In nature, iron (Fe) occurs in abundance and ranks fourth among all elements on Earth's surface. Still, its availability to plants is reduced, once this element is in the form of hydrated oxides, which can limit plant productivity and biomass production. On the other hand, in high concentrations, this essential micronutrient for the plants can become a toxic agent, increasing the environmental contamination. Fe is necessary for the maintenance of essential processes like respiration and photosynthesis, participating in the electron transport chain and in the conversion between Fe 2+ and Fe 3+

At relatively low concentrations, the element manganese (Mn) is essential for plant metabolism, especially for photosynthesis and as an enzyme antioxidant cofactor. However, industrial and agricultural activities have greatly increased Mn concentrations, and thereby contamination, in soils. We tested whether and how growth of Pisolithus tinctorius is influenced by Mn and glucose and compare the activities of oxidative stress enzymes as biochemical markers of Mn stress. We also compared nutrient accumulation, ecophysiology, and biochemical responses in Eucalyptus grandis which had been colonized by the ectomycorrhizal Pisolithus tinctorius with those which had not, when both were exposed to increasing Mn concentrations. In vitro experiments comprised six concentrations of Mn in three concentrations of glucose. In vivo experiments used plants colonized by Pisolithus tinctorius, or not colonized, grown with three concentrations of Mn (0, 200, and 1000 μM). We found that fungal growth and glucose concentration were correlated, but these were not influenced by Mn levels in the medium. The anti-oxidative enzymes catalase and glutathione S-transferase were both activated when the fungus was exposed to Mn. Also, mycorrhizal plants grew more and faster than non-mycorrhizal plants, whatever Mn exposure. Photosynthesis rate, intrinsic water use efficiency, and carboxylation efficiency were all inversely correlated with Mn concentration. Thus, we originally show that the ectomycorrhizal fungus provides protection for its host plants against varying and potentially toxic concentrations of Mn.

Background: The use of plant-based biostimulants to increase crop yield and enhance protection has grown in popularity. We previously manufactured an inoculant combining humic acids with plant growth-promoting bacteria and observed a significant effect on crop yields. However, electrophysiology studies elucidating the mechanisms responsible for enhanced nutrient and water uptake are scarce and are generally restricted to in vitro models. Materials and methods: In this study, a suspension of humic acids isolated from vermicompost along with Herbaspirillum seropedicae was applied to maize seedlings. We measured the root H + flux with an ion-selective vibrating probe system. Furthermore, the transcription of plasma membrane H + ATPase, aquaporin and high-or low-affinity nitrate transporters was measured. Results: Inoculation activated the extracellular H + flux, thus changing the pH and membrane potential of maize root cells and altering the electrochemical potential generated by P-H +-ATPase at the biochemical and molecular level. The overexpression of aquaporins was also observed; however, nitrate transporters were repressed by the inoculants. Conclusion: We demonstrate an increase in the H + efflux in maize root seedlings inoculated with H. seropedicae and concomitant changes in membrane voltage. The inoculation of H. seropedicae in the presence of humic acids decreased the H + flux and surface acidification without changes in the aquaporin transcription level. However, the H + flux in root seedlings in inoculated plants (with or without humic acids) was larger in respect to control plants. These results support the increased water and nitrogen efficiency in plants inoculated with humic acids and H. seropedicae.

Somatic embryogenesis is an important biological process in several plant species, including sugar cane. Proteomics approaches have shown that H pumps are differentially regulated during somatic embryogenesis; however, the relationship between H flux and embryogenic competence is still unclear. This work aimed to elucidate the association between extracellular H flux and somatic embryo maturation in sugar cane. We performed a microsomal proteomics analysis and analyzed changes in extracellular H-flux and H-pump (P-H-ATPase, V-H-ATPase, and H-PPase) activity in embryogenic and non-embryogenic callus. A total of 657 proteins were identified, 16 of which were H pumps. We observed that P-H-ATPase and H-PPase were more abundant in embryogenic callus. Compared to non-embryogenic callus, embryogenic callus showed higher H influx, especially on maturation day 14, as well as higher H-pump activity (mainly, P-H-ATPase and H-PPase activity). H-PPase appears to be the major H pump in embryogenic callus during somatic embryo formation, functioning in both vacuole acidification and PPi homeostasis. These results provide evidence for an association between higher H-pump protein abundance and, consequently, higher H flux and embryogenic competence acquisition in the callus of sugar cane, allowing for the optimization of the somatic embryo conversion process by modulating the activities of these H pumps.

Main conclusionHigher vacuolar proton pump activity may increase plant energy and nutrient use efficiency and provide the nexus between plant inoculation with Herbaspirillum seropedicae and growth promotion.