Iron induced stress signaling in grapevine rootstocks

The project VineLresp (Stress response of grapevine rootstocks to iron deficiency by high bicarbonate soil) investigates the response of iron deficiency and high bicarbonate contents in soils of grapevine rootstocks. Lime induced chlorosis is a huge problem in agronomy and also in viticulture as many viticulture areas worldwide have high lime contents. Grapevine rootstocks show different levels of adaptation to high lime contents, but the biochemical mechanisms behind are not well understood. In model plants like Arabidopsis thaliana the information on iron uptake has been increased dramatically during the last years. VineLresp aims to close the knowledge gap between grapevine and A. thaliana in terms of root exudates composition, endodermal suberization and local and system signals within the plant. Thereby we hypothesize that root exudates in grapevine rootstocks are composed of more complex biomolecules as have been described so far and that the suberization of the endodermis can be an additional factor influencing nutrient uptake. In a step-wise approach we will investigate several aspects of grapevine rootstock physiology. We will combine physiological measurements targeted metabolite analyses, RNASeq and radiotracer analyses. The novelty of our approach is the application of standardized experimetns with high end analyses on different levels and the focus on processes yet unknown in grapevine rootstocks.

The VineLresp project aimed to investigate the different tolerance levels of grapevine rootstocks with regard to the availability of iron, either due to iron deficiency or high lime contents. Yellow leaves (chlorosis) is the symptom of iron deficiency and strongly influences photosynthesis. In a first step, we validated different conditions for pot experiments for their suitability to address the research questions. To induce lime stress with bicarbonate, an inert substrate as sand turned out to be the most efficient solution, while a hydroponic culture suited best to modulate the composition of the nutrient solution. The stress response of the plants was determined by measuring plant physiology, growth, root morphology, production of secondary metabolites and gene expression with RNASeq. All experiments were carried out with the tolerant grapevine rootstock Fercal and the susceptible rootstock 3309C. Iron deficiency (-Fe) and low iron availability through high bicarbonate content (+Fe+BIC) led to the following key results: (a) significantly stronger foliar symptoms in 3309C (as expected), although the concentration of iron in leaves was similar to Fercal, (b) disturbed plant growth in both rootstocks, but Fercal enhanced root growth under +Fe+BIC stress, supported by increased expression of root development genes, c) the profile of organic acids in the roots was strongly influenced by the type of stress and genotype, underlining their importance for nutrient uptake, and d) increased expression of genes for iron mobilisation and transport in the root tips of Fercal, which may indicate a more efficient transport from roots to shoots. The experiment was repeated as a split-root system and further analyses will provide additional results on plant signaling. The incorporation of suberin into the endo- and exodermis of the root was analysed with cross sections and also showed differences between Fercal and 3309C, with Fercal incorporating suberin earlier and more strongly. To further understand Fe uptake in plants, we performed a hydroponic experiment with iron deficiency in combination with nitrogen supplied either as nitrate or in combination as nitrate and ammonium. The main results are: a) Fercal invests in root growth and ferric reductase activity under Fe deficiency, b) nitrogen only as nitrate increases the severity of chlorosis symptoms, c) different genes for iron uptake, regulation and transport are strongly affected in their expression and emphasize the influence of different rootstocks. In summary, our experiments identified a strong influence of rootstocks on outcomes at different levels, both due to iron deficiency and carbonate stress. In conclusion, Fercal has a higher adaptive capacity to maintain the balance of iron in the plant under iron deficiency compared to 3309C. Our results contribute significantly to the understanding of phenotypic differences in grapevine rootstocks, but further studies are needed to understand the hierarchy of processes.