Glandular trichomes are specialized hairs found on the surface of about 30% of all vascular plants and are responsible for a significant portion of a plant’s secondary chemistry. Glandular trichomes are an important source of essential oils, i.e., natural fragrances or products that can be used by the pharmaceutical industry, although many of these substances have evolved to provide the plant with protection against herbivores and pathogens. The storage compartment of glandular trichomes usually is located on the tip of the hair and is part of the glandular cell, or cells, which are metabolically active. Trichomes and their exudates can be harvested relatively easily, and this has permitted a detailed study of their metabolites, as well as the genes and proteins responsible for them. This knowledge now assists classical breeding programs, as well as targeted genetic engineering, aimed to optimize trichome density and physiology to facilitate customization of essential oil production or to tune biocide activity to enhance crop protection. We will provide an overview of the metabolic diversity found within plant glandular trichomes, with the emphasis on those of the Solanaceae, and of the tools available to manipulate their activities for enhancing the plant’s resistance to pests.

Background Plants are hotbeds for parasites such as arthropod herbivores, which acquire nutrients and energy from their hosts in order to grow and reproduce. Hence plants are selected to evolve resistance, which in turn selects for herbivores that can cope with this resistance. To preserve their fitness when attacked by herbivores, plants can employ complex strategies that include reallocation of resources and the production of defensive metabolites and structures. Plant defences can be either prefabricated or be produced only upon attack. Those that are ready-made are referred to as constitutive defences. Some constitutive defences are operational at any time while others require activation. Defences produced only when herbivores are present are referred to as induced defences. These can be established via de novo biosynthesis of defensive substances or via modifications of prefabricated substances and consequently these are active only when needed. Inducibility of defence may serve to save energy and to prevent self-intoxication but also implies that there is a delay in these defences becoming operational. Induced defences can be characterized by alterations in plant morphology and molecular chemistry and are associated with a decrease in herbivore performance. These alterations are set in motion by signals generated by herbivores. Finally, a subset of induced metabolites are released into the air as volatiles and function as a beacon for foraging natural enemies searching for prey, and this is referred to as induced indirect defence.

Summary: Plants respond to herbivory by mounting a defense. Some plant‐eating spider mites (Tetranychus spp.) have adapted to plant defenses to maintain a high reproductive performance. From natural populations we selected three spider mite strains from two species, Tetranychus urticae and Tetranychus evansi, that can suppress plant defenses, using a fourth defense‐inducing strain as a benchmark, to assess to which extent these strains suppress defenses differently. We characterized timing and magnitude of phytohormone accumulation and defense‐gene expression, and determined if mites that cannot suppress defenses benefit from sharing a leaf with suppressors. The nonsuppressor strain induced a mixture of jasmonate‐ (JA) and salicylate (SA)‐dependent defenses. Induced defense genes separated into three groups: ‘early’ (expression peak at 1 d postinfestation (dpi)); ‘intermediate’ (4 dpi); and ‘late’, whose expression increased until the leaf died. The T. evansi strains suppressed genes from all three groups, but the T. urticae strain only suppressed the late ones. Suppression occurred downstream of JA and SA accumulation, independently of the JA–SA antagonism, and was powerful enough to boost the reproductive performance of nonsuppressors up to 45%. Our results show that suppressing defenses not only brings benefits but, within herbivore communities, can also generate a considerable ecological cost when promoting the population growth of a competitor.

The transformation of the ovary into a fruit after successful completion of pollination and fertilization has been associated with many changes at transcriptomic level. These changes are part of a dynamic and complex regulatory network that is controlled by phytohormones, with a major role for auxin. One of the auxin-related genes differentially expressed upon fruit set and early fruit development in tomato is Solanum lycopersicumAUXIN RESPONSE FACTOR 9 (SlARF9). Here, the functional analysis of this ARF is described. SlARF9 expression was found to be auxin-responsive and SlARF9 mRNA levels were high in the ovules, placenta, and pericarp of pollinated ovaries, but also in other plant tissues with high cell division activity, such as the axillary meristems and root meristems. Transgenic plants with increased SlARF9 mRNA levels formed fruits that were smaller than wild-type fruits because of reduced cell division activity, whereas transgenic lines in which SlARF9 mRNA levels were reduced showed the opposite phenotype. The expression analysis, together with the phenotype of the transgenic lines, suggests that, in tomato, ARF9 negatively controls cell division during early fruit development.

Plants are nutritious and hence herbivores and phytopathogens have specialized to attack and consume them. In turn, plants have evolved adaptations to detect and withstand these attacks. Such adaptations we call ‘defenses’ and they can operate either directly between the plant and the plant consumer or indirectly i.e. when taking effect via other organisms such as predators and parasitoids of herbivores. Plant defenses put selection pressure on plant-consumers and, as a result, herbivores and pathogens have evolved counter-adaptations to avoid, resist, or manipulate plant defenses. Here we review how plant consumers have adapted to cope with plant defenses and we will put special emphasis on the phenomenon of suppression of plant defenses.