Protective and defensive roles of non-glandular trichomes against multiple stresses: structure-function coordination

As superficial structures, non-glandular trichomes, protect plant organs against multiple biotic and abiotic stresses. The protective and defensive roles of these epidermal appendages are crucial to developing organs and can be attributed to the excellent combination of suitable structural traits and chemical reinforcement in the form of phenolic compounds, primarily flavonoids. Both the formation of trichomes and the accumulation of phenolics are interrelated at the molecular level. During the early stages of development, non-glandular trichomes show strong morphological similarities to glandular ones such as the balloon-like apical cells with numerous phenolics. At later developmental stages, and during secondary wall thickening, phenolics are transferred to the cell walls of the trichomes. Due to the diffuse deposition of phenolics in the cell walls, trichomes provide protection against UV-B radiation by behaving as optical filters, screening out wavelengths that could damage sensitive tissues. Protection from strong visible radiation is also afforded by increased surface light reflectance. Moreover, the mixtures of trichome phenolics represent a superficial chemical barrier that provides protection against biotic stress factors such as herbivores and pathogens. Although the cells of some trichomes die at maturity, they can modulate their quantitative and qualitative characteristics during development, depending on the prevailing conditions of the external biotic or abiotic environment. In fact, the structure and chemical constituents of trichomes may change due to the particular light regime, herbivore damage, wounding, water stress, salinity and the presence of heavy metals. Hence, trichomes represent dynamic protective structures that may greatly affect the outcome of many plant-environment interactions.