UV‐B impact on aphid performance mediated by plant quality and plant changes induced by aphids

Plants face various abiotic and biotic environmental factors and therefore need to adjust their phenotypic traits on several levels. UV‐B radiation is believed to impact herbivorous insects via host plant changes. Plant responses to abiotic challenges (UV‐B radiation) and their interaction with two aphid species were explored in a multifactor approach. Broccoli plants [Brassica oleracea L. convar. botrytis (L.), Brassicaceae] were grown in two differently covered greenhouses, transmitting either 80% (high UV‐B) or 4% (low UV‐B) of ambient UV‐B. Three‐week‐old plants were infested with either specialist cabbage aphids [Brevicoryne brassicae (L.), Sternorrhyncha, Aphididae] or generalist green peach aphids [Myzus persicae (Sulzer), Sternorrhyncha, Aphididae]. Plants grown under high‐UV‐B intensities were smaller and had higher flavonoid concentrations. Furthermore, these plants had reduced cuticular wax coverage, whereas amino acid concentrations of the phloem sap were little influenced by different UV‐B intensities. Cabbage aphids reproduced less on plants grown under high UV‐B than on plants grown under low UV‐B, whereas reproduction of green peach aphids in both plant light sources was equally poor. These results are likely related to the different specialisation‐dependent sensitivities of the two species. The aphids also affected plant chemistry. High numbers of cabbage aphid progeny on low‐UV‐B plants led to decreased indolyl glucosinolate concentrations. The induced change in these glucosinolates may depend on an infestation threshold. UV‐B radiation considerably impacts plant traits and subsequently affects specialist phloem‐feeding aphids, whereas aphid growth forces broccoli to generate specific defence responses.     

Photobiology: Light signal transduction and photomorphogenesis

正Light is crucial for plants, not only because of photosynthesis, but also because of photomorphogenesis. As one of the most important environmental cues, light influences multiple responses in plants,including seed germination, seedling de-etiolation,shade avoidance, phototropism, stomata and chloroplast movement, circadian rhythms, and flowering     

Temperature‐dependent shade avoidance involves the receptor‐like kinase ERECTA

Plants detect the presence of neighbouring vegetation by monitoring changes in the ratio of red (R) to far‐red (FR) wavelengths (R:FR) in ambient light. Reductions in R:FR are perceived by the phytochrome family of plant photoreceptors and initiate a suite of developmental responses termed the shade avoidance syndrome. These include increased elongation growth of stems and petioles, enabling plants to overtop competing vegetation. The majority of shade avoidance experiments are performed at standard laboratory growing temperatures (>20°C). In these conditions, elongation responses to low R:FR are often accompanied by reductions in leaf development and accumulation of plant biomass. Here we investigated shade avoidance responses at a cooler temperature (16°C). In these conditions, Arabidopsis thaliana displays considerable low R:FR‐mediated increases in leaf area, with reduced low R:FR‐mediated petiole elongation and leaf hyponasty responses. In Landsberg erecta, these strikingly different shade avoidance phenotypes are accompanied by increased leaf thickness, increased biomass and an altered metabolite profile. At 16°C, low R:FR treatment results in the accumulation of soluble sugars and metabolites associated with cold acclimation. Analyses of natural genetic variation in shade avoidance responses at 16°C have revealed a regulatory role for the receptor‐like kinase ERECTA.     

Effects of Red and Blue LED Light on the Growth and Photosynthesis of Barley (Hordeum vulgare L.) Seedlings

According to the action spectrum of photosynthesis, photosynthetic efficiency is highest for red light. However, long-term growth with only red light leads to unfavorable changes in plant morphology, decrease in photosynthetic capacity and plant productivity. Detailed mechanisms behind these changes are still poorly understood. We studied the effects of narrow-band red (RL) and blue (BL) LED lighting on the morphology and photosynthesis of barley (Hordeum vulgare L.) seedlings at 9 days old, when energy for plant growth comes mostly from the endosperm, and light has a mainly morphogenic effect on plant growth. Plants grown with white fluorescent lamps (WL) were used as a control. At this developmental stage, light spectrum had small but significant effects on most morphometric parameters, which may become more prominent as the plant grows. These effects were more pronounced in RL-grown plants and were similar to the ‘shade-avoidance response’, which is unusual as in nature it occurs when the fraction of red light in the spectrum is low. RL-grown plants also had impaired photosynthetic photochemical efficiency (as assessed by PAM-fluorometry and leaf absorption). BL-grown plants had a stronger similarity to control plants in their morphology and photosynthetic characteristics than RL-grown plants; however, they had higher NPQ and different NPQ induction kinetics than WL- and RL-grown plants. Our results suggest that photoregulation of plant morphology and photosynthesis evolutionarily adapted to natural light is miscoordinated in narrow-band LED light. We discuss possible reasons for this miscoordination and for the formation of observed phenotypes on the level of photoreceptors.

