— Anthocyanins are water-soluble pigments found in all plant tissues throughout the plant kingdom. Our understanding of anthocyanin biosynthesis and its molecular control has greatly improved in the last decade. The adaptive advantages of anthocyanins, especially in non-reproductive tissues, is much less clear. Anthocyanins often appear transiently at specific developmental stages and may be induced by a number of environmental factors including visible and UVB radiation, cold temperatures and water stress. The subsequent production and localization of anthocyanins in root, stem and especially leaf tissues may allow the plant to develop resistance to a number of environmental stresses. This article reviews the environmental induction of anthocyanins and their proposed importance in ameliorating environmental stresses induced by visible and UVB radiation, drought and cold temperatures.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1751-1097.1999.tb01944.x

Environmental Significance of Anthocyanins in Plant Stress Responses

MicroRNAs (miRNAs) are a class of regulatory RNAs of ∼21 nucleotides that posttranscriptionally regulate gene expression by directing mRNA cleavage or translational inhibition. Increasing evidence points to a potential role of miRNAs in diverse physiological processes. miR398 targets two closely related Cu/Zn superoxide dismutases (cytosolic CSD1 and chloroplastic CSD2) that can detoxify superoxide radicals. CSD1 and CSD2 transcripts are induced in response to oxidative stress, but the regulatory mechanism of the induction is unknown. Here, we show that miR398 expression is downregulated transcriptionally by oxidative stresses, and this downregulation is important for posttranscriptional CSD1 and CSD2 mRNA accumulation and oxidative stress tolerance. We also provide evidence for an important role of miR398 in specifying the spatial and temporal expression patterns of CSD1 and CSD2 mRNAs. Our results suggest that CSD1 and CSD2 expression is fine-tuned by miR398-directed mRNA cleavage. Additionally, we show that transgenic Arabidopsis thaliana plants overexpressing a miR398-resistant form of CSD2 accumulate more CSD2 mRNA than plants overexpressing a regular CSD2 and are consequently much more tolerant to high light, heavy metals, and other oxidative stresses. Thus, relieving miR398-guided suppression of CSD2 in transgenic plants is an effective new approach to improving plant productivity under oxidative stress conditions.

/pdf/posttranscriptional-induction-of-two-cu-zn-superoxide-2opg49qip8.pdf

Posttranscriptional Induction of Two Cu/Zn Superoxide Dismutase Genes in Arabidopsis Is Mediated by Downregulation of miR398 and Important for Oxidative Stress Tolerance

The phytotoxin coronatine and the plant growth regulator methyl jasmonate (MeJA) caused similar growth-inhibitory effects on Arabidopsis seedlings. To test whether these two compounds have similar action, 14 independent coi1 (coronatine-insensitive) mutants of Arabidopsis were selected. The mutants segregated as single recessive Mendelian markers, and all were alleles at the coi1 locus. All coi1 mutants were also insensitive to MeJA and were male sterile. Both coronatine and MeJA inhibited root growth, stimulated anthocyanin accumulation, and increased the level of two proteins of ~31 and ~29 kD detected in SDS-polyacrylamide gels of wild-type Arabidopsis but caused none of these effects in the coi1 mutant. Coronatine and MeJA also induced the systemic appearance of proteinase inhibitor activity in tomato. The male-sterile flowers of the coi1 mutant produced abnormal pollen and had reduced level of an ~31-kD protein, which was abundant in the wild-type flowers. A coronatine-producing strain of Pseudomonas syringae grew in leaves of wild-type Arabidopsis to a population more than 100 times greater than it reached in the coi1 mutant. We conclude that coronatine mimics the action of MeJA and that coi1 controls a step in MeJA perception/response and in flower development.

/pdf/arabidopsis-mutants-selected-for-resistance-to-the-2wwgghtwl7.pdf

Arabidopsis Mutants Selected for Resistance to the Phytotoxin Coronatine Are Male Sterile, Insensitive to Methyl Jasmonate, and Resistant to a Bacterial Pathogen.

