Cultivar-Specific Oxidative Modification of Proteins, Proteolytic Activity and Alterations in Proteomes of Rice Seedlings to Simultaneous Water Deficit and Aluminum Toxicity

Water deficit (WD) and aluminum toxicity (Al) are two key constraints to crop production, especially in upland acid soils. We examined the combined effects of the two stresses in comparison to individual stresses in the seedlings of rice (Oryza sativa L.) cultivars with contrasting stress tolerance, on its antioxidant capacity, oxidative modification of proteins, status of proteolytic activity and proteomic alterations, when grown in sand cultures. Seedlings exposed to WD and Al-excess for 24–72 h showed growth reduction, increased protein carbonylation in the tissues and increased proteolysis. In WD/Al-sensitive cv. Malviya-36, the combined stresses caused more reduction in growth compared to the individual stresses. Immunoblot analysis of protein carbonyls from root/shoot extracts revealed increased oxidatively modified proteins due to WD and Al-excess. When exposed to either WD, Al-excess or combined stresses, cv. Malviya-36 seedlings showed increased proteolysis, more activity bands corresponding to proteolysis and more oxidized proteins than cv. Vandana (WD /Al-tolerant) seedlings. The antioxidant capacity appeared to be greater in cv. Vandana (tolerant) than cv. Malviya-36 (sensitive) measured by DPPH assay. Proteins from rice shoots, separated by 2DE, when analyzed by PDQuest, revealed nearly 200 clear spots, among which 55 were differentially abundant under individual and combined stresses. MALDI-TOF mass spectrometry of differentially abundant proteins revealed that many chloroplast and photosynthesis-related proteins were downregulated, 6 proteins disappeared and the expression of calmodulin and ABA/stress-induced proteins was specifically induced in the sensitive cultivar under combined stresses indicating that photosynthesis was affected in this cultivar under combined stresses.