     

UV‐B radiation delays flowering time through changes in the PRC2 complex activity and miR156 levels in Arabidopsis thaliana

UV‐B is a high‐energy component of the solar radiation perceived by the plant and induces a number of modifications in plant growth and development, including changes in flowering time. However, the molecular mechanisms underlying these changes are largely unknown. In the present work, we demonstrate that Arabidopsis plants grown under white light supplemented with UV‐B show a delay in flowering time, and this developmental reprogramming is mediated by the UVR8 photoreceptor. Using a combination of gene expression analyses and UV‐B irradiation of different flowering mutants, we gained insight into the pathways involved in the observed flowering time delay in UV‐B‐exposed Arabidopsis plants. We provide evidence that UV‐B light downregulates the expression of MSI1 and CLF, two of the components of the polycomb repressive complex 2, which in consequence drives a decrease in H3K27me3 histone methylation of MIR156 and FLC genes. Modification in the expression of several flowering time genes as a consequence of the decrease in the polycomb repressive complex 2 activity was also determined. UV‐B exposure of flowering mutants supports the involvement of this complex in the observed delay in flowering time, mostly through the age pathway.     

Keeping up with the neighbours: phytochrome sensing and other signalling mechanisms

Plants ‘forage’ for light in plant canopies using a variety of photosensory systems. Far-red radiation (FR) reflected by neighbours is an early signal of competition that elicits anticipatory shade-avoidance responses. In Arabidopsis and cucumber, perception of reflected FR requires phytochrome B. Horizontal blue (B) light gradients also guide plant shoots to canopy gaps in patchy vegetation, and these B light signals are perceived by specific photoreceptors. When plants are shaded by neighbours they undergo extensive reprogramming of their morphological development. Although phytochromes and B light receptors are certainly involved in these responses to shading, other sensory systems probably play important roles in the field. Recent studies of plant–plant signalling are unveiling a paradigm of sensory diversity and sophistication, which has important implications for understanding the functioning of plant populations and communities.     

Tissue‐specific and light‐dependent regulation of phytochrome gene expression in rice

Phytochromes are red‐ and far red light photoreceptors in higher plants. Rice (Oryza sativa L.) has three phytochromes (phyA, phyB and phyC), which play distinct as well as cooperative roles in light perception. To gain a better understanding of individual phytochrome functions in rice, expression patterns of three phytochrome genes were characterized using promoter‐GUS fusion constructs. The phytochrome genes PHYA and PHYB showed distinct patterns of tissue‐ and developmental stage‐specific expression in rice. The PHYA promoter‐GUS was expressed in all leaf tissues in etiolated seedlings, while its expression was restricted to vascular bundles in expanded leaves of light‐grown seedlings. These observations suggest that light represses the expression of the PHYA gene in all cells except vascular bundle cells in rice seedlings. Red light was effective, but far red light was ineffective in gene repression, and red light‐induced repression was not observed in phyB mutants. These results indicate that phyB is involved in light‐dependent and tissue‐specific repression of the PHYA gene in rice.     

Light signaling in plants

Light signals have profound morphogenic effects on plant development. Signals perceived by the red/far‐red absorbing phytochrome family of photoreceptors and the blue/green/ UV‐A absorbing cryptochrome photoreceptor converge on a group of pleiotropic gene products defined by the COP/DET loci to control the pattern of development. The signaling pathway, although still undefined, includes several classic signaling molecules, such as G‐proteins, calcium, calmodulin, and cGMP. A separate signaling pathway is involved in the modulation of the phototropic response. Additional mutants have been identified that affect subsets of light signaling responses. This review provides an overview of our current understanding of the light signaling process, in particular recent genetic and biochemical advances.     

