Abstract
Potato, S. tuberosum, is one of the most important global crops, but has high levels of waste due to tuber greening under light, which is associated with the accumulation of neurotoxic glycoalkaloids. However, unlike the situation in de-etiolating seedlings, the mechanisms underlying tuber greening are not well understood. Here, we have investigated the effect of monochromatic blue, red, and far-red light on the regulation of chlorophyll and glycoalkaloid accumulation in potato tubers. Blue and red wavelengths were effective for induction and accumulation of chlorophyll, carotenoids and the two major potato glycoalkaloids, α-solanine and α-chaconine, whereas none of these accumulated in darkness or under far-red light. Key genes in chlorophyll biosynthesis (HEMA1, encoding the rate-limiting enzyme glutamyl-tRNA reductase, GSA, CHLH and GUN4) and six genes (HMG1, SQS, CAS1, SSR2, SGT1 and SGT2) required for glycoalkaloid synthesis were also induced under white, blue, and red light but not in darkness or under far-red light. These data suggest a role for both cryptochrome and phytochrome photoreceptors in chlorophyll and glycoalkaloid accumulation. The contribution of phytochrome was further supported by the observation that far-red light could inhibit white light-induced chlorophyll and glycoalkaloid accumulation and associated gene expression. Transcriptomic analysis of tubers exposed to white, blue, and red light showed that light induction of photosynthesis and tetrapyrrole-related genes grouped into three distinct groups with one group showing a generally progressive induction by light at both 6 h and 24 h, a second group showing induction at 6 h in all light treatments, but induction only by red and white light at 24 h and a third showing just a very moderate light induction at 6 h which was reduced to the dark control level at 24 h. All glycoalkaloid synthesis genes showed a group one profile consistent with what was seen for the most light regulated chlorophyll synthesis genes. Our data provide a molecular framework for developing new approaches to reducing waste due to potato greening.
Highlights
Potato is the leading non-grain commodity in the global food system and the largest tuber food crop in terms of human consumption (Birch et al, 2012)
We have investigated the effect of monochromatic B, R and FR light on the regulation of chlorophyll and GA accumulation in tubers of a commercial potato cultivar, King Edward (KE), which is susceptible to tuber greening (Garnett et al, 2018), and demonstrate how the pathway is controlled via the induction of key biosynthesis genes for both chlorophyll and GA synthesis under these light conditions
Chlorophyll accumulation was detected in tubers irradiated with white light (WL) for more than two days and chlorophyll levels increased with increasing periods of light exposure (Figure 1A; Supplementary Figure S1B)
Summary
Potato is the leading non-grain commodity in the global food system and the largest tuber food crop in terms of human consumption (Birch et al, 2012). Previous experiments have established the HEMA1, CHLH and GUN4 genes as part of a small group of key regulatory genes for the pathway in Arabidopsis (McCormac et al, 2001; McCormac and Terry, 2002; Matsumoto et al, 2004; Kobayashi and Masuda, 2016) This includes regulation via the phytochrome family of red (R) and far-red (FR) light photoreceptors in Arabidopsis seedlings (McCormac et al, 2001; McCormac and Terry, 2002; Kobayashi and Masuda, 2016) as well as in rice (Inagaki et al, 2015). Evidence to date suggests that both B and R wavelengths are active in inducing chlorophyll in the cultivar Sebago (Petermann and Morris, 1985) with phytochrome proposed as the dominant photoreceptor (Morris et al, 1979)
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