AMR1, an arabidopsis gene that coordinately and negatively regulates the Mannose/L-galactose ascorbic acid biosynthetic pathway

Abstract

Ascorbic acid (AsA) biosynthesis in plants occurs through a complex, interconnected network with mannose (Man), myoinositol, and galacturonic acid as principal entry points. Regulation within and between pathways in the network is largely uncharacterized. A gene that regulates the Man/L-galactose (L-GaI) AsA pathway, AMR1 (for ascorbic acid mannose pathway regulator 1), was identified in an activation-tagged Arabidopsis (Arabidopsis thaliana) ozone-sensitive mutant that had 60% less leaf AsA than wild-type plants. In contrast, two independent T-DNA knockout lines disrupting AMR1 accumulated 2- to 3-fold greater foliar AsA and were more ozone tolerant than wild-type controls. Real-time reverse transcription-polymerase chain reaction analysis of steady-state transcripts of genes involved in AsA biosynthesis showed that AMR1 negatively affected the expression of GDP-Man pyrophosphorylase, GDP-L-GaI Phosphorylase, L-Gal-1-phosphate phosphatase, GDP-Man-3',5'epimerase, L-Gal dehydrogenase, and L-galactono-1,4-lactone dehydrogenase, early and late enzymes of the Man/L-Gal pathway to AsA. AMRl expression appears to be developmentally and environmentally controlled. As leaves aged, AMRl transcripts accumulated with a concomitant decrease in AsA. AMR1 transcripts also decreased with increased light intensity. Thus, AMR1 appears to play an important role in modulating AsA levels in Arabidopsis by regulating the expression of major pathway genes in response to developmental and environmental cues. © 2009 American Society of Plant Biologists.