Evidence of Salicylic Acid-Mediated Protein Degradation and Amino Acid Transport in Mature Leaves of Brassica napus

Brassica napus is characterized by low nitrogen remobilization efficiency from source leaves to seed filling, which is mainly due to poor hydrolysis of foliar proteins. To investigate the roles of salicylic acid (SA) in the proteolytic process in source organ at the early pod-filling stage, the SA concentration, protein profile, protease activity, and amino acid transporter genes were measured in mature leaves sprayed with SA or control at four and 30 days after treatment. Exogenous SA application resulted in an increase in SA concentration, accompanied with a significant decrease in protein with concomitant increase in amino acid concentration. Rubisco proteins were largely degraded in the SA-applied leaves. A significant increase in protease activity (up to 98 % at day 30) in the SA-applied leaves was observed. The increased proteolytic activity was confirmed in-gel staining of protease. Three amino acid permeases, BnAAP1, BnAAP2, and BnAAP6, were identified in mature leaves. Exogenous SA application enhanced the expression of three amino acid transporter genes. The present study clearly indicates that the SA level increased by exogenous SA application could improve N remobilization efficiency by up-regulating protein degradation and amino acid transport in source leaves at the pod-filling stage.