Cyclobutane pyrimidine dimer (CPD) photolyase repairs ultraviolet-B-induced CPDs in rice chloroplast and mitochondrial DNA

Plants use sunlight as energy for photosynthesis; however, plant DNA is exposed to the harmful effects of ultraviolet‐B (UV‐B) radiation (280–320 nm) in the process. UV‐B radiation damages nuclear, chloroplast and mitochondrial DNA by the formation of cyclobutane pyrimidine dimers (CPDs), which are the primary UV‐B‐induced DNA lesions, and are a principal cause of UV‐B‐induced growth inhibition in plants. Repair of CPDs is therefore essential for plant survival while exposed to UV‐B‐containing sunlight. Nuclear repair of the UV‐B‐induced CPDs involves the photoreversal of CPDs, photoreactivation, which is mediated by CPD photolyase that monomerizes the CPDs in DNA by using the energy of near‐UV and visible light (300–500 nm). To date, the CPD repair processes in plant chloroplasts and mitochondria remain poorly understood. Here, we report the photoreactivation of CPDs in chloroplast and mitochondrial DNA in rice. Biochemical and subcellular localization analyses using rice strains with different levels of CPD photolyase activity and transgenic rice strains showed that full‐length CPD photolyase is encoded by a single gene, not a splice variant, and is expressed and targeted not only to nuclei but also to chloroplasts and mitochondria. The results indicate that rice may have evolved a CPD photolyase that functions in chloroplasts, mitochondria and nuclei, and that contains DNA to protect cells from the harmful effects of UV‐B radiation.     

Genome-wide analysis of light-dependent transcript accumulation patterns during early stages of Arabidopsis seedling deetiolation

Light is among the most important exogenous factors that regulate plant development. To sense light quality, intensity, direction, and duration, plants have evolved multiple photoreceptors that enable the detection of photons from the ultraviolet B (UV-B) to the far-red spectrum. To study the effect of different light qualities on early gene expression, dark-grown Arabidopsis (Arabidopsis thaliana) seedlings were either irradiated with continuous far-red, red, or blue light or received pulses of red, UV-A, or UV-A/B light. The expression profiles of seedlings harvested at 45 min and 4 h were determined on a full genome level and compared with the profiles of dark controls. Data were used to identify light-regulated genes and to group these genes according to their light responses. While most of the genes were regulated by more than one light quality, a considerable number of UV-B-specific gene expression responses were obtained. An extraordinarily high similarity in gene expression patterns was obtained for samples that perceived continuous irradiation with either far-red or blue light for 4 h. Mutant analyses hint that this coincidence is caused by a convergence of the signaling cascades that regulate gene expression downstream of cryptochrome blue light photoreceptors and phytochrome A. Whereas many early light-regulated genes exhibited uniform responses to all applied light treatments, highly divergent expression patterns developed at 4 h. These data clearly indicate that light signaling during early deetiolation undergoes a switch from a rapid, but unspecific, response mode to regulatory systems that measure the spectral composition and duration of incident light.     

Environmentally Modified Organisms – Expanding Genetic Potential with Light

Light is a potent regulator of plant growth and development, impacting gene expression to global physiology and metabolism. Plants sense a broad range of wavelengths, from UV to far-red, through separate photoreceptors. These light sensors direct adaptive responses under changing environmental conditions and specifically activate precise downstream signaling pathways. Research studies in photobiology, mostly in Arabidopsis thaliana, have characterized light effects on many plant behaviors, along with the genetic mechanisms that control them. Transferring this knowledge to crops has opened a new field in plant science where variation in light quantity, quality, duration or combinations can be used to change plant growth, development or metabolism to influence a desired final product. With the potential of easily being introduced into production chains, and given its safety and relative low cost, this approach can be combined with, or used as an alternative to, breeding or genetic engineering. We review how light has been used in 22 different crop species to manipulate growth characteristics, nutritional value, and yield. We also discuss future opportunities in using light to control produce quality or timing of plant product production.     

