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 Fe²⁺ and Fe³⁺, being a key element for carbon dioxide (CO₂) fixation and, therefore, important for crop production of cultivated or natural species. The balance of Fe should be strictly controlled, because both its deficiency and its toxicity affect the physiological process of plants. In aerated soils Fe is present in the form of Fe³⁺, which is the oxidized form and is less available to plants, so these organisms have developed different strategies for absorption, transport and storage of Fe. Deficiency and excess of Fe correlate with local soil conditions and with the care adopted in plant nutrition during the phenological phases and/or in the course of its cultivation. In situations of excessive accumulation of Fe in tissues, an enhancement of hydroxyl radical generation (OH^•) occurs by Fenton reaction. Here, we review the nutritional, genetic and ecophysiological aspects of uptake, translocation and accumulation of Fe ions in plants growing under conditions of deficiency or toxicity of this metal.

Background: The humic acid (HA) fraction of soil organic matter (SOM) exerts an effective plant growth promotion through a complex mechanism involving a coordinated activation of several key ion transport and signaling systems. We investigated the effects of HA on H⁺and Ca²⁺cellular dynamics at the early stages of lateral plant root development.Results: Emergence of lateral root in rice seedlings were related to specific H⁺and Ca²⁺fluxes in the root elongation zone underlying an activation of the plasma membrane H⁺-ATPase and of the Ca²⁺-dependent protein kinase (CDPK). The latter was coupled with an increased expression of the voltage-dependentOsTPC1Ca²⁺channels and two stress responsive CDPK isoforms, such asOsCPK7andOsCPK17.Conclusions: HA act as molecular elicitors of H⁺and Ca²⁺fluxes, which seem to be upstream of a complex CDPK cell-signaling cascade. These findings shed light on the first ion signaling events involved in the mechanism of action of HA on plant growth and development.

Background Common beans are a staple food in many developing countries. However, changes in global precipitation patterns, particularly short droughts during the rainy season, have affected crop production because nodulation is greatly affected by water shortages. Plant growth-promoting bacteria, together with humic substances, can alleviate the negative effects of soil stresses, including drought. Aims This work aimed to evaluate the effects of co-inoculations of rhizobia and Herbaspirillum seropedicae in the presence of humic acid-like substances isolated from vermicompost on the recovery of common beans after a controlled water stress. Methods Two independent experiments were conducted in a greenhouse using Phaseolus vulgaris cv. Grafite and cv. Bonus, which originated in Brazil and Mozambique, respectively. The soil humidity was maintained at field capacity in pots, and the water suppression was induced at the pre-flowering stage. After 12 days, the water was restored, and the beans were evaluated. Results In the first experiment with ‘Grafite’ beans, the relative water content of the foliar disks was significantly higher in the co-inoculated treatment, as were the numbers and masses of nodules. The phenylalanine ammonia lyase activity was induced by drought, and its activity was higher in co-inoculated plant leaves. In the second experiment, the rate of net photosynthesis and stomatal conductance were also evaluated. The recuperation of common beans after water stress was improved by co-inoculation with rhizobia and H. seropedicae in the presence of humic acid-like substances.