Little is known about how plants regulate their folate content,including whether the expression of folate biosynthesis genes is orchestrated during development or modulated by folate levels.Nor is much known about how folate levels impact the expression of other genes.These points were addressed using wild-type tomato fruit and fruit engineered for high folate content.In wild-type fruit,the expression of genes specifying early steps in folate biosynthesis declined during development but that of other genes did not.In engineered fruit overexpressing foreign GTP cyclohydrolase I and aminodeoxychorismate synthase genes,the expression of the respective endogenous genes did not change,but that of three downstream pathway genes-aminodeoxychorismate lyase,dihydroneopterin aldolase,and mitochondrial folylpolyglutamate synthase-respectively increased by up to 7.8-,2.8-,and 1.7-fold,apparently in response to the build-up of specific folate pathway metabolites.These results indicate that,in fruit,certain folate pathway genes are developmentally regulated and that certain others are subject to feedforward control by pathway intermediates.Microarray analysis showed that only 14 other transcripts(of 11 000 surveyed) increased in abundance by two-fold or more in high-folate fruit,demonstrating that the induction of folate pathway genes is relatively specific.
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