Comparative transcriptomics to understand variability of salinity tolerance in white poplar

Soil salinity is a major abiotic stress that strongly reduces the fertility of agricultural and forest lands. The desertic and saline soils are even extending in the Mediterranean region as a consequence of global climate changes. White poplar (Populus alba L.) is an interesting agroforestry species in the Mediterranean habitats due to a good potential for fast growth and adaptation to dry environments. Moreover, the native mediterranean germoplasm of this species maintains a wide genetic variability and represents a genetic resource to improve tolerance to drought and salinity. The objective of this study is to identify the genes involved in the intra-specific variability of salinity tolerance. These genes would be valuable in breeding programmes. Most transcriptome studies are conducted on one genotype and do not explain the heritable variation. In this study, two Populus alba genotypes with different response to salinity were compared in a salt stress experiment to study the transcriptome regulation induced by salt stress in leaves. Rooted cuttings were grown in pots filled with sand and irrigated with half strength Hoagland's nutrient solution. A shock salt stress treatment was applied by addition of sodium chloride (200 mM) to the irrigation solution. Leaf samples were collected three, 27, and 75 hours after the starting of the stress treatment in order to analyse the gene expression and mineral uptake. The transcriptome regulation in leaves was analysed by cDNA microarrays with about 8000 Populus euphratica transcript sequences selected from subtracted cDNA libraries. In a first step, a RNA pooling design was applied to obtain a time course analysis of transcriptome response induced by the stress. For both genotypes the number of stress-regulated genes increased from three to 75 hours. Thus, a detailed microarray analysis was performed on individual plant replicates collected at 75 hours, according to a balanced block design. Many genes were down-regulated in both genotypes, whereas a small proportion of genes were regulated by salt stress in a genotype-specific manner. Relatively few genes were differentially regulated between the two genotypes. Cluster analyses of these genes revealed groups with opposite transcription profile. These genes are candidates for the intra-specific variability of salinity tolerance.