Plasma membrane H+ pump at a crossroads of acidic and iron stresses in yeast-to-hypha transition.

Iron is an essential nutrient but is toxic in excess mainly under acidic conditions. Yeasts have emerged as low cost, highly efficient soil inoculants for the decontamination of metal-polluted areas, harnessing an increasing understanding of their metal tolerance mechanisms. Here, we investigated the effects of extracellular iron and acid pH stress on the dimorphism of Yarrowia lipolytica. Its growth was unaffected by 1 or 2 mM FeSO4, while a strong cellular iron accumulation was detected. However, the iron treatments decreased the hyphal length and number, mainly at 2 mM FeSO4 and pH 4.5. Inward cell membrane H+ fluxes were found at pH 4.5 and 6.0 correlated with a pH increase at the cell surface and a conspicuous yeast-to-hypha transition activity. Conversely, a remarkable H+ efflux was detected at pH 3.0, related to the extracellular microenvironment acidification and inhibition of yeast-to-hypha transition. Iron treatments intensified H+ influxes at pH 4.5 and 6.0 and inhibited H+ efflux at pH 3.0. Moreover, iron treatments inhibited the expression and activities of the plasma membrane H+-ATPase, with the H+ transport inhibited to a greater extent than the ATP hydrolysis, suggesting an iron-induced uncoupling of the pump. Our data indicate that Y. lipolytica adaptations to high iron and acidic environments occur at the expense of remodelling the yeast morphogenesis through a cellular pH modulation by H+-ATPases and H+ coupled transporters, highlighting the capacity of this non-conventional yeast to accumulate high amounts of iron and its potential application for bioremediation.

Inoculation With Piriformospora indica Is More Efficient in Wild-Type Rice Than in Transgenic Rice Over-Expressing the Vacuolar H+-PPase.

Achieving food security in a context of environmental sustainability is one of the main challenges of the XXI century. Two competing strategies to achieve this goal are the use of genetically modified plants and the use of plant growth promoting microorganisms (PGPMs). However, few studies assess the response of genetically modified plants to PGPMs. The aim of this study was to compare the response of over-expressing the vacuolar H+-PPase (AVP) and wild-type rice types to the endophytic fungus; Piriformospora indica. Oryza sativa plants (WT and AVP) were inoculated with P. indica and 30 days later, morphological, ecophysiological and bioenergetic parameters, and nutrient content were assessed. AVP and WT plant heights were strongly influenced by inoculation with P. indica, which also promoted increases in fresh and dry matter of shoot in both genotypes. This may be related with the stimulatory effect of P. indica on ecophysiological parameters, especially photosynthetic rate, stomatal conductance, intrinsic water use efficiency and carboxylation efficiency. However, there were differences between the genotypes concerning the physiological mechanisms leading to biomass increment. In WT plants, inoculation with P. indica stimulated all H+ pumps. However, in inoculated AVP plants, H+-PPase was stimulated, but P- and V-ATPases were inhibited. Fungal inoculation enhanced nutrient uptake in both shoots and roots of WT and AVP plants, compared to uninoculated plants; but among inoculated genotypes, the nutrient uptake was lower in AVP than in WT plants. These results clearly demonstrate that the symbiosis between P. indica and AVP plants did not benefit those plants, which may be related to the inefficient colonization of this fungus on the transgenic plants, demonstrating an incompatibility of this symbiosis, which needs to be further studied.