Comparative Proteomic Analysis Reveals That Antioxidant System and Soluble Sugar Metabolism Contribute to Salt Tolerance in Alfalfa (Medicago sativa L.) Leaves

Soil salinity poses
a serious threat to alfalfa (Medicago
sativa L.) productivity. To characterize
the molecular mechanisms of salinity
tolerance in Medicago, the comparative
proteome of leaves from Medicago sativa cv. Zhongmu No.1 (ZM1, salt-tolerant) and Medicago
truncatula cv. Jemalong A17 (A17, salt-sensitive)
was performed using the iTRAQ approach. A total of 438 differentially
expressed proteins (DEPs) were identified, among which 282 and 120
DEPs were specific to A17 and ZM1, respectively. In salt-tolerant
ZM1, key DEPs were primarily enriched in antioxidant system, starch
and sucrose metabolism, and secondary metabolism. ZM1 possessed a
greater ability to remove reactive oxygen species and methylglyoxal
under salt stress, as demonstrated by enhancement of the antioxidant
system and secondary metabolism. Moreover, ZM1 orchestrated starch
and sucrose metabolism to accumulate various soluble sugars (sucrose,
maltose, glucose, and trehalose), which in turn facilitate osmotic
homeostasis. Salt stress dramatically inhibited photosynthesis of
A17 due to the downregulation of the light-harvesting complex and
photosystem II related protein. Quantitative analyses of photochemical
efficiency, antioxidant enzyme activities, hydrogen peroxide, malondialdehyde,
and soluble sugar contents were consistent with the alterations predicted
on the basis of DEP functions. These results shed light on our understanding
of the mechanisms underlying the salt tolerance of alfalfa.