A novel type of phytosulfokine, PSK-epsilon, positively regulates root elongation and formation of lateral roots and root nodules in Medicago truncatula

Phytosulfokines (PSKs) are a class of tyrosine-sulfated pentapeptides. PSK-alpha, PSK-gamma, and PSK-delta are three reported PSK members involved in regulating plant growth, development, and resistance to biotic and abiotic stresses. Here, we reported a novel type of PSK, PSK-epsilon with the sequence YSO3VYSO3TN, and its precursor proteins (MtPSK epsilon, LjPSK epsilon, and GmPSK epsilon), specifically from legume species. PSK-epsilon peptide differs from PSK-delta by one amino acid and is close to PSK-delta in the phylogenetic relationship. Expression profile analysis showed that MtPSK epsilon was highly expressed in Medicago truncatula roots, especially in root tips and emerged lateral roots. Application of the synthetic sulfated PSK-epsilon peptide and overexpression of MtPSK epsilon significantly promoted M. truncatula root elongation and increased lateral root number, probably by inducing cell division and expansion in roots. Furthermore, MtPSK epsilon expression was induced by rhizobia infection and was detected in root nodules including nodule primordia. Both PSK-epsilon peptide treatment and MtPSK epsilon overexpression significantly increased nodule number in M. truncatula. Taken together, these results demonstrate that PSK-epsilon, a novel type of phytosulfokine, positively regulates root elongation and formation of lateral root and root nodule in M. truncatula.