Regulation of carotenoid and flavonoid biosynthetic pathways in Lactuca sativa var capitate L. in protected cultivation

Abstract

In the face of a growing world population and limited land, there is an urgent demand for higher productivity of food crops, and cultivation systems must be adapted to future needs. Sustainable crop production should aim for not only high yields, but also high nutritional values. In particular, the consumption of bioactive compounds such as carotenoids and flavonoids is associated with a reduced incidence of non-transmissible diseases. Modulating environmental conditions by improving cultivation systems can lead to the adaption of plant metabolisms and the accumulation of bioactive compounds. The present study investigates the regulation of carotenoid and flavonoid metabolisms in lettuce (Lactuca sativa var capitate L.) grown in a protected environment (polytunnels) compared to plants grown without polytunnels. Carotenoid, flavonoid and phytohormone (ABA) contents were determined using HPLC-MS and transcript levels of key metabolic genes were analyzed by RT-qPCR. In this study, we observed inverse contents of flavonoids and carotenoids in lettuce grown without or under polytunnels. Flavonoid contents on a total and individual level were significantly lower, while total carotenoid content was higher in lettuce plants grown under polytunnels compared to without. However, the adaptation was specific to the level of individual carotenoids. For instance, the accumulation of the main carotenoids lutein and neoxanthin was induced while the β-carotene content remained unchanged. In addition, our findings suggest that the flavonoid content of lettuce depends on transcript levels of the key biosynthetic enzyme, which is modulated by UV light. A regulatory influence can be assumed based on the relation between the concentration of the phytohormone ABA and the flavonoid content in lettuce. In contrast, the carotenoid content is not reflected in transcript levels of the key enzyme of either the biosynthetic or the degradation pathway. Nevertheless, the carotenoid metabolic flux determined using norflurazon was higher in lettuce grown under polytunnels, suggesting posttranscriptional regulation of carotenoid accumulation, which should be an integral part of future studies. Therefore, a balance needs to be found between the individual environmental factors, including light and temperature, in order to optimize the carotenoid or flavonoid contents and to obtain nutritionally highly valuable crops in protected cultivation.

Influence of UV radiation-absorbing foils on secondary plant metabolites in three lettuce cultivars (Lactuca sativa L. and Cichorium intybus L.)

“…Larissa, an effect of the treatments was observed for β-carotene. This is in contrast to Caldwell and Britz (2006) and Harbart et al (2023), who found increased β-carotene, lutein, and neoxanthin concentrations after a supplemental UVA and UVB treatment of 16 lettuce cultivars, and under polytunnel treatment.…”

“…In a previous study, different shading intensities (based on the cardinal directions) had little to no effect on carotenoids in kale (Bayer et al, 2022). It is noteworthy that Harbart et al (2023) reported no differences between foil treatments and their control on a transcriptional level (phytoene synthase), suggesting a posttranscriptional regulated effect that increased carotenoid concentrations but was not observed in our study. However, such post-transcriptional regulation might explain the diverse results reported in the literature.…”

“…Thus, those foil systems can increase the productivity of lettuce production when facing unfavorable environmental conditions (Santos et al, 2009;Wallace et al, 2012). However, temperature differences between foil treatments and direct sunlight are usually limited to 2 °C-3 °C (Harbart et al, 2023). During our study, differences in dry matter were widely irrelevant, and contrary results were also published by Santos et al (2009), who found higher dry matter in lettuce cultivated under foils.…”

“…More recently, dicaffeoyltartaric acid reductions under consistent UVB exposure have been reported, while quercetin and kaempferol glycosides increased 3-7 fold under the same conditions (Weiland et al, 2023). The accumulation of polyphenols in lettuce under natural UV exposure is due to a regulation of the transcriptional level, namely, a higher chalcone synthase expression, compared to lettuce grown under foil (Harbart et al, 2023). In contrast to polyphenol accumulation, posttranscriptional regulation has been assumed for carotenoids (Harbart et al, 2023).…”

“…The accumulation of polyphenols in lettuce under natural UV exposure is due to a regulation of the transcriptional level, namely, a higher chalcone synthase expression, compared to lettuce grown under foil (Harbart et al, 2023). In contrast to polyphenol accumulation, posttranscriptional regulation has been assumed for carotenoids (Harbart et al, 2023). High concentrations of polyphenols are regarded as an important factor for a healthy lifestyle, contributing to the prevention of metabolic-, aging-, or lifestyle-associated illnesses (Leri et al, 2020).…”