Perspectives

Since the complexity of the network controlling iron bioavailability (uptake, transport, storage and delivery) in the plant cell, any approach to improve the iron content in crops to ameliorate the nutritional quality of foods must take into consideration the multiple steps connecting iron metabolism with the primary cell metabolism. It should be noted that, an uncontrolled iron excess can result in tissue damage by an overproduction of ROS and a consequent yield diminution and/or decrease of the product quality. The finding that both Strategy I and Strategy II plants can grow healthier under iron-deficient conditions if supplemented with exogenous NO could imply the development of molecular tools designed to improve crop yield in soils with iron limiting conditions.

It has been shown that in mammals [56] as well as in plants [57] and in the unicellular green alga Chlorella sorokiniani [58], mitochondria reduce nitrite to NO. It was also reported that nitrite is the major source of NO produced by A. thaliana when challenged by the pathogen Pseudomonas syringae [25]. In parallel, Vanin et al. [23] found that the nitrite/nitrate ratio together with the endogenous level of superoxide control the concentration of NO in plants. Interestingly, it has been found that ascorbate plays an important role by producing more than 90% of the NO yield from peroxynitrite (ONOO-),

Here, it is interesting to point out that the 80% of cellular iron and the 80% of cellular ascor-bate are thought to be localized in chloroplast where chlorosis, one of the first symptoms of iron deficiency, is developed. The questions are, could Fe be released from DNICs in chloro-plast in a process that involves NO and ascorbate? Might iron and ascorbic acid be crucial for chlorophyll biosynthesis through the maintenance of NO homeostasis in chloroplasts?

As a final comment, we would like to emphasize about the fact that, most of the biological processes are a result of the interaction between numerous cellular constituents forming complex networks that are precisely coordinated and synchronized. There, metabolic compensations as well redundancy activities are operating to maintain cellular homeostasis. In this scenario, NO, iron, DNICs, GSNO and ascorbic acid are probably working together in the same act of the play that describes the mechanisms controlling plant iron homeostasis.

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