Title: Xylem exudate composition and root-to-shoot nickel translocation in Alyssum species
Authors: Centofanti T, Sayers Z, Cabello-Conejo MI, Kidd P, Nishizawa NK, Kakei Y, Davis AP, Sicher Jr RC, Chaney RL
Journal: New Phytol
Accepted date: 2013 May 15
Interpretive summary: Improved understanding of the physiology of nickel hyperaccumulator plants is expected to improve the development of phytoextraction/phytomining technology for nickel. The chemicals involved in nickel translocation remains one of the unsettled questions about how Alyssum species achieve such extreme nickel accumulation in their shoots which is required for commercial phytoextraction of nickel. Early studies suggested nickel was chelated to malate, a common organic acid in plants. But the weak chelation of nickel by malate indicated that citrate was more likely chelating xylem nickel. Subsequently, a report indicated that histidine was greatly increased in xylem exudate of Alyssum hyperaccumulating nickel. But additional studies suggested that this outcome may have been an artifact of the experimental method.To clarify nickel translocation chemistry in Alyssum, we grew several hyperaccumulator and non-accumulator species in nutrient solutions with varied nickel levels, or in naturally Ni rich serpentine soils, and used modern methods of analysis of trace levels of organic compounds suspected to be involved. Further, we grew the plants for months with the high nickel solutions or soils to attain a steady-state in nickel translocation. Xylem exudate from these plants commonly contained 3000 micromolar nickel, but only less than 100-300 micromolar citrate, malate or histidine. Another suspected nickel chelator, nicotianamine, was also not high enough to chelate much of the exudate nickel. We conclude that most of the nickel in stem exudate of nickel hyperaccumulating Alyssum species is the free cation rather than chelated forms.
Publication date: 2013 Dec