Submitted to: Molecular Plant
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/7/2009
Publication Date: 2/12/2010
Citation: Waller, J., Akhtar, T., Lara-Nunez, A., Gregory, J., Giovannoni, J.J., Hanson, A. 2010. Developmental and feedforward control of the expression of folate biosynthesis genes in tomato fruit. Molecular Plant. 3:66-77. Interpretive Summary: We report here a comprehensive study of transcriptional control of the folate pathway during fruit development and in response to endogenous folate overproduction. Previous investigations of folates and transcription in plants, human cells, and protists all focused on folate depletion. To achieve this, they often used folate synthesis inhibitors, which are not entirely specific to the folate pathway and can have major secondary effects (e.g., disruption of one-carbon metabolism, thymidine starvation) due to the folate deficiency that they create. Our data provide evidence for a blend of developmental and feedforward control of folate pathway transcript levels. Such feedforward induction of downstream pathway genes by overexpression of an upstream transgene has also been reported for carotenoid biosynthesis in tomato fruit. It is thus reasonable to hypothesize that folate synthesis in tomato fruit is regulated, in part, by two interlocking mechanisms: developmental control of the expression of enzymes at the start of the pathway, and feedforward control of expression of downstream enzymes by pathway intermediates and products.
Technical Abstract: 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 gens did not. In engineered fruit overexpression 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 buildup 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 13 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.