Physiological and iTRAQ-Based Proteomic Analyses Reveal the Function of Spermidine on Improving Drought Tolerance in White Clover

Endogenous spermidine interacting
with phytohormones may be involved
in the regulation of differentially expressed proteins (DEPs) associated
with drought tolerance in white clover. Plants treated with or without
spermidine (50 μM) were subjected to 20% PEG 6000 nutrient solution
to induce drought stress (50% leaf-relative water content). The results
showed that increased endogenous spermidine induced by exogenous spermidine
altered endogenous phytohormones in association with improved drought
tolerance, as demonstrated by the delay in water-deficit development,
improved photosynthesis and water use efficiency, and lower oxidative
damage. As compared to untreated plants, Spd-treated plants maintained
a higher abundance of DEPs under drought stress involved in (1) protein
biosynthesis (ribosomal and chaperone proteins); (2) amino acids synthesis;
(3) the carbon and energy metabolism; (4) antioxidant and stress defense
(ascorbate peroxidase, glutathione peroxidase, and dehydrins); and
(5) GA and ABA signaling pathways (gibberellin receptor GID1, ABA-responsive
protein 17, and ABA stress ripening protein). Thus, the findings of
proteome could explain the Spd-induced physiological effects associated
with drought tolerance. The analysis of functional protein–protein
networks further proved that the alteration of endogenous spermidine
and phytohormones induced the interaction among ribosome, photosynthesis,
carbon metabolism, and amino acid biosynthesis. These differences
could contribute to improved drought tolerance.