A novel type of phytosulfokine, PSK-ε, positively regulates root elongation and formation of lateral roots and root nodules in <i>Medicago truncatula</i>

Phytosulfokines (PSKs) are a class of tyrosine-sulfated pentapeptides. PSK-α, PSK-γ, and PSK-δ 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-ε with the sequence Y_SO3VY_SO3TN, and its precursor proteins (MtPSKε, LjPSKε, and GmPSKε), specifically from legume species. PSK-ε peptide differs from PSK-δ by one amino acid and is close to PSK-δ in the phylogenetic relationship. Expression profile analysis showed that <i>MtPSKε</i> was highly expressed in <i>Medicago truncatula</i> roots, especially in root tips and emerged lateral roots. Application of the synthetic sulfated PSK-ε peptide and overexpression of <i>MtPSKε</i> significantly promoted <i>M. truncatula</i> root elongation and increased lateral root number, probably by inducing cell division and expansion in roots. Furthermore, <i>MtPSKε</i> expression was induced by rhizobia infection and was detected in root nodules including nodule primordia. Both PSK-ε peptide treatment and <i>MtPSKε</i> overexpression significantly increased nodule number in <i>M. truncatula</i>. Taken together, these results demonstrate that PSK-ε, a novel type of phytosulfokine, positively regulates root elongation and formation of lateral root and root nodule in <i>M. truncatula</i>.