Lights, P450, action! Metabolite formation in chloroplasts

The capacity of plants to convert light into sugar and, consequently, to build specialized compounds of vast complexity is a well-established phenomenon. In their study in this issue of Journal of Experimental Botany, Gnanasekaran et al. (pages 2495–2506) show that the most versatile catalysts of specialized metabolism can be fueled directly by photosynthetic electron flow. This finding might have great implications for future plant metabolic engineering endeavors.Plants, being sessile organisms, rely on myriad chemicals as the means of interaction with their environment. Collectively referred to as ‘specialized compounds’, they comprise structurally variable metabolites from diverse classes, such as terpenoids, phenylpropanes and alkaloids, to name only a few major groups. Besides helping plants to attract pollinators or deter enemies, many of these compounds have become directly important to us. Applied as fragrances, flavors, fine chemicals or medicines, they are being extracted in large amounts, traded and widely utilized – they have been rightly dubbed ‘plant natural products’ (PNPs). Understanding the biosynthetic capacity behind specialized compound accumulation and exploiting this natural ‘warehouse’ has, therefore, become a major scientific goal. Ultimately, elucidation of metabolite formation will facilitate targeted engineering of pathways – modifying plants to spur production of compounds of interest in defined amounts and at desired time points and, further, generate novel structures (Staniek et al., 2013).Dhurrin, the metabolite in question in the study of Gnanasekaran et al. (2016) is a cyanogenic glucoside originating from Sorghum spp. Although probably of no direct use to people, it confers enormous advantage to its host plants by deterring feeding insects (Tattersall et al., 2001). For delineation of plant metabolism the pathway leading to dhurrin seems to provide an ideal playground. First, it takes only three enzymatic steps (from tyrosine, the primary metabolism-derived amino acid) to yield the final product, making it a relatively short and straightforward metabolic route. Second, the pathway enzymes have to act in a tightly regulated and choreographed manner. Since intermediates on the way from tyrosine to dhurrin are toxic to the cell, the catalysts need to operate in a kind of ‘bucket chain’, handing over the product of one reaction to the following actor, thereby minimizing leakage and preventing toxicity. While unequivocally demonstrated for dhurrin biosynthesis, the so-called metabolon principle is postulated to be relevant to many more pathways (Moller, 2010). Third, the three actors in this short pathway are members of large enzyme families – cytochrome P450 monooxygenases (‘P450s’) and UDP-glucosyl transferases – whose relatives can be found in almost all pathways of plant metabolism. Therefore, the findings of Gnanasekaran et al. could possibly be extrapolated to numerous PNP pathways and might help engineer synthetic metabolic pathways for enhanced product formation.