Plants are not alone in their perception of and sensitivity towards UV wavelengths, and the diverse multi-biotic nature of global agro-ecosystems continues to pose challenges for crop production, largely as a consequence of the ‘moving target’ of pest and disease attack. Actual losses attributed to the varied impacts of pest and disease have been estimated at 26–40% for sugar beet, barley, soybean, wheat, cotton, maize and rice (Oerke & Dehne, 2004), with potential losses estimated by the same authors at 50–80% across selected key crops. It is also widely believed that pest and disease pressures may be exacerbated by climatic change (Percy et al., 2002; Jump & Peñuelas, 2005). Many of the diverse and dynamic aspects of plant photomorphogenesis are composed of physical and chemical endpoints which are commonly associated with pest and disease resistance, thus indicating an enhanced and as yet underexplored role for UV radiation within integrated pest and disease management strategies (Paul & Gwynn-Jones, 2003; Mazza et al., 2012). The role of plant secondary metabolism as a constitutive and inducible defence against insect feeding has been characterized extensively (Baldwin et al., 2001; Kliebenstein, 2004), and the development of such defences as a key plant strategy to deter pest foraging and/or reduce palatability could be considered as an exaptation of the UV phytochemical response, developed against the transition of plants from water to land in a background of high UV flux. In a similar manner to the aforementioned commonly observed aspects of plant UV photomorphogenesis, deterrence in insect pest feeding or attack on plants exposed to UV-B is now a frequently observed phenomenon (Ballaré et al., 1996; Lindroth et al., 2000; Rousseaux et al., 2004), with the majority of observations based on feeding choice experiments carried out under field conditions using selective solar UV filtration, or in the laboratory, with a common focus on the Lepidoptera. Despite the generalized correlation between UV exposure, induction of secondary metabolism and consequences for pest defences, specific mechanisms have remained somewhat obscure, with past studies highlighting the difficulties in attributing anti-feedant responses to particular UV-induced metabolites (Hatcher & Paul, 1994); yet, progress is now being made. For example, significant convergence in the composition of phenolic response to both UV-B and stimulated herbivory has been demonstrated in the model system Nicotiana, with chlorogenic acid and dicaffeoylspermidine isomers the commonly associated responders to both stimuli (Izaguirre et al., 2007); interestingly, the key flavonoid observed during the study, rutin, was singularly induced by UV-B and not herbivory. The identification of signalling initiators and intermediates in the UV–herbivory response presents a desirable aim, and it has been hypothesized that the key signalling component of the wounding response, jasmonic acid (JA), could be a primary regulator. In support of JA as a mediator of UV-induced plant defence, adult moths were observed by Caputo et al. (2006) to preferentially deposit eggs on plants which had not received supplementary UV-B exposure, yet this deterrence effect was diminished in plants with the jar1-1 mutation, which were hence deficient in JA signalling. Furthermore, silencing of the lipoxygenase-3 (LOX3) gene in Nicotiana attenuata plants led to greatly reduced UV-B-mediated resistance to insect attack in the field and, despite no observed increase in endogenous JA levels in wild-type plants, increased sensitivity of jasmonate-responsive genes, such as trypsin proteinase inhibitor (TPI), occurred in response to UV-B (Demkura et al., 2010). Thus, a pattern is emerging whereby the role of UV-B in pest resistance is not driven in its entirety by particular signalling pathways or the elicitation of specific secondary metabolites, such as the flavonoids, for example. Further progress is needed to elucidate the role of UV photomorphogenic signalling cascades in the regulation of the herbivory response (i.e. UVR8–COP1–HY5), yet there are early indications that UVR8 may form an element of plant herbivore defence. For example, Fig. 2(a) summarizes a herbivore choice study in which the generalist Spodoptera littoralis preferentially selected uvr8 mutant plants rather than wild-type plants, following early propagation in the absence of UV-B (Fig. 2a). It is clear that the absence of UVR8 function in the mutant may result in certain constitutive differences in phenotype; for example, referring again to the study by Favory et al. (2009), a comparison of gene expression in uvr8 plants relative to wild-type plants in the absence of UV-B indicates significant up-regulation of several functional gene categories regulating cell wall processes (Supporting Information Table S1) and the down-regulation of a selection of RNA transcription and DNA synthesis gene groups. When uvr8 and wild-type plants were then exposed to a high, but globally realistic, UV-B dose for just 12 h, S. littoralis larvae exhibited no significant preference for either genotype, indicating a declining preference for uvr8 plants (Fig. 2a). It is probable that the higher UV-B dose employed in this element of the study may have led to the expression of UVR8-independent responses in the uvr8 mutant, as indicated by the parallel induction of several functional gene groups in the uvr8 mutant and wild-type in the study by Brown et al. (2005) discussed earlier. The regulatory effects of UVR8-specific UV-B photomorphogenesis on herbivory defence are yet to be fully characterized, but this example suggests further that our knowledge is still very much limited regarding the possible overlaps between UVR8-dependent and acute UV-B signalling in the context of varied field UV-B fluxes. In recent years, the prominence of strategies, such as Integrated Pest Management (IPM), to achieve sustainable routes forward in crop production has received a great deal of attention (Vet & Dicke, 1992; Cook et al., 2007), with the use of the biological control of crop pests with natural enemies, for example, insectivorous parasitoids and predators, rapidly becoming a vital component of agronomic practice. Although the consequences of the UV-B plant response for herbivore feeding are