Eating barbed wire: Direct and indirect defensive roles of non‐glandular trichomes

Abstract

Phytochemical toxins have been considered as the ultimate weapon in plants to defend against herbivorous insects (Howe & Jander, 2008). The inter-and intraspecific variations along with the tremendous diversity of these secondary metabolites with their broad, but often species-specific functions have led to rich and diverse research in this area. However, structural defenses such as trichomes, spines, waxy cuticle, thorns, and raphides that also armor the plants, have received less attention (Kariyat, Smith, Stephenson, De Moraes, & Mescher, 2017). Trichomes, a major group among these, are abundantly distributed on various aerial plant parts including leaves, stem, and fruits. Trichomes are broadly divided into two types: glandular and nonglandular trichomes. Non-glandular trichomes are epidermal appendages that mechanically trap and impale herbivores (Liu, Liu, Jiao, Lu, & Xu, 2017), thus restricting or delaying access to the epidermis (Bellota, Medina, & Bernal, 2013; Kariyat et al., 2018; Figure 1). In contrast, glandular trichomes secrete defensive compounds which either attract herbivores' natural enemies (Weinhold & Baldwin, 2011), produce anti-herbivore proteinase inhibitors (Peiffer, Tooker, Luthe, & Felton, 2009), or antibiotic compounds (Dereboylu et al., 2012). The role of trichomes has been predominantly examined through the lens of chemical defense. Because of this emphasis on glandular trichomes, the exact roles of non-glandular trichomes in biotic stress responses are less understood. Recent work by Andama, Mujiono, Hojo, Shinya, and Galis (2020) published in this issue of Plant, Cell & Environment challenges these assumed roles. Through a series of field and lab experiments, they demonstrated that silicified non-glandular trichomes are partly responsible for defending rice plants against chewing herbivores. In rice, popular hybrid varieties NERICA (New Rice for Africa) have been developed by crossing Asian rice WAB56-104 (Oryza sativa L.) with African rice CG14 (Oryza glaberrima Steud). These hybrid varieties are

The Multifunctional Roles of Polyphenols in Plant-Herbivore Interactions

“…Herbivores attack plants by scratching, mining, chewing, biting, sucking, galling, wounding, parasitizing, and even evading the leaf surface using their secretions [4,100,101]. This attack in turn initiates a suite of defenses in form of physical (spines, trichomes and sclerophylly) and chemical defenses in plants [102][103][104] through signaling molecules [105], phytohormone pathways [106], and the initiation and synthesis of physical defense structures and/or formation of defensive chemical compounds such as alkaloids, [4] or activating the compounds already present in inactive form such as cyanogenic glycosides [107,108] and benzoxazinoids [109,110] to name a few (Figure 3). For example., two major weeds in the Solanaceae-Solanum carolinense L. and Solanum elaeagnifolium Cav.-possess a diverse suite of constitutive defenses including non-glandular stellate trichomes that negatively affect herbivore feeding [6], internode spines that deter herbivores [7].…”

The push-pull intercrop Desmodium does not repel, but intercepts and kills pests

“…Second, trichomes on Desmodium hinder movement, damage the cuticle and even entirely immobilize larvae on the plant, increasing developmental time, exposure to natural enemies and overall mortality 39,40 . Third, the ingestion of trichomes will damage the intestinal lining and affect digestion, development and survival 40,41 . Indeed, while first instar larvae easily fed around the trichomes, larger larvae did ingest trichomes as evidenced by trichomes found in larval frass.…”

Morphological characterization of trichomes shows enormous variation in shape, density and dimensions across the leaves of 14 Solanum species

“…Trichomes, in general, are considered as one of the first line of defences possessed by plants to protect against abiotic stresses such as UV rays, water loss, temperature extremities ( Ehleringer 1982 ; Li et al 2018 ; Oksanen 2018 ) and herbivore damage ( Kaur and Kariyat 2020a, b ; Watts and Kariyat 2021 ). Moreover, the leaf trichomes can also act as mechanoreceptors for detection of insects on leaf surface as observed in case of Arabidopsis ( Zhou et al 2017 ).…”