The abundance of cacti in arid lands is, at least in part, due to several biochemical and anatomical adaptations that cacti have for minimizing water loss. To what degree members of the Cactaceae have evolved defenses against microbial attack or insect feeding is not as well studied. In this chapter we will examine how one biosynthetic pathway, the route leading to flavonoid synthesis, responds to conditions which simulate bacterial infection. The chemical and biochemical studies have focused on a popular ornamental species, Cephaloceveus senilis (old-man-cactus)
Most members of the Cactaceae are xerophytes living in desert or semiarid habitats in which rainfall is low and the amount of water present in the soil is very limited for much of the year. These perennial dicots, with specialized water-saving properties, are well suited for such water-stressed conditions. They are capable of storing water by way of their succulent stems and a specialized cuticle cover, which is rich in waxes, thereby reducing evaporation. In addition, cacti possess a metabolic adaptation, crassulacean acid metabolism (CAM), designed to lessen water loss during photosynthesis. This latter adaptation minimizes water loss from diurnal transpiration by allowing stomata to only open at night when moisture levels are higher. While the stomata are open, carbon dioxide is taken up into the cytosol, converted to maleic acid, and stored in the vacuole. During the day, the acid is enzymatically converted to pyruvic acid with the liberation of carbon dioxide. This massive maleic acid fluctuation can store or supply carbon-dioxide independently of moisture conditions. In severe drought, cacti can use up to 80% of their total water content without lethal consequences.
It is generally accepted that cacti are indigenous to the New World. There is one genus, Rhipsalis, which grows wild in equatorial Africa, Madagascar, and the Mascarene Islands, but it is thought to have been introduced by birds. Cacti can easily be identified by round or elongated patches of felt, hair, or wool which are called areoles. From the upper part of these areoles the flowers and branches originate, and from the lower section the spines develop. Since the spines are not directly connected to the stem tissue, this tissue is not wounded when a spine is removed, unlike the case in the easily misidentified succulent Euphorbia species.
C. senilis is of the subfamily Cactoideae in the tribe Pachycereae, which grows in southern Mexico. It is native to the states of Hidalgo and Guanajuato, where its columnar stem grows up to 15 m high and 40 cm diameter. It is commonly grown horticulturally as a novelty because of the beard-like hair which envelopes the young plant. In addition to being readily available for whole plant phytochemical investigations, it has proved to be an excellent source for a liquid suspension culture system to study induction of secondary compounds, since cultures accumulate a red pigmentation with elicitation, which provides a rapid cue that the culture has responded to the elicitation. This accumulation of pigment in the medium correlates with increased activities of enzymes in the phenyl-propanoid pathway and the accumulation of flavonoids in the cells.
Flavonoid chemical analysis of fresh stem material from C. senilis has resulted in the isolation and elucidation of two kaempferol monoglycosides, kaempferol 7-O-α-L-rhamnopyranoside (1) and kaempferol 3-O-α-L-rham-nopyransoyl (l→6)-β-D-galactopyranoside-7-0-α-L-rhamnopyranoside (2), as well as a kaempferol tetraglycoside, kaempferol 3-O-β-D-glucopyranosyl (1→2)-O-α-L-rhamnopyranosyl (1→6)]-β-D-galactopyranoside-7-O-α-L-rham-nopyranoside. Similar flavonol glycosides, based on quercetin kaempferol and isohamnetin skeletons, have been reported in Opuntia, Echincereus, and Neochilenial species.
Unlike traditional crop plants, the growth of cacti in arid regions may be restricted by microbial infections rather than by the absence of water. An understanding and selection for defense mechanisms in cacti may provide one route for reducing the susceptibility of such plants to disease. Research with C. senilis has, thus far, opened up a new class of phytoalexins, providing insight into an inducible chemical defense mechanism that may be operative in cacti. In addition to the aurone phytoalexin, cephalocerone, several other flavonoids with this rare B-ring unsubstituted pattern have been found. Though the biosynthetic pathway to unsubstituted B-ring flavonoids has not been elucidated, enzyme studies have clarified that with elicited cactus cultures, hydroxy-cinnamate: CoA ligase can directly activate cinnamic acid to the cinnamoyl-CoA. Whether high PAL activity is coupled with low CA4H activity, to cause the accumulation of B-ring unsubstituted flavonoids, is now under investigation.
Selections from the book: “Medicinal and Aromatic Plants IX” (1996).