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

Abstract

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.

Magnesium stress signaling in plant: Just a beginning

“…11 These results suggest that ABA may play a critical role in response to MgT in plants. Our results support this hypothesis.…”

“…uORF significantly inhibits AtMHX expression by degradation of AtMHX mRNA, 11,23 and these processes may be regulated by ABA and auxins. 11 The novel literatures presented that overexpression of AtMHX increased expression and activity of the vacuolar H C -ATPase, moreover, transgenic plants were sensitive to high Mg 2C level, and got severe necrotic lesions, 24,25 consistent with our results that the expression of AtMHX was inhibited by MgT. 1 These possibly indicate that AtMHX mainly maintains metal and proton homeostasis in cells, the homeostasis is essential for photosynthesis and numerous enzymatic reactions.…”

“…11 Interestingly, its cis-regulons are within the leader intron 22 and 5 0 untranslated region. The later one includes an upstream open reading frame (uORF) to the initiation codon of ORF.…”

“…1 These possibly indicate that AtMHX mainly maintains metal and proton homeostasis in cells, the homeostasis is essential for photosynthesis and numerous enzymatic reactions. 11 However, AtMHX and its upstream regulators in the signal transduction responding to MgSs are still unclear.…”

“…[8][9][10] However, the signal transduction responding to MgSs is largely unknown, although some reports indicated that expression levels of many genes related to phytohormone biosynthesis and signalings are significantly changed by MgSs. [8][9][10][11] Our previous studies discovered that at least 2 hormones, namely abscisic acid (ABA) and gibberellins (GA), and their signaling factors, DELLA and ABI1, are responding to magnesium toxicity (MgT). 1 These results suggest that the discovery of signal transduction for MgT in plants could be a gateway to explore the regulatory mechanisms of responses to MgSs.…”