Post-termination-induced and hormonally dependent expression of low-molecular-weight heat shock protein genes in Douglas fir.

We have isolated and sequenced two cDNA clones (PM 18.2A; PM 18.2B) from Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) which encode for the low-molecular-weight heat shock proteins (LMW HSPs) of 18.2 kDa. The predicted amino acid sequences of the two Douglas fir proteins are 97.5% identical. A phylogenetic tree of class I LMW HSPs showed that the PM LMW HSPs are found within a subgroup consisting exclusively of dicot species indicating that class I LMW HSPs evolved from a common ancestor predating the divergence of gymnosperms and angiosperms. Northern blots of RNA from dry, imbibed, stratified and germinated seeds revealed a notable induction of LMW HSP transcripts during post-germination and early seedling growth. Unlike previous reports, the expression of these HSPs appears to be primarily restricted to seedlings as mRNA transcripts were detected at very low levels during seed development and desiccation. Maximum induction of LMW HSPs in seedlings occurred during heat shock treatment at 38-40 degrees C, whereas cold shock or wounding failed to induce HSP transcripts. The transcription of HSP genes is up regulated by GA, MeJA and auxin and is down regulated by ABA. Methyl jasmonate treatment induced expression of these genes in dormant seeds of Douglas fir. The expression of class I cytoplasmic LMW HSPs in seedlings and their regulation by plant growth regulators suggests specific roles in plant development other than desiccation tolerance.