Phosphorus and iron acquisition strategies in crops: common and unique features

The ever-increasing request of food necessitates developing more sustainable agricultural practices in order to improve productivity while decreasing the environmental and economic costs. It is therefore necessary to increase the nutrient use efficiency (NUE) of crops; however to achieve this goal a deeper comprehension of soil-plant relationships and of plant mechanisms involved in nutrient acquisition is needed. It is known that phosphorus (P) and iron (Fe) are among the main nutrients responsible for determining the quality and quantity of the yield. In fact, both P and Fe are poorly available in well-aerated soils, including acid (for P) and alkaline (P and Fe) soils. To cope with these nutritional problems, some plants have evolved the capacity to release in the soil a wide range of molecules, called root exudates, which can modify the chemical, biological and physical characteristics of the rhizosphere, allowing the mobilization of the two nutrients. The aim of the present PhD thesis was to deepen the knowledge about plant mechanisms involved in the acquisition of these two nutrients and to unravel possible links between the two responses in different plant species. Maize (Zea mays, L.) is one of the most widespread cultivated cereals, both for human and animal consumption. For this reasons, this plant has been used in this PhD thesis to study the capacity to take up Fe, supplied as a sparingly soluble form (Ferrihydrite) or as soluble complexes made up between Fe and natural chelating agents normally occurring in the rhizosphere (Fe-phytosiderophores (Fe-PS) or Fe-citrate). Moreover, the effects of Fe deficiency on the mechanisms involved in P acquisition have been studied. Transcriptomic data showed that Fe deficiency induced the expression of genes coding for proteins involved in the so-called Strategy II, which enhance extra-radical availability of Fe and its acquisition. Moreover, genes encoding proteins involved in the methionine cycle and in the synthesis and release of MAs were positively modulated. The treatments of Fe-deficient plants with three natural sources (Fe-citrate, Fe-PS and Ferrihydrite) determined a downregulation of Strategy II mechanisms, indicating that the sources were used by plants with different efficiency. At the physiological level, the nutritional status influenced the acquisition and allocation of the micronutrient. Indeed, roots of Fe-deficient maize plants accumulated more Fe than the sufficient ones. An opposite behavior was detected in shoots, suggesting that the translocation system might be readily active in Fe-sufficient plants. The capability of Fe-deficient plants to acquire P from readily- or scarcely-available forms was also investigated. In –Fe plants a higher P content in roots and shoots was detected in comparison with the Fe-sufficient plants. In addition, it emerged from transcriptional data that genes encoding P transporters were upregulated in –Fe plants. In particular, ZmPHT1;7, which is putatively involved in the high affinity transport and translocation of P, seems to take part in the Fe starvation response. Indeed, when Fe-deficient plants were treated (re-supplied) with natural Fe-sources, its transcription level was repressed down to that recorded in Fe-sufficient plants. A fundamental step of the plant response to P and Fe deficiency is the release of root exudates, which are able to mobilize these poorly available nutrients in soils. White lupin (Lupinus albus L.) is considered a model plant for root exudation studies, especially when related to P deficiency. Using molecular, physiological and metabolic approaches the response to Fe-deficiency and P-deficiency was characterized with the aim to highlight possible links between the two responses. Analyzing the root transcriptoma in the two different growth conditions (-Fe or –P), a reciprocal interaction was observed between the plant adaptation to each of the two nutritional stresses. Indeed, low Fe availability triggered the positive modulation of P-deficiency-responsive genes and vice versa. Furthermore, the study of the root exudation patterns showed that compounds such as isoflavonoids (genistein and its derivatives) and coumarins (scopoletin and its derivatives) were released in response to either Fe or P deficiency. The chemical characterization of the root content showed that the same compounds were present in any growth condition but in glycosylated or malonate-glycoside forms. From previous studies, a gene called LaMATE2, putatively involved in the genistein release was identified and functionally characterized. This gene was overexpressed in roots of P-deficient lupin plants and, in particular, in cluster root tissues. Silencing of LaMATE2 transcript severely affected the genistein release from roots. The LaMATE2 protein was localized in the plasmalemma, as shown by the expression of a GFP-LaMATE2 construct in Arabidopsis protoplast. Finally, the transport of different compounds in LaMATE2-transformed yeast microsomes was studied, demonstrating that LaMATE2 protein catalyzed the transmembrane transport of genistein, and potentially also of other flavonoids, via a co-transport with H+. Finally, a study of plant response to Fe and P deficiency was also extended to a tree plant species, Malus x domestica Borkh. RNA sequencing was carried out on apple tree roots and highlighted that the deficiency of Fe and P affected different metabolic pathways, thus suggesting that apple tree plants adopt different strategies to cope with these two nutritional disorders.