Characterization of alkaline stress tolerance mechanisms in Lotus forage species modulated by Pantoea eucalypti

Abstract

This study was designed to elucidate the physiological responses of three Lotus forage accessions to alkaline stress and the influence of the inoculation of a Pantoea eucalypti endophyte strain on its mitigation. One-month-old diploid accessions of Lotus corniculatus (Lc) and Lotus tenuis (Lt), and the interspecific hybrid LtxLc obtained from these parental accessions, were exposed to alkaline stress (pH 8.2) by the addition of NaHCO3 10 mM to the nutrient solution for 2 weeks. The results indicated that Lt and the LtxLc hybrid are alkaline-tolerant compared to Lc, based on the observation that their dry mass is not reduced under stress, and symptoms of chlorosis do not appear on leaf blades, in contrast to observations of the Lc accession subjected to identical growth and stress conditions. In Lc and LtxLc accessions, the Fe2+ concentration decreased in the aerial part under stress and increased in the roots. Interveinal chlorosis observed in the youngest leaves of Lc during alkaline treatment was accompanied with a higher reduction of Fe2+ levels in shoots and a higher increment of Fe2+ in roots, compared to the other accession. Plant inoculation also tended to acidify the medium under alkalinity, contributing to Fe accumulation in the roots. Moreover, the inoculation caused a considerable increase in Fe2+ content in shoots in all three Lotus forage species under alkaline treatment. Fv/Fm and PIABS were only reduced in Lc under alkaline treatment. Inoculation reverted this effect and improved the ABS/RC and DIo/RC ratios in all three accessions. In addition, under alkaline conditions, Lc dissipated more energy than control plants. Expression of the metal-transporting gene NRAMP1 increased in the inoculated Lc accession under stress, while remaining unmodified in Lt and LtxLc hybrid. Altogether, the results obtained make clear the importance of inoculation with P. eucalypti, which contributed significantly to the mitigation of alkaline stress. Thus, all the results provide useful information for improving alkaline tolerance traits in Lotus forage species and their interspecific hybrids.