Ascorbic acid (vitamin C) is a major antioxidant in plants, and fruits are the major source of this vitamin for humans. While the pathways of synthesis, recycling and degradation are well characterized, their regulation is still poorly understood. We have used reverse genetic approaches to target two key steps of the Wheeler & Smirnoff synthesis pathway: GDP-D-mannose epimerase (GME) and GDP-L-galactose phosphorylase (GGP) in tomato fruits. In addition to a reduction of ascorbic acid content, RNAi-silenced gme tomato lines exhibited growth phenotypes resulting from cell division and expansion defects, exacerbated fragility and loss of fruit firmness related to modifications of the cell wall structure and composition [1,2]. These findings help to explain observed links between seemingly unrelated quality traits such as fruit firmness and ascorbic acid content. Two ggp knockout lines were identified from TILLING (EMS) mutant populations. These have reduced ascorbic content and show bleaching and leaf necrosis after short exposure to high light [3,4]. Integration of transcriptomic, proteomic, and metabolomic data from wild type and mutant tomato fruits identified candidate genes involved in the regulation of the ascorbate pathway [5]. A forward genetic screen has identified four mutant lines which have 2.5 to 5-fold higher levels of ascorbic acid than wild-type, and one candidate gene has been mapped using next generation sequencing approaches.
[1] Gilbert L, Alhagdow M, Nunes-Nesi A, Quemener B, Guillon F, Bouchet B, Faurobert M, Gouble B, Page D, Garcia V, Petit J, Stevens R, Causse M, Fernie AR, Lahaye M, Rothan C and Baldet P. (2009). Plant J. 60, 499-508.
[2] Voxeur A, Gilbert L, Rihouey C, Driouich A, Rothan C, Baldet P and Lerouge P (2011). Journal of Biological Chemistry, 286: 8014-8020.
[3] Okabe Y, Asamizu E., Saito T., Matsukura C., Ariizumi T., Bres C., Rothan C., Mizoguchi T. and Ezura H. (2011). Plant Cell Physiol. 52(11): 1994–2005.
[4] Just D., Garcia V., Fernandez L., Bres C., Mauxion JP., Petit J., Jorly J., Assali J., Bournonville C., Ferrand C., Baldet P., Lemaire-Chamley M., Mori K., Okabe Y., Ariizumi T., Asamizu E., Ezura H., and Rothan C. (2013). Plant Biotech. 30, 225-231.
[5] Garcia V., Stevens R., Gil L., Gilbert L., Gest N., Petit J., Faurobert M., Maucourt M., Deborde C., Moing A., Poessel JL., Jacob D., Bouchet JP., Giraudel JL., Gouble B., Page D., Alhagdow M., Massot C., Gautier H., Lemaire-Chamley M., de Daruvar A., Rolin D., Usadel B., Lahaye M., Causse M., Baldet P. and Rothan C. (2009). Compte rendu de Biologie. 332, 1007-1021.
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