The role of a vacuolar zinc transporter in tolerance and accumulation in zinc hyperaccumulators

Jeffery L Gustin, Purdue University

Abstract

Zinc (Zn) is one of a group of elements generated by fusion reactions within the cores of stars and is an essential element for living organism on the planet. Zn is the 23rd most abundant element in the earth's crust, and occurs naturally almost exclusively as Zn ion with oxidation state +2 (Zn II). In this naturally occurring state, the electron shell is completely full making Zn an extremely stable ion. Because of its stability in ionic form, Zn is believed to have been used for biochemical reactions in the very reducing conditions of earth's early oceans. Given this hypothesized early adoption in biological systems, it is not surprising that Zn is now a critical component of hundreds of physiological processes. In terrestrial plants Zn is acquired from soil solutions by the root and transported throughout the plant to maintain the viability of the Zn-requiring processes. However, Zn can also be toxic if accumulated to excessive levels. Therefore, plants must maintain a homeostatic balance of Zn to provide for critical functions and yet guard against excess accumulation. In this dissertation, I address both high Zn accumulation and Zn deficiency in plants. I begin by introducing a group of plants that can accumulate extremely high levels of Zn into their leaves when growing in their native habitat. High Zn accumulation is typically toxic to most plants, however these hyperaccumulating plants have evolved mechanism that actively acquire and tolerate high Zn levels in the leaves. I have adapted a reciprocal grafting technique, which has allowed us to address fundamental questions regarding these plants' extreme Zn physiology. Studies suggest that one way in which Zn hyperaccumulators are able to accumulate high levels of Zn into the leaves by partitioning the normally toxic Zn levels away from critical cellular mechanisms through sequestration into the vacuoles of leaf cells. In the second chapter, I characterize a vacuolar Zn transporter, MTP1, from the Cation Diffusion Facilitator (CDF) family and find that this protein is likely to contribute significantly to the Zn hyperaccumulation phenotype of these plants by efficiently sequestering Zn into the vacuole. In fact, MTP1 is so efficient at sequestering Zn away from other cellular components that, when heterologously expressed in a nonaccumulating plant Arabidopsis thaliana, it induces Zn deficiency responses. Creation of this genetically induced Zn deficiency allowed us to readdress an old, but potentially important proposition in chapter three which suggest that Zn deficiency alters auxin regulated growth and developmental phenotypes. The conclusions from our study support the proposition and expand upon previous observations using current knowledge of the roles of auxin in plant growth and development. Lastly, I characterize the phylogenetic structure of the plant CDF family of vacuolar cation transporters and find, among other conclusions, that the origins of the ancestral Zn transporter extend back to an age when prokaryotic ancestors were evolving in highly reducing seas.

Degree

Ph.D.

Advisors

Salt, Purdue University.

Subject Area

Agronomy|Horticulture|Botany

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