Identification of adaptation-specific differences in mRNA expression of sessile and pedunculate oak based on osmotic-stress-induced genes.

Quercus petraea (Matt.) Liebl. and Q. robur L. hybridize frequently and occupy similar, though distinct, ecological niches. So far, genetic discrimination between these species at the molecular level has been based mainly on neutral markers. Because such markers often exhibit low species differentiation because of high genetic compatibility and exchange between Q. robur and Q. petraea at these loci, we used adaptation-related expressed genes as markers. Accordingly, we identified osmotic-stress-induced genes in a Q. petraea cell line grown under moderate osmotic stress conditions. Two subtraction libraries were established from callus cells cultured under hyperosmotic stress for 1 or 48 h. Thirty-three differentially expressed sequence tags (ESTs) (from 70 originally isolated) were classified according to their putative functions. At least five of these gene products may contribute to osmotic-stress tolerance in oak: betaine aldehyde dehydrogenase, two trans-acting transcription factors (one abscsic acid (ABA)-responsive, the other ABA-independent), a glutathione-S- transferase and a heat-shock cognate protein. Seven genes were selected based on their putative function and their expression monitored in vivo. Leaf tissue from Q. petraea and Q. robur plantlets grown hydroponically under hyperosmotic conditions was harvested after 0, 1, 6, 24 or 72 h and analyzed by real-time polymerase chain reaction (PCR). We found indications of osmotic stress adaptation in Q. petraea based on up-regulation of genes related to protective functions, whereas down-regulation of these genes was evident in Q. robur. Thus, genetic markers related to adaptive traits may be useful for differentiating Q. petraea and Q. robur genotypes.