AtMHX is an auxin and ABA-regulated transporter whose expression pattern suggests a role in metal homeostasis in tissues with photosynthetic potential

AtMHX is a vacuolar transporter encoded by a single gene in Arabidopsis thaliana (L.) Heynh. It exchanges protons with Mg²⁺, Zn²⁺, and Fe²⁺ ions. Proper homeostasis of these metals is essential for photosynthesis and numerous enzymatic reactions. In particular, very little is known about mechanisms involved in Mg²⁺ homeostasis in plants. Expression analysis using reporter-gene constructs suggested that AtMHX functions in metal homeostasis mainly in tissues with photosynthetic potential. This balancing is conducted by expression in the vascular region, the cortex of stems, trichomes, and hydathodes. Expression in stems is developmentally regulated, suggesting that minerals are accumulated in the upper regions of young stems, and are released during silique development. Mineral content in different stem parts was consistent with this possibility. Expression was induced by auxin and ABA, but not by the metal content of the growth medium, suggesting that expression is mainly regulated by endogenous developmental programs. AtMHX exhibits two distinguished regulatory properties. Its leader intron is absolutely essential for expression, and mediates an 86-fold enhancement of expression. This enhancement is the highest reported thus far for any dicot intron. Another remarkable feature is that a repetitive genomic element of 530 bp (or part of it) functions as an enhancer.