PLANT DEFENSE, THE COSTS OF SECONDARY METABOLITES, AND ECOLOGICAL STRATEGIES

2.4 PLANT DEFENSE, THE COSTS OF SECONDARY METABOLITES, AND ECOLOGICAL STRATEGIES

2.4.1 Trade-off between direct and indirect defense

Most secondary metabolites are involved in the defense against generalist herbivores and opportunistic disease organisms. For instance, Capsicum annuum, the most common of the five domesticated species of peppers (Ince et al., 2010), includes a range of secondary metabolites such as carotenoid pigments (which attract animal seed dispersal vectors) and polyphenols (quercetin, luteolin, and capsaicinoids—it is the capsaicinoids that vary most between varieties and give hot chillies their heat; Saga and Sato, 2003). These polyphenols are antioxidants, which incidentally have promising antimutagenic properties (Nazzaro et al., 2009), and in nature have a range of toxic effects on micro-organisms (Cichewicz and Thorpe, 1996) and insects (Cardoza and Tumlinson, 2006). However, there exist cases in which animals have evolved resistance to defense compounds and actually prefer to eat plants containing these compounds because there are fewer herbivores competing for that particular food source. Thus, humans are not the only animals that seek chillies—the American pepper weevil (Anthonomus eugenii) can smell the plume of volatile compounds given off by the plant and use this to navigate to its food source (Addesso and McAuslane, 2009). (For the record, birds are the main seed dispersal vector of chillies in nature, are attracted to the brightly colored fruits, and do not have the same pain/heat receptors as mammals and thus are not deterred by their pungency—this is important for successful seed dispersal as mammals grind seeds whereas birds swallow food whole and let the seeds pass through the gut.)

Despite the apparently disadvantageous effect of advertising to insects, the main function of this system of volatile emission is actually to attract insects, but typically as part of an inducible defense system. Herbivore damage, particularly by generalist invertebrates such as spider mites, stimulates the emission of volatile terpenes, which attract specific carnivorous insects (van den Boom et al. 2004). Thus in addition to direct defense (i.e., simply being full of noxious chemicals) peppers also use a form of indirect defense mediated by a third party; the predator. Plants thus face a trade-off between investment of resources in direct and indirect defenses. Despite exhibiting moderately strong direct defense, peppers actually invest more in indirect defense, which has been demonstrated to have a greater effect on herbivore behavior (Van den Boom et al., 2004). In contrast, a number of plants such as Ginkgo biloba invest extremely heavily in direct defense, with indirect defense having a relatively weak effect on herbivores.

2.4.2 Trade-off between primary and secondary metabolism

The concept of trade-offs is essential to plant ecology because the resources available to plants are limited and plant survival depends on optimizing resource use. Thus, another important trade-off exists with regard to resource investment in either primary metabolism (favoring growth) or secondary metabolism (favoring resistance). Indeed, Harborne (1997) states that plants can be considered as belonging to three main groups: (1) perennial “growth-dominated” plants with rapid growth, poor chemical defense but with a highly inducible defense system, (2) perennial plants with slow growth rates that are inherently well defended (often including spines and physical defenses) but with less responsive facultative defenses, and (3) annual plants for which defense is not as important. These three extremes in survival strategy reflect the three main directions in which all organisms may be adapted to survive. Indeed, Grime and Pierce (2011) have recently shown that organisms from every branch of the tree of life face a universal three-way trade-off between C-selection (competition selects for a strategy that invests resources in acquiring more resources), S-selection (stress selects for the investment of resources in the maintenance of individuals), and R-selection (disturbance selects for survival based on regeneration and a rapid lifecycle). Thus, the extent to which a plant species invests in facultative (induced) or obligate defenses reflects the overall survival strategy.

Soure: Giacinto Bagetta, Marco Cosentino, Marie Tiziana Corasaniti, Shinobu Sakurada (2012); Herbal Medicines: Development and Validation of Plant-derived Medicines for Human Health; CRC Press