In the Spotlight: Salt and peppers - two spices meet to turn up the heat

A world in which the plant genus Capsicum does not exist would be a de-spiced dystopia. Some of the most beloved foods worldwide, from jambalaya to jollof to jalfrezi, incorporate the fruits of Capsicum plants. The genus includes only five species but contains an estimated 50,000 cultivars (Antonio et al., 2018) ranging from the versatile and dependable bell pepper (Capsicum annuum) to the small but fiery piri-piri (Capsicum frutescens). For nearly 70 years, the standard for measuring the heat of a pepper was the Scoville test (Scoville, 1912). Chemical extracts from a pepper were repeatedly diluted and taste-tasted by human subjects until the spice was no longer detectable: the higher the dilution, the higher the Scoville score. With the development of high-performance liquid chromatography (HPLC) technologies in the 1970s, food chemists began to determine spiciness by measuring the absolute quantity of the chemical compounds responsible for heat or pungency, a class of phenolic compounds called capsaicinoids. Two compounds in particular, capsaicin (CAP) and dihydrocapsaicin (DHC), account for over 90% of the pungency of hot peppers (Bennett & Kirby, 1968) and are therefore of substantial interest to plant breeders hoping to produce spicier (or milder) cultivars. Breeding new and more flavorful cultivars of plants takes time, but a recent study of two cultivars of thyme (Thymus vulgaris and Thymus daenensis) showed that simply salt stressing plants caused an increase in the production of the phenolic compounds responsible for their flavor and aroma (Bistgani et al., 2019). Might the production of capsaicinoids, which are also phenolic compounds, be increased in pepper plants upon salt stress? In this issue of Physiologia Plantarum, Shams et al. (2023) have sought to answer this question by measuring the effects of salt stress on the accumulation of capsaicinoids in two hot pepper cultivars: Capsicum chinense cv. Habanero and Capsicum annuum cv. Maras. Interestingly, when the levels of capsaicinoids were measured in the fruits of salt-stressed pepper plants, both CAP and DHC showed statistically significant increases in both cultivars. In Maras peppers, salt stress caused an increase in CAP and DHC content of ~10% and 50%, respectively while in salt-stressed Habanero peppers, CAP and DHC content increased by ~27% and 19%, respectively. These increases in CAP and DHC levels were even more pronounced in plants that were only salt-stressed for 30 days, although the absolute levels of capsaicinoids were lower at 30 days than at 60 days. Nevertheless, both Maras and Habanero peppers from salt-stressed plants contained higher levels of DHC and CAP (Figure 1). In addition to the fruits, the leaves and roots of pepper plants also contain capsaicinoids, albeit in much smaller quantities than in fruits. Interestingly, the authors noticed that salt stress also caused an increase in the accumulation of CAP and DHC in these organs (Figure 1), indicating that the increase in capsaicinoid content in response to salt stress is a plant-wide response that is not specifically localized to fruits. To understand why the levels of capsaicinoids were higher in salt-stressed plants, Shams and colleagues measured the transcript levels of capsaicinoid biosynthesis genes. They found that the transcript levels of some of these genes were upregulated in response to salt stress and in specific tissue types. Interestingly, the gene expression patterns are not identical between Maras and Habanero peppers, indicating these two cultivars respond slightly differently to salt stress. In both cultivars, however, it appears that increased transcription of capsaicinoid biosynthesis genes may be at least partly responsible for salt-induced increases in capsaicinoid content. The discovery by Shams and coauthors that the heat of a pepper can be increased by simply watering plants with slightly salty water could have far-reaching implications. Many farmers are interested in maximizing yields of crops, but some farmers might be willing to grow plants with reduced biomass as long as they produce more pungent herbs or spices. Moreover, the cost of freshwater for agriculture is likely to increase as its availability continues to decline (Rodell et al., 2018). Obtaining more pungent food products while also potentially saving money by using marginal, moderately saline water sources would seem to be a solution that many farmers would welcome. Future research will determine whether there is an optimal level of salinity for maximizing biomass and pungency as well as the molecular mechanisms allowing plants to increase capsaicinoid content in response to salt stress. For now, however, we can all be content in knowing that a Capsiscum- and capsaicinoid-free future is not one that we will have to experience any time soon.