Overexpression of a bacterial 1-deoxy-D-xylulose 5-phosphate synthase gene in potato tubers perturbs the isoprenoid metabolic network: implications for the control of the tuber life cycle

Abstract

Potato tubers were engineered to express a bacterial gene encoding 1-deoxy-D-xylulose 5-phosphate synthase (DXS) in order to investigate the effects of perturbation of isoprenoid biosynthesis. Twenty-four independent transgenic lines out of 38 generated produced tubers with significantly elongated shape that also exhibited an early tuber sprouting phenotype. Expression analysis of nine transgenic lines (four exhibiting the phenotype and five showing a wild-type phenotype) demonstrated that the phenotype was strongly associated with dxs expression. At harvest, apical bud growth had already commenced in dxs-expressing tubers whereas in control lines no bud growth was evident until dormancy was released after 56-70 d of storage. The initial phase of bud growth in dxs tubers was followed by a lag period of approximately 56 d, before further elongation of the developing sprouts could be detected. Thus dxs expression results in the separation of distinct phases in the dormancy and sprouting processes. In order to account for the sprouting phenotype, the levels of plastid-derived isoprenoid growth regulators were measured in transgenic and control tubers. The major difference measured was an increase in the level of trans-zeatin riboside in tubers at harvest expressing dxs. Additionally, compared with controls, in some dxs-expressing lines, tuber carotenoid content increased approximately 2-fold, with most of the increase accounted for by a 6-7-fold increase in phytoene.

Carotenoid Biosynthesis in Arabidopsis: A Colorful Pathway

“…Virtually all the Arabidopsis genes encoding enzymes involved in carotenoid biosynthesis, including those of the MEP pathway, are upregulated during light-triggered deetiolation (Ghassemian et al, 2006;Rodríguez-Concepción, 2006;Meier et al, 2011). The coordination at the gene expression level between the MEP pathway and the carotenoid pathway is consistent with the observation that carotenoid biosynthesis in light-grown plants can be limited by the availability of GGPP precursors, as deduced from the increases observed in carotenoid levels upon overexpression of flux-controlling enzymes of the MEP pathway such as DXS or HDR (Estévez et al, 2001;Botella-Pavía et al, 2004;Enfissi et al, 2005;Carretero-Paulet et al, 2006;Morris et al, 2006;Muñoz-Bertomeu et al, 2006;. To ensure that the GGPP pre-cursors required for carotenoid biosynthesis will be supplied when necessary, an increased expression of genes encoding MEP pathway enzymes has been shown to occur just prior to, or concomitant with, the production of carotenoids not only during during deetiolation but also in other development processes associated with an increased production of carotenoids or apocarotenoids (Bouvier et al, 1998b;Lois et al, 2000;Walter et al, 2000;Botella-Pavía et al, 2004;Hans et al, 2004;Cordoba et al, 2009;Toledo-Ortiz et al, 2010).…”

“…Interestingly, the coordination appears to be strongest with genes encoding flux-controlling steps of both pathways such as DXS and PSY, which are highly and coordinately up-regulated when carotenoid biosynthesis is boosted during early light-triggered deetiolation in Arabidopsis ( Figure 5). A down-regulated accumulation of transcripts encoding PSY has also been reported when the MEP pathway is chemically or genetically blocked in Arabidopsis seedlings (Laule et al, 2003), whereas PSY expression is induced when DXS expression and/ or activity is upregulated in potato tubers and tomato fruit (Lois et al, 2000;Rodríguez-Concepción et al, 2001;Enfissi et al, 2005;Morris et al, 2006). Additional evidence that there is an active communication between the MEP and carotenoid pathways in Arabidopsis and other plant systems is available in other recent reviews (Giuliano et al, 2008;Cordoba et al, 2009;Rodriguez-Concepcion, 2010).…”

Feedback Inhibition of Deoxy-d-xylulose-5-phosphate Synthase Regulates the Methylerythritol 4-Phosphate Pathway

“…Posttranscriptional regulation of DXS was observed by the level of the end product of the pathway (59). Higher or lower accumulation of isoprenoid end products in transgenic species having an over-or underexpressed DXS gene, respectively, was observed in both bacteria (60 -62) and plants (63)(64)(65). DXS gene expression is strongly regulated with different developmental stages and strongly correlated with carotenoid accumulation in tomato fruits (58).…”

Regulation of potato tuber sprouting

“…Moreover, these substances reduced the insensitivity against cytokinin that has been described during the early phases of dormancy (Turnbull and Hanke 1985b;Suttle 1998b), but did not eliminate it. Similarly, transgenic potato tubers which express a bacterial 1-desoxy-d-xylose-5-phosphate synthase showed increased level of trans-zeatin riboside and were characterised by broken dormancy at the time point of harvest, but further elongation of sprouts began only after a lag phase (Morris et al 2006). The authors presumed hormonal and/or metabolic imbalances as reasons for this phenotype.…”