Hormonal responses of nodulated Medicago ciliaris lines differing in salt tolerance

Hormonal changes in two Medicago ciliaris lines differing in salt tolerance (TNC 1.8 being more tolerant than TNC 11.9) were studied as possible regulators of growth and symbiotic nitrogen fixation (SNF). After 21 days of saline treatment (100 mM NaCl), four major phytohormones (abscisic acid, ABA; the cytokinin trans-zeatin, t-Z; the auxin indole-3-acetic acid, IAA; and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid, ACC) were analysed. Salinity differently affected hormone concentrations in vegetative tissues and nodules in both lines. Principal component analysis (PCA) revealed that growth and SNF parameters under salinity were inversely correlated to the amount of reduced nitrogen (Nred) allocated to the roots and nodules, to the sucrolytic activity (TSA) in the roots, and to the t-Z and ABA concentration in the nodules. However, those parameters were positively associated along PC1 to the Nred and TSA in the leaves and all the hormones analysed in the roots. Interestingly, the ACC concentration of all organs was positively associated with vegetative growth and with SNF under salinity, as a putative regulator of the symbiotic-parasitic relation with the bacteria. The influence of hormonal changes in relation to plant growth, SNF and symbiotic relation is discussed.

► Hormonal changes between nodules and vegetative tissues control symbiotic relations, N-fixation and growth under salinity. ► N-reduction in control conditions seems regulated by cytokinins, abscisic acid and sucrolytic activity in the nodules. ► Salt sensitivity was related to the retention of N compounds in the roots in detriment of the leaves. ► The ABA/cytokinins ratio seems to be a master regulator of the nodule activity respect the rest of the plant. ► Salt-tolerance was related to the ethylene-precursor ACC as a measure to maintain the symbiotic over a parasitic relation.