Phytometallophore-Mediated Nutrient Acquisition by Plants

Abstract

Soils in many agricultural areas are alkaline and have high amounts of calcium carbonate, resulting in low availability of micronutrients. Plant nutrient uptake is influenced by root architecture, the presence of mycorrhizal fungi, proton exudation from roots, and release of phytometallophores. Some genotypes are able to release phytometallophores from roots to the surrounding rhizosphere soil, which increases the solubility of nutrients and as a result, its availability for plant uptake. Phytomettalophore (PM) release occurs under nutrient deficiencies in representative Poaceae and has been speculated to be a general adaptive response to enhance the acquisition of micronutrient metals. Many vascular plant species are unable to colonize calcareous sites. The inability of calcifuge plants to establish in limestone sites could be related to a low capacity of such plants to solubilize and absorb Fe from these soils. Under Fe deficiency, species of Poaceae enhance their Fe uptake by releasing non-proteinogenic amino acids, phytometallophores (PM), from their roots which mobilize Fe from the soil by forming a chelate that is then taken up by the root. Phytometallophore release and uptake is thought to be specific for Fe deficiency. However, a universal role of phytometallophores in the acquisition of micronutrient metals is established. Since PM form stable chelates with Zn, Mn, and Cu, they extract considerable amounts of Zn, Mn, and Cu from calcareous soils and deficiencies of Zn, Mn, and Cu are quite common on calcareous and non-calcareous soils. Fe deficiency in the shoot triggers the production of phytometallophores. Although PM release under nutrient deficiency is not a specific response, it could still have ecological significance. Studying metal extraction by PM from a wide range of calcareous and non-calcareous soils indicated that PM preferentially mobilize Fe but also significant quantities of Zn and Cu from soils. Considering that plant Cu demand is much lower than Fe demand, the amounts of Cu mobilized appeared sufficient to meet plant requirements for this metal. This suggests that PM release would be an advantage for plants growing on soils low in available Cu. PM release in response to deficiencies of micronutrient metals is not restricted to Fe and Zn, but might be more widespread than previously thought. The mechanism seems to be specific for Fe and Cu deficiencies, but not for Zn or Mn deficiency in crop. The release of PM under nutrient deficiency could have an ecological significance, regardless of whether it is indirectly caused by impaired metabolism or as a specific response mechanism. Also, it remains to be examined whether nutrient-deficiency-induced PM release is a general phenomenon in crop species.