The mystery of apocarotenoid catabolism in plants

Carotenoids are essential pigments for photosynthesis in leaves, contribute to the vivid yellow or orange coloration of flowers and fruits, and some are important for human nutrition. In plants, carotenoids can be catabolized by enzymatic and nonenzymatic oxidation of the double bonds in their backbones, resulting in a broad spectrum of cleavage products known asapocarotenoids. Some phytohormones including strigolactone and abscisic acid (ABA) are derived from apocarotenoids. Additionally, apocarotenoids serve as signaling molecules in many physiological processes such as the excess light response (Hou et al., 2016). Although the biosynthetic pathway of carotenoids has been well-established (Wurtzel, 2019), our understanding of carotenoid catabolism has lagged behind.

Carotenoid accumulation in plants is determined by rates of both biosynthesis and degradation. Since the pathway flux provides a higher potential level of carotenoid biosynthesis than the actual accumulated amounts, carotenoid degradation must play a critical role in maintaining the steady-state levels of carotenoids (Koschmieder and Welsch, 2020). However, the primary apocarotenoid products are not sufficient to account for the proportion of carotenoid degradation (Schaub et al., 2017; Schaub et al., 2018). Thus, further unknown metabolic steps may also be involved in carotenoid catabolism. Moreover, most apocarotenoids contain reactive α, β-unsaturated carbonyls, which are toxic for plants, raising additional questions about their catabolism.

In this issue of Plant Physiology, Koschmieder et al. (2021) used transcriptomic and metabolic studies to investigate carotenoid catabolism, and showed that plants use defense mechanisms against reactive xenobiotic carbonyls to deal with apocarotenoid accumulation.

In order to investigate carotenoid catabolism, it is necessary to start with a sufficient level of carotenoids as substrates. Previously, overexpression of PHYTOENE SYNTHASE (PSY), which encodes the enzyme that catalyzes the first committed step of carotenoid biosynthesis in Arabidopsis thaliana (Arabidopsis), has been shown to induce carotenoid accumulation in callus and roots as well as to simultaneously increase β-apocarotenoid levels (Maass et al., 2009; Schaub et al., 2018). Therefore, Koschmieder et al. (2021) used PSY-overexpressing Arabidopsis root tissue for this study. They found that β-carotene accounted for the major carotenoid increase, consistent with their previous finding (Maass et al., 2009). Most known β-carotene breakdown products, including β-apocarotenoids, apocarotene-dialdehydes, and volatile cyclic β-apocarotenoids, also increased in PSY-overexpressing roots compared to wild-type roots.

Through transcriptomic and metabolite studies of these PSY-overexpressing roots, the authors asked several questions to try to understand which signaling pathways and/or metabolic processes contribute to the transcriptional changes they observed. First, they investigated to what extent overaccumulation of apocarotenoids might affect the accumulation of the apocarotenoid-derived hormones, strigolactones, and ABA. No global upregulation of strigolactone or ABA synthesis was detected. The authors conclude that strigolactone and ABA biosynthesis are unlikely to contribute to the catabolism of excess carotenoids.

The authors next examined possible lipid stress responses that might arise due to overaccumulation of β-carotene. First, the authors compared their transcriptome data with previously reported lipid stress response transcriptome (Mueller et al., 2008) and found little overlap in differently expressed genes (DEGs). Further analysis of marker molecules and genes that increase under lipid stress also showed no change. Therefore, lipid stress is not the main cause of the transcriptome changes.

The next question is, since some apocarotenoids can serve as signal molecules, to what extent does apocarotenoid-triggered signaling lead to the observed transcriptome changes? For instance, the carotenoid oxidation product β-cyclocitral exhibits signaling functions in response to high light and high oxidative stresses. To address this question, the authors compared the DEGs affected by β-cyclocitral treatment and found little overlap with DEGs in apocarotenoid accumulating roots. This result argues against apocarotenoid signaling playing as a major contributor to the transcriptional responses.

The authors next investigated to what extent the toxicities of overaccumulated apocarotenoids contribute to the transcriptional effects. DEGs from several reported transcriptomes representing different chemical-induced detoxification mechanisms were compared to apocarotenoid-accumulating DEG sets. Interestingly, DEGs from xenobiotic-treated plants showed a high overlap with apocarotenoid-accumulating root DEGs. Since detoxification of xenobiotics also involves enzymatic reactions, enzymes with the potential to reduce the xenobiotic carbonyl reactivity of β-apocarotenoids were further investigated from the overlapping DEGs. Several enzymes, including NADPH-dependent aldehyde reductase ChlADR, the aldo-keto reductases AKR4C8 and AKR4C9, and the aldehyde dehydrogenases ALDH3H1 and ALDH3I1 were selected and used for further in vitro enzymatic activity assays. It turns out that most β-apocarotenoids except β-cyclocitral can be further metabolized by those enzymes (Figure 1).

Figure 1.

Potential metabolization of β-apocarotenoids by carbonyl-detoxifying enzymes. ChlADR, chloroplastic NADPH-dependent aldehyde reductase; AKR4C8 and AKR4C9, NADPH-dependent aldo-keto reductases; ALDH3H1 and ALDH3I1, aldehyde dehydrogenases. Figure by Tianhu Sun.

In summary, Koschmieder et al. (2021) provide a unique model to study carotenoid catabolism and reveal the mystery of apocarotenoid metabolism in plants. This work also provides some insights into catabolism of other carotenoids such as xanthophyll and gives some clues to explore potential transportation and compartmentation processes of apocarotenoid metabolites. Identifying additional catabolic enzymes will require more genetic and in vivo evidence in the long run.

Considering the complexity of apocarotenoid functions in plants, catabolism studies will be challenging without a proper model organism. In recent years, saffron crocus (Crocus sativus) stigmas with abundant red-colored crocins have been used as a natural model for apocarotenoid catabolism and transportation (Demurtas et al., 2019). Together with the innovation of omics assays and advances in detection methods, the mysteries of apocarotenoid catabolism will be fully revealed in the future.