Saponinsare valuable compounds to humans due to their use in pharmacy, industry, cosmetics, agriculture and the food market (Balandrin1996; Tanaka et al.
1996; Cheeke 1998, Fenwick and Oakenfull 2006; Skene and Sutton 2006; Sun etal. 2009). Moderate and severe water stress increasedsaponin content of borage leaf and inflorescence(Table 5). This increase could be related to its protective role againstoxidative stress (Lin et al.
2009). The hostplant benefits from saponin production by their participation in mechanisms of defenseand protection as well as in environmental interactions (Hoagland et al. 1996;Osbourn 1996; Agrell et al. 2003; Golawska et al. 2006).
Increasein saponin levels in response to stress is often mediated by thetranscriptional activation of biosynthetic genes through a complex signalingcascade involving the jasmonate and salicylate hormones. Hence, thebiosynthesis of these molecules can be induced using elicitors and this featurehas been exploited in several plant species to improve saponin yields (Shabaniet?al., 2009; Yendo et?al., 2010). Stress- ordevelopmentally regulated changes in saponin composition contribute to thetotal plant secondary metabolite pool where they may ensure protection of plantorgans and overall plant survival.
However, as described above, in many plants,saponins and their biosynthetic intermediates accumulate during normal growthand development. Manipulating the biosynthesis of these molecules causesmorphological and physiological effects, suggesting additional functions (Abeet?al., 1988; Guhling et?al., 2006). It could be stated that in nearly all plantsanalyzed, the concentrations of secondary plant products are significantlyenhanced under drought stress conditions.
In fact, the stress related decreasein biomass generally over-compensates the increase in accumulation of relevantnatural products (Al-Gabbieshet al., 2015).This is the main reason forenhancing saponin and tannin contents of borage leaves and inflorescences underdrought stress (Table 5).