Phytochrome is a plant regulatory photoreceptor that mediates red light effects on a wide variety of physiological and molecular responses. DNA blot analysis indicates that the Arabidopsis thaliana genome contains four to five phytochrome-related gene sequences. We have isolated and sequenced cDNA clones corresponding to three of these genes and have deduced the amino acid sequence of the full-length polypeptide encoded in each case. One of these proteins (phyA) shows 65-80% amino acid sequence identity with the major, etiolated-tissue phytochrome apoproteins described previously in other plant species. The other two polypeptides (phyB and phyC) are unique in that they have low sequence identity (approximately 50%) with each other, with phyA, and with all previously described phytochromes. The phyA, phyB, and phyC proteins are of similar molecular mass, have related hydropathic profiles, and contain a conserved chromophore attachment region. However, the sequence comparison data indicate that the three phy genes diverged early in plant evolution, well before the divergence of the two major groups of angiosperms, the monocots and dicots. The steady-state level of the phyA transcript is high in dark-grown A. thaliana seedlings and is down-regulated by light. In contrast, the phyB and phyC transcripts are present at lower levels and are not strongly light-regulated. These findings indicate that the red/far light-responsive phytochrome photoreceptor system in A. thaliana, and perhaps in all higher plants, consists of a family of chromoproteins that are heterogeneous in structure and regulation.

/pdf/novel-phytochrome-sequences-in-arabidopsis-thaliana-1w8da8ma5w.pdf

Novel phytochrome sequences in Arabidopsis thaliana: structure, evolution, and differential expression of a plant regulatory photoreceptor family

This study explored reflectance indices sampled with a 'leaf reflectometer' as measures of pigment content for leaves of contrasting light history, developmental stage and functional type (herbaceous annual versus sclerophyllous evergreen). We employed three reflectance indices: a modified normalized difference vegetation index (NDVI), an index of chlorophyll content; the red/green reflectance ratio (R RED :R GREEN ), an index of anthocyanin content; and the change in photochemical reflectance index upon dark-light conversions (APRI), an index of xanthophyll cycle pigment activity. In Helianthus annuus (sunflower), xanthophyll cycle pigment amounts were linearly related to growth light environment; leaves in full sun contained approximately twice the amount of xanthophyll cycle pigments as leaves in deep shade, and at midday a larger proportion of these pigments were in the photoprotective, de-epoxidized forms relative to shade leaves. Reflectance indices also revealed contrasting patterns of pigment development in leaves of contrasting structural types (annual versus evergreen). In H. annuus sun leaves, there was a remarkably rapid increase in amounts of both chlorophyll and xanthophyll cycle pigments along a leaf developmental sequence. This pattern contrasted with that of Quercus agrifolia (coast live oak, a sclerophyllous evergreen), which exhibited a gradual development of both chlorophyll and xanthophyll cycle pigments along with a pronounced peak of anthocyanin pigment content in newly expanding leaves. These temporal patterns of pigment development in Q. agrifolia leaves suggest that anthocyanins and xanthophyll cycle pigments serve complementary photoprotective roles during early leaf development. The results illustrate the use of reflectance indices for distinguishing divergent patterns of pigment activity in leaves of contrasting light history and functional type.

https://nph.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1469-8137.1999.00424.x

Assessing leaf pigment content and activity with a reflectometer

Temperature affects the total amount, the time course, and the red/far-red effectiveness ratio of light-dependent anthocyanin production in Brassica oleracea L. seedlings. Some of the effects of temperature on anthocyanin production in cabbage are in agreement with the predictions of a model proposed by JK Wall and CB Johnson (1983 Planta 159: 387-397) for the effects of temperature on the state of phytochrome and on the expression of phytochrome-mediated high irradiance responses, but others are not. The lack of a complete agreement between experimental results and model predictions might be due to factors related to the experimental system used or to limitations of the model or both.

https://www.jstor.org/stable/pdfplus/4270070.pdf

Light, temperature, and anthocyanin production.

Plants possess the capability of adapting their patterns of growth and development to changes in the light conditions of the environment. The action of light on plant growth and development (photomorphogenesis) is mediated by specific photoreceptors. Light-induced changes in the state of the photo-receptors provide the signal for the induction/modulation of plant responses to light. A major plant photomorphogenic photoreceptor is phy tochrome, a biliprotein pigment (Chapter 4.1) which is reversibly interconverted by light between an inactive form, Pr, and a physiologically active one, Pfr, with peaks of absorption in the red (R) and far-red (FR) regions of the spectrum, respectively.

