A proteomic analysis of drought and salt stress responsive proteins of different sorghum varieties
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Sorghum (Sorghum bicolorï, a drought tolerant cereal crop, is not only an important food source in the semi arid/arid regions but also a potential model for studying and gaining a better understanding of the molecular mechanisms of drought and salt stress tolerance in cereals. This study reports on a proteomic analysis of sorghum proteomes in response to salt and hyperosmotie stresses. Two-dimensional gel electrophoresis (2DE) in combination with mass spectrometry (MS) was used to separate, visualise and identify sorghum proteins using both sorghum cell suspension cultures and whole plants. The sorghum cell suspension culture system was used as a source of culture filtrate (CF) proteins. Of the 25 visualised CBB stained CF spots, 15 abundant and well-resolved spots were selected for identification using a combination of MALDI- TOF and MALDI- TOFTOF MS, and database searching. Of these spots, 14 were positively identified as peroxidases, germ in proteins, oxalate oxidases and alpha-galactosidases with known functions in signalling processes, defense mechanisms and cell wall metabolism. Following 200 mM NaCl and 400 mM sorbitol stress treatments, the expression/abundance of a protein spot similar to a rice wall-associated protein kinase was upregulated in the sorghum secretome in response to both stresses. Amino acid sequence alignment of the matching peptides between these two proteins showed that the sorghum CF spot possesses a protein kinase domain. Therefore, this protein could possibly participate in cell signalling functions, which link the external environment with the cell's cytoplasm. Using whole plant systems, a comparative study of leaf protein expression between two sorghum varieties, AS6 (salt sensitive) and MN1618 (salt tolerant) was conducted. Forty well resolved spots of varying abundances were picked for MS analysis. Of these, 28 were positively identified, representing proteins with functions in carbohydrate metabolism (60.7%), proton transport (17.9%), protein synthesis (7.1%), hydrolytic functions (7.1%), nucleotide metabolism (3.6%) and detoxification (3.6%). Using PDQuest™ Advanced 2D Analysis Software version 8.0.1 (BIO-RAD), a comparative analysis of leaf proteome expression patterns between the two sorghum varieties was conducted. The results indicated proteins with similar expression patterns as well as qualitative and quantitative differences between the two leaf proteomes. The effect of 100 mM NaCI on leaf proteome expression between the two sorghum varieties was also studied. Western blotting analysis of leaf, sheath and root tissues using Hsp70 antibodies showed that this treatment induced Hsp70 expression, a known stress protein, in both varieties. Thereafter, the partially annotated leaf proteome map was used to landmark other salt responsive proteins. Examples of differential expression patterns included glutathione S transferase and hydroxynitrile lyase proteins whose abundances were upregulated in both varieties, while the large subunit of RuBisCo was downregulated in AS6 but upregulated in MN1618. Qualitative spot expression differences in response to salt stress were also observed between the two sorghum varieties but these remained unidentified after both MALDI-TOF and MALDI-TOF-TOF MS, possibly indicating novel and previously uncharacterised sorghum proteins. The results of this study can be used as reference tools by proteomics researchers worldwide as well as a foundation for future studies.