Dormancy-breakage in seeds of Douglas fir (Pseudotsuga menziesii(Mirb.) Franco). Support for the hypothesis that LEA gene expression is essential for this process

In Douglas fir seeds (Pseudotsuga menziesii(Mirb.) Franco), dormancy-breakage is associated with quantitative and qualitative changes in gene expression. Specific examples of genes that are up-regulated during dormancy-breakage include those encoding LEA proteins (DF6, DF65 and DF77) (Jarvis et al., J. Plant Physiol. 147: 559–566, 1996). In particular, DF65, a dehydrin, was specifically expressed during stratification and DF6 and DF77 were significantly up-regulated. On examination of the tissue distribution of the LEA gene transcripts during chilling (which breaks dormancy) and warm-incubation, LEA transcripts were detected in both embryo and megagametophyte tissues. Expression of DF65 was lower, and the transcript appeared later, in the embryo than in the megagametophyte. With DF6 and DF77, transcripts reached maximal levels in the megagametophyte earlier than in the embryo. While de novosynthesis of ABA was important for the expression of dormancy, four weeks of chilling, only led to a 36% reduction in endogenous ABA and did not change its distribution between the embryo and megagametophyte, compared with warm-incubated seeds. However, following chilling, seeds showed a reduced sensitivity to applied ABA. To determine if there was a link between the expression pattern of LEA genes and the dormancy status of the seeds and/or abiotic stresses, seeds were treated with the plant growth regulators ABA and Me-JA, or subjected to osmotic, salt, and heat stresses. ABA treatment prevented germination of non dormant seeds, and did not induce expression of LEA genes. Me-JA stimulated dormant seeds to germinate and induced low level expression of all three LEA genes after 24 hours, but not after one week. Expression of all three LEA genes was detected in seeds which had been subjected to osmotic and salt stresses expression. However, under heat stress only DF6 and DF77 were expressed. The degree of induction by these treatments was much less than that induced by chilling. We conclude that the primary role of these gene products is connected with changes that occur during the cold, moist treatment that leads to dormancy-breakage in seeds.