Transcriptional Profile Analysis of Young and Mature Leaves of Citrus Trees Acclimated to Salinity

Abstract

While the molecular response of model plants to salt stress in the short-medium term (hours-days) has been broadly studied, the knowledge about the nature of genes involved in maintaining homeostatic conditions in the long term (months-years) in woody perennial trees has not been addressed yet. We have analyzed physiological parameters and the transcriptome profiles of photosynthetically active leaves from citrus trees acclimatized to moderate salinity (NaCl 30 mM) after 2 years treatment. Through functional genomics, global gene expression in response to NaCl treatment in mature (8 months-old) and young (2 months-old) leaves has been analyzed and compared. Although young leaves (YL) accumulated low levels of chloride (0.51%+/- 0.06), they exhibited a much stronger response to salinity in term of the number of differentially expressed genes (1,211 genes) compared with mature leaves (ML), which accumulated higher chloride levels (1.05%+/- 0.01), and exhibited a much lower number of differentially-responsive genes (100 genes). In this work, a number of responses have been observed that differ from those described in previous studies of citrus plants non-acclimatized to salt stress (Brumos et al., 2009), whose principal manifestation was the lack of repression of primary metabolism in leaves at the molecular and physiological levels. Results describing enriched functional categories of differentially expressed genes are presented and discussed highlighting how the long-term acclimation to NaCl stress involves drastically different molecular strategies depending on the developmental stage of plant leaves.