How to preserve and enhance Brassicaceae nutritional and pytotherapeutic benefits ?

Brassicaceae include a wide range of plant varieties (e.g. Broccoli, Rapeseed, Cabbage, Mustard) exploited around the world for their nutritional and bioactive phytochemicals. They have high levels of primary metabolites, such as amino acids, carbohydrates and pigments, which constitute a first defense system against external damage. Some of these compounds are precursors of secondary metabolites, such as glucosinolates or phytoalexins, which provide more specific protection, supplemented by the action of polyphenols and steroids. These substances give plants antioxidant, anti-tumor and anti-microbial properties, but some of them also have anti-nutritional qualities. Many genetic and environmental parameters (e.g. soil composition, sunshine, herbivores, pathogens) affect Brassicaceae metabolome during plant growth. Under biotic stress conditions, plants emit signaling molecules (e.g. salicylic acid) capable of modulating and adapting the biosynthesis of defense metabolites against different types of pests. After harvesting, secondary metabolites content is still likely to fluctuate depending on storage and heat treatment conditions. Based on Jahangir and al.’s work, this article identifies the main abiotic stresses and multitrophic relationships that may affect the composition of Brassicaceae.

Context

Brassicaceae are very rich in anti-oxidation and anti-carcinogenesis compounds. They are also studied as part of cardiovascular risk prevention. Some of their compounds have anti-nutritional properties. However, these plants occupy a huge place in the world’s population diet, which makes them a relevant agronomic model. Cultivated plants are generally less rich in metabolites of interest than wild plants. However, they are the most consumed. In view of public health challenges linked to Brassicaceae production, it seems essential to produce healthy plants, free of phytosanitary products and of excellent nutritional quality, in order to maximize their beneficial effects. The objective is to understand modalities allowing plants natural enrichment, without deleterious impact on their taste, odor and toxicology. These studies are also part of the prospect of extreme environmental conditions due to global warming.

Defense mecanisms of Brassicaceae

Substances involved in Brassicaceae defense mechanisms come from primary and secondary metabolisms. A number of primary metabolites, such as pigments and vitamins, act as antioxidants. Others, like amino acids, play a role in heavy metals detoxification. Glucosinolates, secondary metabolites derived from amino acids, are toxic to pests that ingest them. Phytoalexins are defense molecules with antifungal and antibacterial properties; in Brassicaceae, their structure derives mainly from tryptophan. Phenolic compounds have antioxidant properties acting as a relay to the primary detoxification system provided by ascorbic acid. Their levels increase in the event of bacterial and herbivorous attack. Finally, steroids, derived from membrane sterols, confer broad spectrum resistance to biotic and abiotic stresses, such as high and low temperatures, drought, salt excess, pathogenic attack and heavy metals exposure.

Alteration of the benefits of plants

Many parameters can affect Brassicaceae benefits. On the one hand, their richness in anti-nutritional factors can be an obstacle to their consumption; S-methylcysteine sulphoxide, erucic acid, polyphenols and glucosinolates affect Brassicaceae taste, as well as their consumers metabolic processes. On the other hand, metabolites rapid degradation after harvest affects their bioavailability. Some conditions can prematurely activate glucosinolates hydrolysis, inhibit myrosinase enzyme or block their aglycones reorganization into active compounds. -85C freezing gives good results on phenols contents preservation but the cell fracture induced results in glucosinolates activation. Conservation under a modified atmosphere high in CO2 tends to reduce glucosinolate contents. Therefore, room temperature storage seems to be the most suitable way to maintain Brassicaceae composition. Consumption of raw or slightly heated plants is preferable for their metabolites' preservation.

