Evolution of Alkaloidal Defense Systems

Alkaloids are apparently well-adapted molecules that can serve plants as potent defense chemicals which are used on their own or together with other mostly 

Secondary metabolites with similar structural types and pharmacophoric groups can be seen in several bacteria (where they are often termed antibiotics if they have antimicrobial or cytotoxic properties). Since eukaryotic cells had taken up a-proteobacteria (which became mitochondria) and cyanobacteria (which became chloroplasts), they also inherited a number of genes that encode enzymes for pathways leading to secondary metabolites. Therefore, we may speculate that early plants already had the capacity of building defense compounds and that alkaloids were among the first. Since the numbers and types of herbivores and other enemies have increased within the last 100 million years, angiosperms have had to face more enemies and as a consequence have developed a more complex pattern of defense and signal compounds. 

Several plants produce milk juice sequestered in laticifers in several plant genera alkaloids are mainly stored in latex vesicles, such as isoquinoline alkaloids in Papaver and Chdidonium, or piperidine alkaloids in Lobelia. If herbivores wound such a plant, the latex will spill out and the herbivore will immediately be confronted with alkaloids. Since most of them are strong poisons, a deterrent effect is usually achieved. Another strategic way to store alkaloids is their sequestration in epidermal vacuoles or in trichomes. These tissues have to ward off not only herbivores (especially small ones) but also microorganisms in the first place. Several classes of alkaloids have been found in epidermal tissues, such as quinohzidine and tropane alkaloids [2,3]. 

Most plants produce an alkaloid in one organ and transport the alkaloids after synthesis, via either xylem or phloem (Table 1.3), to other plant tissues in which the alkaloids are stored for defense or signaling [2,11,21]. 

Lupanine Quinolizidine Lupinus, Genista, Cytisus Laburnum, Spartium Yes No 

---