Suberin structural role

The phenylpropanoid pathway (Fig. 3.1) is responsible for the production of many natural products that are of interest in the context of plant growth and development, human health, and ecology. For example, flavonoids are necessary for pollen viability in maize and petunia, and have been suggested to play a role in directed auxin transport. Flavonoids and sinapate esters have been found to be important UV-protectants in many species, including Arabidopsis. Furthermore, wall-bound phenolics are thought to impart control over cell wall expansion, and hydroxycinnamic acids are an important structural component of the hydrophobic barrier polymer suberin. Finally, lignin is a phenylpropanoid polymer ubiquitous in higher plants, which is necessary for mechanical support and water transport. " ... 

Since in most cases the resistance is caused by factors other than alkaloids, such as differences in cell wall structure, accumulation of phenolic compounds, or an ability to form suberin layers around the infection point, it seems that alkaloids have never played an important role in protecting plants against fungal and bacterial infections. Most alkaloids are without effect on the development of adapted, as well as foreign, fungi in experiments in vitro. Therefore, it seems justified to assume that in cases in which an alkaloid-rich plant is not infected by a certain parasite, the reason for its immunity is not the presence of alkaloids. 

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