UV-B antagonises shade avoidance and increases levels of the flavonoid quercetin in coriander (Coriandrum sativum)

Abstract Despite controlling a diverse array of regulatory processes in plants, UV-B wavelengths (280–315 nm) are attenuated by common greenhouse materials such as glass and polycarbonate and are therefore depleted in many commercial growing environments. In this study, we analysed the architecture, pigment accumulation and antioxidant capacity of coriander (Coriandrum sativum, also known as cilantro) plants grown with and without supplementary UV-B (1.5 µmol m−2 s−1). We demonstrate that UV-B limits stem elongation responses to neighbour proximity perception (shade avoidance), promoting a more compact plant architecture. In addition, UV-B increased leaf quercetin content and total antioxidant capacity. Arabidopsis thaliana mutants deficient in flavonoid biosynthesis were not impaired in shade avoidance inhibition, suggesting that UV-B-induced flavonoid synthesis is not a component of this response. Our results indicate that UV-B supplementation may provide a method to manipulate the architecture, flavour and nutritional content of potted herbs whilst reducing the deleterious impacts of dense planting on product quality

Integration of light and circadian signals that regulate chloroplast transcription by a nuclear-encoded sigma factor

We investigated the signalling pathways that regulate chloroplast transcription in response to environmental signals. One mechanism controlling plastid transcription involves nuclear‐encoded sigma subunits of plastid‐encoded plastid RNA polymerase. Transcripts encoding the sigma factor SIG5 are regulated by light and the circadian clock. However, the extent to which a chloroplast target of SIG5 is regulated by light‐induced changes in SIG5 expression is unknown. Moreover, the photoreceptor signalling pathways underlying the circadian regulation of chloroplast transcription by SIG5 are unidentified. We monitored the regulation of chloroplast transcription in photoreceptor and sigma factor mutants under controlled light regimes in Arabidopsis thaliana. We established that a chloroplast transcriptional response to light intensity was mediated by SIG5; a chloroplast transcriptional response to the relative proportions of red and far red light was regulated by SIG5 through phytochrome and photosynthetic signals; and the circadian regulation of chloroplast transcription by SIG5 was predominantly dependent on blue light and cryptochrome. Our experiments reveal the extensive integration of signals concerning the light environment by a single sigma factor to regulate chloroplast transcription. This may originate from an evolutionarily ancient mechanism that protects photosynthetic bacteria from high light stress, which subsequently became integrated with higher plant phototransduction networks

Manipulation of growth and development in tobacco through targeted inactivation of the Phytochrome Chromophore

The photo-regulatory activity of the plant photoreceptor phytochrome requires covalent attachment of the plastid-synthesised linear tetrapyrrole, phytochromobilin. Constitutive expression of the mammalian enzyme biliverdin reductase (BVR) in plants inactivates chromophore biosynthesis, resulting in a loss of all phytochromes throughout development. Extensive molecular, biochemical and physiological analyses of tobacco plants {Nicotiana tabacum cv. Maryland Mammoth), expressing BVR targeted to both cytosol and plastid, have been used to investigate the consequences of total phytochrome deficiency on the growth and development of a short-day plant.Constitutive expression of BVR in tobacco resulted in reduced germination, elongated hypocotyls, smaller cotyledons and decreased chlorophyll content in light-grown seedlings. Impaired photomorphogenic responses under continuous far-red and red wavelengths are consistent with deficiencies in both phytochrome A and phytochrome B activities. Mature plants with plastid-targeted BVR displayed elongated intemodes, altered leaf morphology and reduced chlorophyll levels, consonant with deficiencies in multiple phytochromes. Photoperiodic sensitivity was reduced in transgenic plants with all lines flowering early in photoperiods above 10.5 hours. This result is consistent with an inhibitory role for phytochrome in controlling flowering in short-day plants. Cytosol-targeted BVR lines showed an intermediate phenotype between that of plastid-targeted BVR lines and wild-type plants, a result consistent with a partial reduction in photoactive holophytochrome. Targeting of BVR to plastids also resulted in perturbations of the tetrapyrrole pathway and altered plastid morphology. A severe protochlorophyllide deficiency was recorded in dark-grown seedlings, despite increased levels of ALA synthesis. Transcription of Mg-chelatase subunits was unaffected in these plants and feeding studies revealed no post-transcriptional impairment of Mg-chelatase activity. In contrast, a marked inhibition of ALA synthesis was observed in light-grown seedlings, an effect resulting in pronounced cotyledon bleaching. Decreased levels of chlorophyll a/b binding protein {CAB) transcript and reduced granal stacking were also observed. Altered expression patterns of tetrapyrrole synthesis enzymes were recorded in plastid-targeted BVR seedlings grown both in the dark and the light and are consistent with a role for tetrapyrroles in plastid-nuclear signalling

Temperature-regulation of plant architecture

As sessile organisms, plants have evolved great plasticity to adapt to their surrounding environment. Temperature signals regulate the timing of multiple developmental processes and have dramatic effects on plant architecture and biomass. The modulation of plant architecture by temperature is of increasing relevance with regard to crop productivity and global climate change. Unlike many other organisms, the mechanisms through which plants sense changes in ambient temperature remain elusive. Multiple studies have identified crosstalk between ambient temperature sensing, light signaling, cold acclimation and pathogen response pathways. The regulation of plant architecture by temperature appears to involve the complex integration of multiple hormone signaling networks. Gibberellin (GA), Salicylic Acid (SA) and cytokinin have been implicated in the regulation of plant growth during chilling, whilst a predominant role for auxin is observed at high temperatures. This mini-review summarizes current knowledge of plant growth regulation by temperature and crosstalk with other abiotic and biotic stress signaling pathways

Light signals, phytochromes and cross-talk with other environmental cues

Plants have evolved highly complex sensory mechanisms to monitor their surroundings and adapt their growth and development to the prevailing environmental conditions. The integration of information from multiple environmental cues enables the coordination of development with favourable seasonal conditions and, ultimately, determines plant form. Light signals, perceived via the phytochrome, cryptochrome and phototropin photoreceptor families, are especially important environmental signals. Redundancy of function among phytochromes and their interaction with blue light photoreceptors enhance sensitivity to light signals, facilitating the accurate detection of, and response to, environmental ¯uctuations. In this review, current understanding of Arabidopsis phytochrome functions will be summarized, in particular, the interactions among the phytochromes and the integration of light signals with directional and temperature sensing mechanisms. Key words: Arabidopsis, environmental cues, light signals, photoreceptors, temperature sensing mechanisms