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Photo-regulation of anthocyanin synthesis in …

Light is the key stimulus for anthocyanin biosynthesis among numerous other environmental cues such as temperature, nutrient deficiency, water status, wounding and pathogen attack. The production of anthocyanin in young seedlings requires prolonged exposure to visible and near-visible wavelengths of light at a relatively high photon flux, and the extent of the plant response to light is a function of light quality and quantity. High-light conditions trigger the accumulation of anthocyanin in vegetative tissues, which serves as a means to safeguard against the detrimental effects of excess light on the photosynthetic apparatus, which can lead to photo-inhibition. Sugar is a common regulator of a number of genes involved in photosynthesis, carbohydrate metabolism and pathogenesis. It also induces anthocyanin biosynthesis in Arabidopsis seedlings in the form of disaccharide sugars such as sucrose (Suc) and maltose. Plant hormones such as abscisic acid, jasmonic acid, cytokinin and gibberellic acid act in concert with sugar in the presence of light to regulate anthocyanin accumulation in either a positive or negative manner. Thus, light, sugar and hormone signals interact in an intricate signaling network that simultaneously coordinates plant homeostasis and regulates anthocyanin pigmentation. Here, we review recent advances in our understanding of these interactions between light, sugar and ethylene and how they regulate anthocyanin pigmentation in Arabidopsis.

Regulation of anthocyanin and stilbene biosynthesis in Vitis vinifera L.

Carbohydrates are potent inducers of anthocyanin pigmentation in different organs in several plant species through their ability to transcriptionally activate anthocyanin biosynthesis genes. For instance, Suc promotes anthocyanin accumulation in the developing corollas of Petunia hybrida, Vitis vinifera cell suspensions and radish (Raphanus sativus) hypocotyls. In Arabidopsis, anthocyanins are produced in cotyledons or leaves when plants are grown on a sugar-containing medium, as evidenced by purple coloration of the organs. More recently, genetic elements through which Sucspecific induction of anthocyanin biosynthesis is mediated have been identified in Arabidopsis seedlings., Interactions or crosstalk, among hormones and sugars form a complex network of overlapping signals that coordinate overall plant growth and development. Ethylene markedly suppresses anthocyanin accumulation. Several studies have shown that inhibition of ethylene biosynthesis by Co2+ and prevention of ethylene action by silver increases anthocyanin content in maize (Zea may) seedlings. Similarly, in transgenic tobacco (Nicoatiana tabacum) plants expressing a mutated melon (Cucumis melo) ethylene receptor gene, ethylene response 1 (ETR1H69A), higher levels of anthocyanin accumulate in flower petals as compared to control plants. In contrast, the concentration of anthocyanin in constitutive triple response 1 (ctr1) mutants, which exhibit a constitutive response to ethylene, is similar to wild-type (WT) plants in the presence of a high concentration of Suc., Subsequent finding in Arabidopsis confirmed that ethylene is a negative regulator of anthocyanin biosynthesis, and ethylene signaling is mitigated through a triple response pathway.

Regulation of anthocyanin biosynthesis in ..

In Arabidopsis, light signaling is perceived and transduced by photoreceptors, including UV-B photoreceptor, cryptochrome 1 (CRY1) and phytochrome (PHY) A and B. The plant transcription factors HY5 and PIF3 positively regulate anthocyanin biosynthesis through direct binding to the promoters of anthocyanin structural genes, including CHS, CHI, F3H, F3'H, DFR and LDOX. However, negative regulation of pigmentation by ethylene was shown to be independent of HY5. In addition to photoreceptors, photosynthesis also contributes to the formation of anthocyanin. In turnip seedlings and non-chlorophyllous corn leaf, light-dependent anthocyanin accumulation was significantly inhibited by treatment with diuron, 3-(3,4-dicrhlorophenyl)-1, 1-dimethylurea (DCMU), a photosynthetic inhibitor., Consistent with this, in Arabidopsis, DCMU treatment of seedlings suppressed the accumulation of anthocyanin pigmentation in photosynthetically active leaf tissues in wild-type Columbia seedlings (shown in ), as well as in the ethylene-insensitive mutant etr1-1. This suppression of anthocyanin accumulation was shown to be mediated through regulation of the transcription factors of the MYB-bHLH-WD40 (MBW) complex such as PAP1, Gl3 and EGL3. Additionally, there was an inverse relationship between the activity of the MBW complex and the level of MYBL2, which inhibits the formation of active MYB complexes. Ethylene maintains anthocyanin pigmentation in Arabidopsis leaves through the regulation of MYBL2 at the transcriptional level.

