Comparative transcriptome profiling of salt stress response in Populus alba L.

Soil salinity is an important limiting factor to tree growth and productivity. Populus alba, an economically and environmentally valuable tree, is moderately salt-tolerant. The P. alba natural populations are adapted to contrasting environments, thus providing the genetic resources needed to identify key genes for tolerance to abiotic stress, such as salinity. Yet, physiological and molecular mechanisms of responses to salinity stress remain mostly unclear in this species. To elucidate the genetic basis of variation for salt tolerance in P. alba, we analyzed the gene expression profiles by using a poplar cDNA microarray. Two contrasting genotypes: 6K3, a salt-sensitive clone, and 14P11, a salt-tolerant clone, originating from North and South Italy, respectively, were challenged with shock salt stress (200mM NaCl). The transcriptome change induced by salinity was analysed in leaves collected from three to 72 hours of stress. Remarkably, the widest changes in number of transcripts were observed at 72 hours after stress initiation, a time that has been further investigated. Approximately 20% of transcripts showed up- or down-regulation at this time point. Down-regulated genes were significantly higher than the up-regulated ones in 6K3 clone, whereas there was no such difference in 14P11 clone. While, most down-regulated transcripts were common to the two genotypes, the up-regulated ones were mostly regulated in a genotype-specific manner. The commonly down-regulated transcripts correspond to 67 genes and are functionally related to a variety of biological processes (Gene Ontology annotation), including photosynthesis, response to stress, carbohydrate metabolic process, and formation of precursor metabolites. The commonly up-regulated transcipts represent 13 genes and are mainly involved in response to stress and transport. The functional annotation of up-regulated transcripts discriminated the molecular response of the two genotypes in terms of molecular function, cellular component, and biological process. Particularly, in 14P11 clone, 21 genes were specifically up-regulated and mainly involved in response to stress, transport, and biosynthetic process, which may contribute to salinity tolerance. But, only 11 genes were specifically up-regulated in 6K3 and were involved in response to stress and nucleoside, nucleotide and nucleic acid metabolic process. The comparative analysis revealed that most genes affected by salt stress behaved in the same manner and were related to the down-regulation of leaf functions, negatively affected by the high salinity. However, the differences in transcriptome between the two contrasting genotypes could address the molecular basis of intra-specific variation in salinity tolerance.