Plant trichomes as microbial habitats and infection sites

Trichomes, also simply referred to as hairs, are fine outgrowths of epidermal cells in many organisms including plants and bacteria. Plant trichomes have long been known for their multiple beneficial roles, ranging from protection against insect herbivores and ultraviolet light to the reduction of transpiration. However, there is increasing evidence that the presence of trichomes may have detrimental consequences for plants. For example, plant pathogenic bacteria can enter hosts through the open bases or broken stalks of damaged trichomes. Similarly, trichomes are considered a preferred site for fungal infection, and in this regard, the colonization and penetration of trichomes by fungi and oomycetes have been visualized using light, fluorescence, and scanning electron microscopy in a variety of plants from grasses to shrubs and trees. In addition to parasitic interactions, trichomes also form a host site for endophytic relationships with fungi, thereby serving as an unusual fungal niche. The replication and presence of plant viruses in trichomes have also been confirmed after inoculation. In contrast, the well-known beneficial Azolla–Anabaena symbiosis is facilitated through epidermal trichomes of the seedless vascular plant Azolla. These observations indicate that plant trichomes are involved in multiple interactions in terms of providing microbial habitats and infection sites as well as functioning as protective structures. Trichome-related microbial parasitism and endophytism can, in many ways, be considered comparable to those associated with root hairs.