Differential Proteins Expressed in Rice Leaves and Grains in Response to Salinity and Exogenous Spermidine Treatments

Exogenous application of spermidine (Spd) has been reported to modulate physiological processes and alleviate salt-induced damage to growth and productivity of several plants including rice. Employing a proteomic approach, we aimed at identifying rice leaf and grain proteins differentially expressing under salt stress, and in response to Spd prior to NaCl treatment. A total of 9 and 20 differentially expressed protein spots were identified in the leaves of salt-tolerant (Pokkali) and salt-sensitive (KDML105) rice cultivars, respectively. Differential proteins common to both cultivars included a photosynthetic light reaction protein (oxygen-evolving complex protein 1), enzymes of Calvin cycle and glycolysis (fructose-bisphosphate aldolase and triose-phosphate isomerase), malate dehydrogenase, superoxide dismutase and a hypothetical protein (OsI_18213). Most proteins were present at higher intensities in Pokkali leaves. The photosynthetic oxygen-evolving enhancer protein 2 was detected only in Pokkali and was up-regulated by salt-stress and further enhanced by Spd treatment. All three spots identified as superoxide dismutase in KDML105 were up-regulated by NaCl but down-regulated when treated with Spd prior to NaCl, indicating that Spd acted directly as antioxidants. Important differential stress proteins detected in mature grains of both rice cultivars were late embryogenesis abundant proteins with protective roles and an antioxidant protein, 1-Cys-peroxiredoxin. Higher salt tolerance of Pokkali partly resulted from higher intensities and more responsiveness of the proteins relating to photosynthesis light reactions, energy metabolism, antioxidant enzymes in the leaves, and stress proteins with protective roles in the grains.