Mitochondria dysfunctions under Fe and S deficiency: is citric acid involved in the regulation of adaptive responses?

Within the last years, extensive information has been accumulated on the reciprocal influence between S and Fe nutrition at both physiological and molecular level in several plant species, but the mechanisms regulating S and Fe sensing and signaling are not fully understood. Fe and S interact for the building of Fe-S clusters, and mitochondria is one of the cellular compartments where Fe-S cluster assembly takes place. Therefore, it would be expected that mitochondria might play a central role in the regulation of Fe and S interaction. The Fe deficiency-induced alteration in the synthesis of mitochondria-derived carboxylic acids, such as citric acid, and the evidence that such molecules have already been identified as important players of metabolite signaling in several organisms, further support this hypothesis. Tomato plants were grown under single or combined Fe and S deficiency with the aim of verifying whether mitochondria activities played a role in Fe/S interaction. Both Fe and S deficiencies determined similar alteration of respiratory chain activity: a general decrease of Fe-S containing complexes as well as an increase of alternative NAD(P)H activities was observed in both Fe and S deficient-plants. However, the content of Krebs cycle-related organic acids in roots was substantially different in response to treatments, being the accumulation of citric acid always increased, while the others (i.e. succinic, malic, fumaric acids) always decreased. Interestingly, citric acid levels significantly correlated with the expression of some Fe and S deficiency induced genes. Our results contribute to existing knowledge on the complexity of the S/Fe interaction, suggesting a model in which endogenous alteration of citric acid content in plant tissues might act as signal molecule for the regulation of some nuclear-encoded and nutrient-responsive genes and also provide a basis for further study of the mechanism underlying S and Fe sensing and signalling.