Glucosinolates : key molecules

Glucosinolates, components at the origin of Brassicaceae typical smell and taste, are highly studied for their anti-cancer and cardio-protective properties. They are found in plants aerial parts and root systems. Depending on the amino acids involved in their biosynthesis, glucosinolates can be aliphatic, aromatic or indolic. The proportion of each type of glucosinolate and the sensitivity induced in pests differ from one Brassicaceae species to another. Indolic glucosinolates are also precursors of phytoalexins and auxins. In the event of a pathogenic attack, Brassicaceae emit signaling molecules stimulating the biosynthesis of glucosinolates. When the cellular integrity of the plant tissue is broken (e.g. wounding, freezing, ingestion), glucosinolates come into contact with myrosinase enzyme that hydrolyses them into unstable intermediate compounds, which then change into active defense molecules (e.g. isothiocyanates, nitriles). These breakdown compounds also act as chemical repellents for surrounding aggressors.

Role of signaling molecules

Under biotic stress, Brassicaceae emit signaling molecules (e.g. jasmonic acid, ethylene), which can be biosynthesized away from the aggression point and stimulate neighboring plants. These substances act on aerial parts of plants and self-regulate secondary defense metabolites production (e.g. glucosinolates, phytoalexins). The type of aggressor determines the type of molecule emitted. Two molecular signals promoting the synthesis of the same metabolite do not necessarily act on the same organs. Signaling molecules can be injected into non-producing organs to increase their level of defense. This technique is also used to bypass the resistance of certain pathogens. However, in the event of multiple attacks, antagonistic molecules can be produced; therefore, injecting a signaling substance can alter the resistance induced by another. Brassicaceae also diffuse other volatile compounds (e.g. terpenes, isothiocyanates) capable of repelling their aggressors and attracting their predators.

Impact of cultural conditions

Brassicaceae chemical composition is influenced by their growing conditions. Brassicaceae accumulate minerals from the soil, including heavy metals. Soil quality is therefore an important parameter to monitor. Maintaining low nitrogen intake and increasing sulfur intake eases glucosinolates and polyphenols production. Water stress and excess salinity increase primary metabolites and glucosinolates contents. UV radiation and high temperatures stimulate carotenoids and secondary metabolites biosynthesis. Allowing mild epidemic spread, or localized on organs that will not be collected, helps to naturally enrich the plant with defense products. Controlled injection of signaling molecules is one way to support this process. Metabolites nature and quantity vary according to organs and their growth stages, influencing harvest time.

Discussion

Brassicaceae metabolites’ richness is representative of the multitude of their predators’ action mechanisms. Precise knowledge of these phenomena will allow agronomists to develop phytosanitary products alternatives, allowing Brassicaceae natural enrichment in bioactive phytochemicals. This is possible in particular through the use of signaling molecules. These techniques’ main obstacle is the inevitable increase in anti-nutritional compounds in the organs consumed. It is possible to remove some of these compounds by selection (e.g. glucosinolates, erucic acid) or heat treatment (e.g. phytic acid, myrosinase). But this also results in reducing the concentration of defense substances. Organoleptic and toxicological studies will therefore be necessary to guarantee these superfoods acceptance by consumers. In addition, studies conducted in extreme conditions (e.g. high temperatures, drought) suggest the definite impact of global warming on the plants’ metabolome.

Conclusion

This document lays foundations for applied research on Brassicaceae natural enrichment in defense molecules. It is clear that further studies will be needed to clarify the interactions of these components with each other. The study of multitrophic underground interactions of Brassicaceae would give a more global view of ecological processes implemented. Ideally, all active, anti-nutritional and potentially toxic substances should be investigated. However, since many environmental parameters can flucturate, it seems difficult to be able to concentrate on one molecule’s enrichment at the expense of one another. Then, preserving bioactive phytochemicals after harvest should involve the education of all stakeholders and consumers. Finally, it would be useful to monitor the plants composition after a phytosanitary products exposure; indeed, like any other environmental aggression, it might impact metabolites nature, quality and quantity.

Leslie Colin

University of Bordeaux, Sciences et Technology College, 2019

Reference :

Healthy and unhealthy plants : The effect of stress on the metabolism of Brassicacea, M. Jahangir et al, Environmental and Experimental Botany, Volume 67, Issue 1, November 2009, Pages 23-33.