now fairly well established, there is still little information regarding the knock-on effects for higher trophic interactions. There is, however, an intriguing indication that UV-B exposure may mediate yet further desirable endpoints in crop protection; Foggo et al. (2007) showed that, during a series of choice experiments, Cotesia plutellae, a parasitic wasp of the diamondback moth (Plutella xylostella), was attracted to brassica plants which previously had been exposed to supplementary UV-B, thus indicating some potential for UV-led trophic interactions in the context of pest control. Although the same study also indicated that moth larvae raised on a diet exclusively composed of UV-B-exposed plant tissue were subject to reduced weight gain by the completion of the larval phase of development, insect foragers may be subject to trade-offs in terms of positive and negative outcomes of consuming UV-B-exposed plants. For example, Fig. 2(b) shows the cuticular UV-absorbing compound content of the generalist herbivore Epiphyas postvittana following larval feeding on L. sativa leaves which were subjected to UV-B exposure. Here, exclusive feeding on UV-B-treated leaves led to increased cuticular absorbance across the UV spectrum, indicating the possibility of additional physiological consequences for herbivores as a result of UV photomorphogenesis, for example photoprotection, and insect health. In terms of the sensory interactions that influence insect foraging, yet are not directly related to feeding, there is currently little information regarding the influence of UV exposure on plant-emitted volatile compounds which often influence insect decision-making, although it is clear that longer wavelength UV, such as the UV-A waveband, is a common component of insect vision, and the use of UV-inclusive and UV-exclusive filters has shown that the UV environment does regulate visual-based behaviour by insect pests (Antignus et al., 2001). Interestingly, although evidence is limited, the role of UV-B wavelengths in insect vision has also been demonstrated recently (Mazza et al., 2010). Arguably the most significant role of ‘beneficial’ insects within an agro-ecosystem is pollination, with the value of pollination as an ecosystem service estimated at $117 × 109 yr−1 based on 1997 values, with more than one-third of the total value of pollination attributable to croplands (Costanza et al., 1997). It is known that insects, such as bees, use UV radiation as a visual cue, yet we know little regarding a direct association with UV photomorphogenesis, inflorescence morphology and pollinator interaction, although there is evidence that UV can influence floral development and may regulate pollinating insect interest as a consequence (Petropoulou et al., 2001; Koti et al., 2005).
Plant pathogen responses to UV radiation remain poorly characterized at a mechanistic level, particularly in terms of understanding the wavelength and dose dependence of the various signal transduction elements involved, with many past studies again focused on unnaturally high UV fluxes. An intriguing prospect of UV-response manipulation for disease control lies in the variability of differing, yet proximate, wavebands in eliciting quite different patho-responses. For example, it is quite well established that the exposure of fungal pathogen spores to UV-B radiation will result in spore death (Paul, 2000; Wu et al., 2000; Paul et al., 2005), almost certainly as a consequence of chronic damage, yet UV-A wavelengths are known to stimulate the sporulation of some pathogens, as has been demonstrated using UV-excluding filters (Elad, 1997). When combined with our understanding of the role of blue light in suppressing the development of some fungi (Reuveni & Raviv, 1997), a somewhat complex picture emerges, yet modification of the light environment of a crop with disease control in mind is fully conceivable (Raviv & Antignus, 2004; Vanninen et al., 2010), and would need to take into account local UV fluxes and spectral balance in order that particular pathosystem responses are targeted effectively. This was demonstrated by Paul et al. (2012), who noted that not only did crops at a mid-northern latitude exposed to a fully UV-inclusive and UV-opaque environment express parallel reductions in infection by Bremia lactucae and Botytis cinerea, but that, in vitro, UV doses required for a 50% reduction in spore germination varied quite markedly between pathogen and phylloplane organisms. In an equal sense to that of insect resistance, the question of host resistance could be a prominent aspect of the UV–pathogen interaction. Plant responses to pathogen attack remain a much explored topic in plant biology (Glazebrook, 2005; Jones & Dangl, 2006), yet there is currently very little information regarding the involvement of the well-defined plant resistance networks in UV-mediated host resistance. However, there are indications that pre-exposure to UV-B subsequently induces resistance to fungal infection at a time point after exposure has ended (Wargent et al., 2006), and that such resistance may be mediated by UVR8 via controlled expression of sinapate compounds (Demkura & Ballaré, 2012). There are additional considerations for the impact of the UV environment beyond pathogens which infect plants; fruit and vegetables are now thought to be a significant source for the introduction of food poisoning bacteria into the food chain (Berger et al., 2010). The use of germicidal, short-wavelength UV has been evaluated for postharvest exposure of fruit and vegetables and water treatment techniques (Chen et al., 2009), and yet, although many food pathogens, such as Salmonella sp., can readily survive on growing plant surfaces, there is little information regarding the consequences of UV morphogenesis for human pathogen survival and growth; for example, the role of the phylloplane in protecting leaf-harboured pathogens from the direct or indirect effects of UV exposure has been little explored. In summary, UV radiation exerts a vast range of influences on diverse biota, including herbivorous and other insect pests and plant pathogens, and there is the potential to greater inform pest and disease control, in addition to managing beneficial agronomic biota, more effectively through an understanding of the UV response.