The physiology of phytochrome action

Streptomycin enhances the synthesis of anthocyanins and inhibits the synthesis of chlorophylls and the development of chloroplasts in dark-grown seedlings of cabbage (Brassica oleracea), mustard (Sinapis alba), tomato (Lycopersicon esculentum), and turnip (Brassica rapa) exposed to prolonged periods of irradiation in various spectral regions. These results suggest that the contribution of photosynthesis to light-dependent high irradiance reaction anthocyanin synthesis in seedlings of cabbage, mustard, tomato, and turnip is minimal, if any at all. So far, phytochrome is the only photoreceptor whose action in the control of light-dependent anthocyanin synthesis in seedlings of cabbage, mustard, tomato, and turnip has been satisfactorily demonstrated.

Photocontrol of Anthocyanin Synthesis: III. The Action of Streptomycin on the Synthesis of Chlorophyll and Anthocyanin.

A comparative study of the spectral sensitivity of anthocyanin production in dark-grown and light-pretreated systems was carried out in Brassica oleracea L., Lycopersicon esculentum Mill., Secale cereale L. and Spirodela polyrrhiza L. Light pretreatments bring about an enhancement of the inductive, red-far red reversible response in all systems, a decrease of the continuous irradiation response in cabbage, rye, and tomato seedlings, and an enhancement of the continuous irradiation response in cabbage leaf disks. Light pretreatments also bring about a marked change in the spectral sensitivity of the continuous irradiation response. The different effect of light pretreatments on the photosensitivity of the response to short and long wavelength irradiations suggests that two photoreceptors, phytochrome and cryptochrome, may be involved in the photoregulation of anthocyanin production.

Photoregulation of Anthocyanin Synthesis VIII. Effect of Light Pretreatments

Plants growing in the natural environment are exposed daily to prolonged periods of high intensity irradiation. Many plant photomorphogenic responses are fully expressed only under prolonged exposures to high irradiances of light. The intensive study of these responses, the “High Irradiance Responses” (HIR) of plant photomorphogenesis, which started about 20 years ago, has been essentially directed—so far—toward the identification of the HIR photoreceptor, or photoreceptors, a problem that has not been satisfactorily and definitively solved, as yet. There is a great deal of evidence in support of the hypothesis that phytochrome, the pigment mediating the redfar red reversible plant photo-responses to low fluences of light, is involved in the photocontrol of the HIR. It seems likely that phytochrome may be the only photomorphogenic receptor responsible for the photocontrol of HIR responses brought about by irradiation at wavelengths longer than 600 nm. Phytochrome is probably also involved in the photocontrol of the HIR effects brought about by irradiation in the 350 to 500 nm region of the spectrum, but it cannot be excluded that other photochemical systems may also be involved. From a theoretical point of view, it does not seem unreasonable that the final expression of an HIR response may involve an interaction between phytochrome and other photochemical systems, with phytochrome probably playing the primary role and being responsible for the control of the activity of the other systems. Numerous “phytochrome only” interpretations (models) of the HIR have been proposed. Some of them have been developed to a fairly high degree of elaboration and have allowed the prediction of at least some of the features of the HIR. These “models,” although not rigorously and completely tested yet, seem to provide a reasonable interpretation for the HIR effects displayed under prolonged far red irradiation and for those HIR responses for which far red is the most effective spectral region. However, they do not provide a satisfactory explanation for the HIR responses for which blue is the most or the only effective spectral region, nor for the high effectiveness of white light. But, in spite of these problems, the “phytochrome only” interpretations of the HIR can be considered more satisfactory than those based on an interaction between phytochrome and other photochemical systems, especially in relation to the fact that the identity of these other photochemical systems has not been defined yet.

Alberto L. Mancinelli

Papers

Light, temperature, and anthocyanin production.

The physiology of phytochrome action

Photocontrol of Anthocyanin Synthesis: III. The Action of Streptomycin on the Synthesis of Chlorophyll and Anthocyanin.

Photoregulation of Anthocyanin Synthesis VIII. Effect of Light Pretreatments

The “High Irradiance Responses” of plant photomorphogenesis