Comparative transcriptome analysis of contrasting foxtail millet cultivars in response to short-term salinity stress

Soil salinity represents a major abiotic stress that adversely affects crop growth and productivity. In this study, 21-day-old seedlings of two foxtail millet (Setaria italica) cultivars differing in salt tolerance were found to also differ in lipid peroxidation, ion balance and activity of antioxidative enzymes (glutathione reductase and catalase) under short-term salinity stress (250 mM NaCl for 1-48 h). With the aim of better understanding the molecular mechanisms underlying plant responses to short-term salinity stress, two suppression subtractive hybridization cDNA libraries (forward and reverse) were constructed of these cultivars. A total of 249 non-redundant ESTs was identified by random EST sequencing and grouped into 11 functional categories. Macroarray analysis of these clones showed that 159 (63.9%) were differentially expressed (≥ 2-fold) in response to salinity stress, with 115 (72.3%) up and 44 (27.7%) down-regulated. A data search of transcriptional profiling under salinity stress in other species revealed that 81 (51%) of the 159 differentially expressed transcripts found in foxtail millet have not been reported in previous studies. Hence, these new transcripts may represent untapped gene sources allowing specific responses to short-term salt-stress in an orphan crop known to possess a natural adaptation capacity to abiotic stress. Quantitative real-time PCR of 21 highly up-regulated (≥ 2.5-fold) transcripts showed temporal variation in expression in both cultivars under salinity. Among them, several transcription factors and signalling genes were preferentially expressed in the tolerant cultivar. These results suggest that the tolerant cultivar possesses more effective signal-perception mechanisms for metabolic adjustments in plants under harsh saline conditions. Our findings provide evidence that the unknown genes identified in this study, in addition to several known genes, may play important roles in stress tolerance mechanisms present in foxtail millet.