Arbuscular mycorrhizal fungus inocula from coastal sand dunes arrest olive cutting growth under salinity stress.

Cultivation of olive trees covers large coastal areas of land in Mediterranean regions, many of them characterized by low soil fertility and exposed to salinity and seasonal drought. In this frame, we developed mixed community inocula of arbuscular mycorrhizal fungi (AMF) derived from the extreme, seasonally arid environments of six Mediterranean sand dunes and evaluated their effects, in the form of community inocula, on rooted semi-woody olive tree cuttings (Olea europaea cv. Koroneiki). The plantlets were grown in the greenhouse for 10 months under 50 mM and 100 mM concentrations of NaCl, successively applied to induce osmotic stress. Inoculation had a positive effect on plant growth and nutrient uptake. However, the three best-performing inocula in early colonization and in plant growth enhancement also resulted in high plant sensitivity to high salinity, which was not observed for the other three inocula. This was expressed by decreased nutrient uptake and drastically lower plant growth, plant photosynthesis, and stomatal conductance (generally an over 50% reduction compared to no salinity application). Amplicon sequencing analysis of the olive plants under salinity stress showed that the AMF communities in the roots were clearly differentiated by inoculation treatment. We could not, however, consistently associate the plant responses observed under high salinity with specific shared AMF community membership or assembly attributes. The observed physiological overreaction to osmotic stress may be an adaptation trait, potentially brought about by host selection coupled to abiotic environmental filtering, in the harsh conditions from which the AMF inocula were derived. The overreaction may, however, be undesirable if conveyed to allochthonous plants at an agronomic level.

Characterization of spermidine and spermine synthases in Lotus japonicus: induction and spatial organization of polyamine biosynthesis in nitrogen fixing nodules.

The biosynthesis of the polyamines spermidine (Spd) and spermine (Spm) from putrescine (Put) is catalysed by the consequent action of two aminopropyltransferases, spermidine synthase (SPDS EC: 2.5.1.16) and spermine synthase (SPMS EC: 2.5.1.22). Two cDNA clones coding for SPDS and SPMS homologues in the nitrogen-fixing nodules of the model legume Lotus japonicus were identified. Functionality of the encoded polypeptides was confirmed by their ability to complement spermidine and spermine deficiencies in yeast. The temporal and spatial expression pattern of the respective genes was correlated with the accumulation of total polyamines in symbiotic and non-symbiotic organs. Expression of both genes was maximal at early stages of nodule development, while at later stages the levels of both transcripts declined. Both genes were expressed in nodule inner cortical cells, vascular bundles, and central tissue. In contrast to gene expression, increasing amounts of Put, Spd, and Spm were found to accumulate during nodule development and after maturity. Interestingly, nodulated plants exhibited systemic changes in both LjSPDS and LjSPMS transcript levels and polyamine content in roots, stem and leaves, in comparison to uninoculated plants. These results give new insights into the neglected role of polyamines during nodule development and symbiotic nitrogen fixation (SNF).