Ascorbic acid (AsA) biosynthesis in plants occurs through a complex, interconnected network with mannose (Man), myoinQsitol, and galacturonic acid as principal entry points. Regulation within and between pathways in the network is largely ancharacterized. A gene that regulates the Man/L-galactose (L-Gal) 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-Gal 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. AMR1 expression appears to be developmentally and environmentally controlled. As leaves aged, AMR1 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. [ABSTRACT FROM AUTHOR]