Light is the ultimate energy source for photo-autotrophs on earth. For green plants, however, it can also be toxic under certain stressful environmental conditions and at critical developmental stages. Anthocyanins, a class of flavonoids, act as an effective screening mechanism that allows plant survival and proliferation under occasional periods of harmful irradiation through modulation of light absorption. Apart from light-sensing through photoreceptors such as phytochrome and cryptochrome, plants use the photosynthetic electron transfer (PET) chain to integrate light information. The redox status of the plastoquinone (PQ) pool of the PET chain regulates anthocyanin biosynthesis genes, together with the plant hormone ethylene and plant hormone-like sugars. A complex signaling apparatus in acyanic cells appears to transduce information to cyanic cells to regulate anthocyanin production through an intercellular signaling pathway that remains largely uncharacterized. This review will highlight recent advances in this field and their implications for the regulation of anthocyanin pigmentation.

Wei Zhang - Flinders University

A feature of anthocyanin pigmentation during development is the transient nature of accumulation when either environmental or developmental changes render the plant more sensitive to environmental conditions. Prolonged accumulation of anthocyanin occurs only in tissues that do not have a photosynthetic carbon assimilation function and is favorable only under conditions of high light or in an arid habitat. The effects of developmental and environmental factors on anthocyanin pigmentation have been extensively reviewed in reference . Plants have evolved such temporal and spatial regulation systems in part because the accumulation and maintenance of anthocyanin involves an investment of energy that may reduce light capture and eventually carbon assimilation., Hence, the regulation of anthocyanin pigmentation seems to be intimately connected to the universal phenomenon of overall homeostasis, wherein negative reciprocity between pathways ensures that anthocyanin is synthesized and greening is suppressed during early stages of seeding growth, when the seedlings are most susceptible to light stress. Anthocyanin suppression appears to be mediated in part through the desensitization of certain structural genes and/or negative regulation of other interrelated pathways., Thus, negative regulation of anthocyanin accumulation by ethylene might be a mechanism whereby the proper balance between carbon assimilation and anthocyanin accumulation is maintained in target tissues, via the suppression of light- and sugar-induced anthocyanin pigmentation. Such a view is supported by the observation that an increase in the level of ethylene in vivo is accompanied by an increase in sugar and light dosage.

The accumulation of anthocyanin is a cell-autonomous response, meaning that color development is controlled at the level of the individual cell., Cotyledons and leaves are comprised of epidermis, mesophyll and vasculature. Light signals for anthocyanin synthesis seem to originate in photosynthetic cells in response to exogenously supplied sugars and then act at a distance on subepidermal and some vasculature cells, suggesting the presence of intercellular signaling molecules (see for details). These mesophyll-derived signals are as-yet unidentified, but signaling molecules could be generated in response to the plastoquinone (PQ) pool redox state and then transferred to the target cells. A similar phenomenon has been observed in petunia seedlings wherein corolla pigmentation is subject to regulation through light-derived signals originating from the calyx and leaves. However, a role for photosynthetic electron transport in the regulation of corolla pigmentation was called into question by a later finding that DCMU treatment resulted in an insignificant change in corolla pigmentation, despite the fact that the cell layer underneath the epidermis contained green chloroplast. Such a discrepancy could arise from differences in the ability and efficiency of the photosynthetic apparatus of the underlying tissues, as well as the involvement of other organ-specific factors in generating the requisite signal., Photosynthesis signals could be induced via the thylakoid kinase STN7 or hydrogen peroxide and then transmitted to epidermal cells after being converted into an intercellular signal. Alternatively, hormones that are known to either positively or negatively regulate anthocyanin biosynthesis, such as the gibberellins, jasmonic acid and abscisic acid, could be involved. In fact, any intercellular signaling protein that is expressed in the inner mesophyll tissue and secreted to act on acyanic epidermal and vasculate cells could be involved in the induction of anthocyanin accumulation. The mesophyll-derived intercellular signaling protein stomagen has been shown to induce stomata development in Arabidopsis. Regardless of which molecules participate in intercellular signaling, the regulation of anthocyanin content in the epidermis would protect inner photosynthetic cells from photoinhibition.

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Professor Centre for Marine Bioproducts Development

Flower colour is a key component for plant signaling to pollinators and a staggering variety of colour variations are found in nature. Patterning of flower colour, such as pigment spots or stripes, is common and is important in promoting pollination success. Developmentally programmed pigmentation patterns are of interest with respect to the evolution of specialised plant–pollinator associations and as models for dissecting regulatory signaling in plants. This article reviews the occurrence and function of flower colour patterns, as well as the molecular genetics of anthocyanin pigmentation regulation. The transcription factors controlling anthocyanin biosynthesis have been characterised for many species and an ‘MBW’ regulatory complex of R2R3MYB, bHLH and WD-Repeat proteins is of central importance. In particular, R2R3MYBs are key determinants of pigmentation intensity and patterning in plants. Progress is now being made on how environmental or developmental signal pathways may in turn control the production of the MBW components. Furthermore, additional regulatory proteins that interact with the MBW activation complex are being identified, including a range of proteins that repress complex formation or action, either directly or indirectly. This review discusses some of the recent data on the regulatory factors and presents models of how patterns may be determined.

Cellulose Chemistry and Technology

Regulation of enzymes involved in anthocyanin biosynthesis in carrot cell cultures in response to treatment with ultraviolet light and fungal elicitors.

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