>Pinus banksiana</i> Lamb.) seedlings

Abstract

ABSTRACT Differential screening of a cDNA library constructed from root mRNA from jack pine ( Pinus banksiana Lamb.) seedlings exposed to two cycles of drought conditioning identified a S ‐adenosylmethionine synthetase ( sam‐s ) cDNA. A cDNA encoding the entire open reading frame of SAM‐S was identified and characterized. Analysis of the full‐length sam‐s cDNA revealed that it was 1675 bp, encoded an open reading frame of 393 amino acids and had a predicted protein mass of 43 kDa. Jack pine sam‐s was found to be highly similar to several other plant sam‐s genes. RNA gel blot analysis showed that sam‐s mRNA abundance increased following two cycles of drought conditioning and remained abundant after 3 d of rewatering. Expression of this gene appears to be root‐specific. Quantitative slot blot analysis showed that two cycles of drought conditioning caused a 6‐fold increase in sam‐s mRNA abundance whereas heat shock, cold stress and anoxia did not result in the accumulation of sam‐s mRNA. SAM‐S enzyme activity increased 2‐fold following two cycles of drought conditioning. The increase in the rate of SAM‐s enzyme activity was also correlated with changes in rates of ethylene and betaine synthesis, biosynthetic pathways that utilize SAM as a substrate. Ethylene evolution and betaine abundance increased following two cycles of drought conditioning.