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Iron (Fe) is an essential micronutrient in plants as it takes part in major metabolic pathways such as photosynthesis and respiration and is linked to many enzymes that accomplish many other cellular functions (DNA synthesis, nitrogen fixation, hormone production). Fe deficiency, a major abiotic stress, reduces crop yields, especially in calcareous soils in which the solubility of Fe is extremely low because of the high soil pH. Parietaria judaica is a wild sinantropic Strategy I plant that implements many integrated mechanisms allowing to successfully complete the life cycle in highly calcareous environments. In this work the main mechanisms by which P. judaica overcomes the low bioavailability of Fe have been identified. P. judaica was subject to direct and induced Fe deficiency growth conditions in hydroponic systems. Strategy I biochemical mechanisms of FeII-reduction and rhizosphere acidification were studied, as well as low organic acids and phenolics in root exudates. It was suggested that the accumulation and exudation of phenolic compounds plays a central role in the adaptive strategy of P. judaica to cope with Fe deficiency conditions. Key enzymes of primary and secondary metabolism were assayed in order to identify the metabolic rearrangement that occurs under Fe deficiency conditions. The data analysed confirm that under Fe deficiency the metabolic rearrangement takes place by modifying allocation of carbon skeletons between primary and secondary metabolism. It was observed that secondary metabolism constitutes the main concern under this stress condition as P. judaica sustains the supply of substrates using non oxidative ways. Phenolic composition was characterized by HPLC analyses in P. judaica focusing on phenolic composition changes due to low Fe availability. Phenolic compounds found in P. judaica belong mainly to the mono- and di- caffeoylchinic acids group. The chlorogenic acid was resulted the most sensible component under Fe deficiency stress. Morphological and architectural modifications of root system were also analysed. P. judaica changes its root system according to the experimental treatment imposed. The differences found in direct and induced Fe-Deficiency conditions were oriented to increase the root contact surface with the medium. In particular, the root architecture reflected the plant nutritional status. Comparing the data obtained from high bicarbonate and highly alkaline buffer conditions it was observed that P. judaica has no problems to acquire Fe in highly alkaline environments, suggesting that in a highly chalky environment the availability of bicarbonate itself constitutes the real factor of stress. |