UV-A/B radiation rapidly activates photoprotective mechanisms in Chlamydomonas reinhardtii

Conversion of light energy into chemical energy through photosynthesis in the chloroplasts of photosynthetic organisms is essential for photoautotrophic growth, and non-photochemical quenching (NPQ) of excess light energy prevents the generation of reactive oxygen species and maintains efficient photosynthesis under high light. In the unicellular green alga Chlamydomonas reinhardtii, NPQ is activated as a photoprotective mechanism through wavelength-specific light signaling pathways mediated by the phototropin (blue light) and ultra-violet (UV) light photoreceptors, but the biological significance of photoprotection activation by light with different qualities remains poorly understood. Here, we demonstrate that NPQ-dependent photoprotection is activated more rapidly by UV than by visible light. We found that induction of gene expression and protein accumulation related to photoprotection was significantly faster and greater in magnitude under UV treatment compared with that under blue- or red-light treatment. Furthermore, the action spectrum of UV-dependent induction of photoprotective factors implied that C. reinhardtii senses relatively long-wavelength UV (including UV-A/B), whereas the model dicot plant Arabidopsis (Arabidopsis thaliana) preferentially senses relatively short-wavelength UV (mainly UV-B/C) for induction of photoprotective responses. Therefore, we hypothesize that C. reinhardtii developed a UV response distinct from that of land plants.

In contrast to land plants, which sense short-wave UV light, the unicellular green alga Chlamydomonas reinhardtii senses long-wavelength UV light for photoprotective responses.     

Focusing on the nuclear and subnuclear dynamics of light and circadian signalling

Circadian clocks provide organisms the ability to synchronize their internal physiological responses with the external environment. This process, termed entrainment, occurs through the perception of internal and external stimuli. As with other organisms, in plants, the perception of light is a critical for the entrainment and sustainment of circadian rhythms. Red, blue, far‐red, and UV‐B light are perceived by the oscillator through the activity of photoreceptors. Four classes of photoreceptors signal to the oscillator: phytochromes, cryptochromes, UVR8, and LOV‐KELCH domain proteins. In most cases, these photoreceptors localize to the nucleus in response to light and can associate to subnuclear structures to initiate downstream signalling. In this review, we will highlight the recent advances made in understanding the mechanisms facilitating the nuclear and subnuclear localization of photoreceptors and the role these subnuclear bodies have in photoreceptor signalling, including to the oscillator. We will also highlight recent progress that has been made in understanding the regulation of the nuclear and subnuclear localization of components of the plant circadian clock.     

Sub‐lethal UV‐C radiation induces callose,hydrogen peroxide and defence‐related gene expression in Arabidopsis thaliana

Exposure of plants to UV‐C irradiation induces gene expression and cellular responses that are commonly associated with wounding and pathogen defence, and in some cases can lead to increased resistance against pathogen infection. We examined, at a physiological, molecular and biochemical level, the effects of and responses to, sub‐lethal UV‐C exposure on Arabidopsis plants when irradiated with increasing dosages of UV‐C radiation. Following UV‐C exposure plants had reduced leaf areas over time, with the severity of reduction increasing with dosage. Severe morphological changes that included leaf glazing, bronzing and curling were found to occur in plants treated with the 1000 J·m^?2 dosage. Extensive damage to the mesophyll was observed, and cell death occurred in both a dosage‐ and time‐dependent manner. Analysis of H₂O₂ activity and the pathogen defence marker genes PR1 and PDF1.2 demonstrated induction of these defence‐related responses at each UV‐C dosage tested. Interestingly, in response to UV‐C irradiation the production of callose (β‐1,3‐glucan) was identified at all dosages examined. Together, these results show plant responses to UV‐C irradiation at much lower doses than have previously been reported, and that there is potential for the use of UV‐C as an inducer of plant defence.     

Mutations in the gene for the red/far-red light receptor phytochrome B alter cell elongation and physiological responses throughout Arabidopsis development.

Phytochromes are a family of plant photoreceptors that mediate physiological and developmental responses to changes in red and far-red light conditions. In Arabidopsis, there are genes for at least five phytochrome proteins. These photoreceptors control such responses as germination, stem elongation, flowering, gene expression, and chloroplast and leaf development. However, it is not known which red light responses are controlled by which phytochrome species, or whether the different phytochromes have overlapping functions. We report here that previously described hy3 mutants have mutations in the gene coding for phytochrome B (PhyB). These are the first mutations shown to lie in a plant photoreceptor gene. A number of tissues are abnormally elongated in the hy3(phyB) mutants, including hypocotyls, stems, petioles, and root hairs. In addition, the mutants flower earlier than the wild type, and they accumulate less chlorophyll. PhyB thus controls Arabidopsis development at numerous stages and in multiple tissues.