Physiological Processes Toward Movement of Micronutrients from Soil to Seeds in Biofortification Perspectives

Abstract

Out of the 17 elements, zinc and iron are the most important, and their less bioavailability in diets causes deficiency symptoms in human often leading to serious physiological disorder. Therefore, biofortified seeds or grains enriched with these nutrients particularly Zn and iron in staple crops can eradicate malnourishment to large extent. Genetic variability in the micronutrients among crop species has been well documented which could be exploited to improve essential micronutrients in the seeds through conventional breeding or biotechnological interventions. The crop species widely differ in distribution of micronutrients in various parts of the plant and also the diverse routes through which micronutrients move and get accumulated in seeds. The major challenges are to divert more nutrients in the target tissue seeds that are edible part of the crops. The phloem loading and unloading into the target tissue of plants are not clearly known, and the process is considered to be major limiting factors toward biofortification. The other limiting factors are the soil itself in which bioavailability to the plants is sufficiently low in spite of adequate availability of these elements in soil as they remained in bound or fixed in one or another form. The physicochemical properties of soil determine the efficiency of the uptake of nutrients. Soil enriched with organic decomposed matters and plant growth promoting rhizobia facilitate the absorption of nutrients. The major channels of nutrient uptake are through extensive network of root hairs that enter into the root vascular tissue through symplastic or apoplastic pathways. Phloem loading of micronutrients primarily in chelated form is an essential process prior to transport into seeds. The iron chelator nicotianamine (NA) plays a major role in binding copper and zinc in addition to iron and other ions. Unloading of micronutrients present in the phloem sap into the seeds takes place through bulk flow created by pressure gradient. The requisite osmotic potential buildup in the source tissue due to sugars, organic acids, and potassium ions is responsible to draw water from sink tissue and in exchange unload the micronutrients dissolved in phloem sap. The movement of micronutrients in the phloem such as zinc and iron takes place along with other major ions like potassium, chloride, and sugars. Foliar application of fertilizer or soil application in soluble forms and bioinoculants of AM fungi and bacteria enhance mobilization of zinc and iron in the plant system. The homeostasis of these elements that is balancing the amount in different tissues and redistribution as per demand is considered to be the important aspects to investigate for enriching nutrient content in the seeds. Several transporter genes have been identified, and several genetically modified crops showed manifold increase in the zinc and iron content. Twelve agriculturally important crops are presently in process of biofortification under the megaproject HarvestPlus. The mobility of the zinc and iron has been elucidated using model plant Arabidopsis thaliana and mutant lacking some genes of transporter family. The present review analyzes the mobility of these elements inside the plant, their distribution, limiting factors, and strategies to improve the mineral content in the seeds.