Sugars and production of phenolic compounds

Acquisition of the red/blue color of grape berries in red varieties along ripening is a visual indicator of the biochemical processes occurring in grape. This reflects more precisely the accumulation of anthocyanins pigments in the vacuoles of skin cells, which does not occur in the white grape varieties. Interestingly, anthocyanins precursors and intermediates are ubiquitously synthesized within the fruit while the final pigments accumulate only in the epidermal tissue of the berries. Recently, the role of a putative anthocyanin carrier has been evidenced that would explain such phenomenon (Braidot et al. 2008). Sucrose-induced production of anthocyanins has been demonstrated in various species, such as petunia (Tsukaya et al. 1991, Weiss 2000), grape (Larronde et al. 1998, Vitrac et al. 2000), and radish (Hara et al. 2004). Expression of CHS is upregulated by sucrose treatment, both in petunia and Arabidopsis (Tsukaya et al. 1991, Ohto et al. 2001).

In V. vinifera, the enhanced expression of various genes of the anthocyanin biosynthetic pathway in the berry skin may be correlated with the concomitant accumulation of sugars in the flesh (Boss et al. 1996). In V. vinifera cell suspensions, sucrose treatment promotes anthocyanin synthesis (Larronde et al. 1998). Indeed, sucrose upregulates the expression of dihydroflavonol reductase (DFR) and antho-cyanin synthase/leucoanthocyanidin dioxygenase (ANS/ LDOX). This upregulation and the accumulation of anthocyanins are sucrose specific in Arabidopsis. However, in grape both glucose and sucrose are able to enhance DFR and ANS expression which suggests different sugar-sensing mechanisms (Gollop et al. 2001, 2002).

In grape, the sucrose signal is transmitted via a pathway involving variation of Ca2+ concentrations and the action of kinases and phosphatases (Vitrac et al. 2000). Interestingly, in Arabidopsis, the sucrose transporter SUC2 is involved in anthocyanin biosynthesis: the pho3 mutation impairs the expression of the correct SUC2 protein, resulting in an impaired phloem loading and thus a concomitant accumulation of sugars in leaves. As a result, high levels of antho-cyanins are accumulated because of the specific action of sucrose on the expression of anthocyanin biosynthesis-related genes as well as on the levels of transcription factors such as AtMYB5, also known as PRODUCTION OF ANTHOCYANIN 1 (PAP1), which is sucrose-inducible (Lloyd and Zakhleniuk 2004, Teng et al. 2005, Solfanelli et al. 2006). Furthermore, recent studies reveal the role of the transporter AtSUC1 in the sucrose-dependent signalling leading to accumulation of anthocyanins in seedlings. AtSUC1 mediates sucrose uptake into germinating pollen and its expression in roots is sucrose-inducible (Stadler et al. 1999, Johnson et al. 2004, Sivitz et al. 2008). Its activity would also trigger a sucrose-dependent transduction pathway leading to antho-cyanin accumulation in cotyledons (Sivitz et al. 2008).

Interestingly, sucrose also accelerates the accumulation of carotenoids in the non-climacteric citrus fruit (Iglesias et al. 2001). Carotenoids are accumulated in chloroplasts-derived plastids, the chromoplasts, through the general iso-

prenoid biosynthetic pathway which is upregulated along fruit maturation (for review, see Cunningham and Gantt 1998). Like polyphenols, carotenoids are antioxidant molecules recommended in human nutrition and may protect against cancer and eye degenerative disease (Jonhson 2002).

Tomato fruit accumulates high levels of carotenoids (mainly lycopene) as it ripens, contributing to its characteristic red and orange color. Light and hormones are involved in the control of carotenoid biosynthesis (Giovannoni 2004); the Never ripe mutant (Nr), affected in ethylene sensitivity accumulates low levels of lycopene (Lanahan et al. 1994). Similarly, this climacteric-associated hormone also enhances anthocyanins production in grape berry (El-Kereamy et al. 2003). Sucrose promotes specifically lycopene and phytoene accumulation in tomato without affecting other carotenoids. Accordingly, sucrose enhances the accumulation of the phytoene synthase (PSY1) mRNAs, without acting on the expression of other carotenoid biosynthesis pathway-related genes (Telef et al. 2006).

Was this article helpful?

0 0
Berry Boosters

Berry Boosters

Acai, Maqui And Many Other Popular Berries That Will Change Your Life And Health. Berries have been demonstrated to be some of the healthiest foods on the planet. Each month or so it seems fresh research is being brought out and new berries are being exposed and analyzed for their health giving attributes.

Get My Free Ebook

Post a comment