Abstract
Drought, elevated air temperature, and high evaporative demand are increasingly frequent during summer in grape growing areas like the Mediterranean basin, limiting grapevine productivity and berry quality. The foliar exogenous application of kaolin, a radiation-reflecting inert mineral, has proven effective in mitigating the negative impacts of these abiotic stresses in grapevine and other fruit crops, however, little is known about its influence on the composition of the grape berry and on key molecular mechanisms and metabolic pathways notably important for grape berry quality parameters. Here, we performed a thorough molecular and biochemical analysis to assess how foliar application of kaolin influences major secondary metabolism pathways associated with berry quality-traits, leading to biosynthesis of phenolics and anthocyanins, with a focus on the phenylpropanoid, flavonoid (both flavonol- and anthocyanin-biosynthetic) and stilbenoid pathways. In grape berries from different ripening stages, targeted transcriptional analysis by qPCR revealed that several genes involved in these pathways—VvPAL1, VvC4H1, VvSTSs, VvCHS1, VvFLS1, VvDFR, and VvUFGT—were more expressed in response to the foliar kaolin treatment, particularly in the latter maturation phases. In agreement, enzymatic activities of phenylalanine ammonia lyase (PAL), flavonol synthase (FLS), and UDP-glucose:flavonoid 3-O-glucosyltransferase (UFGT) were about two-fold higher in mature or fully mature berries from kaolin-treated plants, suggesting regulation also at a transcriptional level. The expression of the glutathione S-transferase VvGST4, and of the tonoplast anthocyanin transporters VvMATE1 and VvABCC1 were also all significantly increased at véraison and in mature berries, thus, when anthocyanins start to accumulate in the vacuole, in agreement with previously observed higher total concentrations of phenolics and anthocyanins in berries from kaolin-treated plants, especially at full maturity stage. Metabolomic analysis by reverse phase LC-QTOF-MS confirmed several kaolin-induced modifications including a significant increase in the quantities of several secondary metabolites including flavonoids and anthocyanins in the latter ripening stages, probably resulting from the general stimulation of the phenylpropanoid and flavonoid pathways.
Highlights
Grapevine (Vitis vinifera L.) is a perennial woody plant with a great impact in the global economy, abundantly cultivated in areas with Mediterranean climates and spreading across temperate to semi-dry areas
The quantities of quercetin, rutin, catechin/epicatechin, procyanidin B2, and peonidin 3galactoside, which were identified by reverse phase LC-QTOF-MS, were all substantially increased in berries from grapevines treated with kaolin (Figure 1)
In our previous report (Dinis et al, 2016a) we showed that the transcript levels of a phenylalanine ammonia lyase gene (VvPAL1), that encodes an enzyme catalyzing the first step in the phenylpropanoid pathway in which trans-cinnamic acid is produced, increased in the final maturation stages by 33% in berries from kaolin-treated plants
Summary
Grapevine (Vitis vinifera L.) is a perennial woody plant with a great impact in the global economy, abundantly cultivated in areas with Mediterranean climates and spreading across temperate to semi-dry areas. In Mediterranean areas, extended summer droughts and higher temperatures are increasingly expected (Fraga et al, 2012; Hannah et al, 2013) and climate change is undoubtedly having a negative impact in viticulture, including changes in grape-growing geographical area, the development and application of stress mitigation strategies and of more sustainable agricultural practices is of utmost importance for grape production and winemaking industry In this context, the application of exogenous compounds that could maintain or even improve plant productivity or fruit quality under such environmental stresses are beginning to be experimented but, despite promising results yielded in some crops (Hose et al, 2000; Li et al, 2004; Seckin et al, 2009; Du et al, 2013; Zhou et al, 2014), in grapevine these strategies have so far been less explored. We recently observed that lower ROS quantities, increased hydroxyl radical scavenging and production of antioxidant compounds, including phenolics, apparently contributing to the protective effect of kaolin in grapevine (Dinis et al, 2016a), but little is known regarding the molecular mechanisms underlying these changes
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