Comparative analyses of three legume species reveals conserved and unique root extracellular proteins

Increasing evidence suggests that root extracellular proteins are involved in interactions between roots and their soil environment. In the present study, exudates released by 6-day-old roots of the three legume species white lupin (Lupinus albus), soybean (Glycine max), and cowpea (Vigna sinensis) were collected under axenic conditions, and their constitutively secreted proteomes were analyzed. Between 42 and 93 unique root extracellular proteins with 2 or more different peptide fragments per protein were identified by LC-MS/MS. Functional annotation of these proteins classified them into 14-16 different functional categories. Among those 14 homologous proteins were identified in at least two legume species. Among the unique proteins, 58 in white lupin, 85 in soybean, and 31 in cowpea were specific for each plant species, and many of them were classified in the same functional categories. Interestingly, in contrast to soybean and cowpea, two protein bands of approximately 16 and 30 kDa were present on the SDS-PAGE gel of white lupin. The identification of these bands revealed a class III chitinase and a thaumatin-like protein. Both belong to the class of pathogenesis-related proteins. The results imply that root extracellular proteins play important roles in the cross-talk between plant roots and the rhizosphere.