Arabidopsis thaliana has been used for the study of diverse biosynthetic pathways. The feasibility of accessing and manipulating mutants makes Arabidopsis a model plant. Anthocyanin biosynthesis has been a case of study in Arabidopsis. A number of mutants have been characterized for the biosynthetic pathway. Arabidopsis anthocyanin mutants form a group named transparent testa (tt) which present reduced or no pigmentation in the seed coat. Several of these mutants have been identified as being defective in genes involved in anthocyanin synthesis. At least 10 loci have been described for tt mutations. They have been found to be associated with the biosynthetic enzymes (Table 1). A review is at Koorn, 1990.
Table 1- tt mutants in Arabidopsis thaliana
| Locus | Enzyme |
| tt3 | dihydroflavonol-4-reductase (DFR) |
| tt4 | chalcone synthase (CHS) |
| tt5 | chalcone isomerase (CHI) |
| tt6 | flavonol synthase |
| tt7 | flavonoid 3’-hydroxilase (F3H) |
| ttg | R, transcriptional factor? |
| tt 1,2,8,9 | unknown |
The study of mutants has contributed not only to the understanding of structural genes in the anthocyanin pathway but also to the understanding of their regulation. A good example is ttg (transparent testa glabrous) mutant, which lacks anthocyanin and trichomes. Transformation of this mutant with a region of the R gene (a transcription factor that regulates anthocyanin in maize) from maize was enough to rescue the production of both anthocyanin and trichomes (Lloyd et al, 1992). This result suggests that R homologs are probably found in Arabidopsis and they may act as transcriptional factors modulating structural genes similar to those in maize.
Light has long been attributed to regulate anthocyanin production . However, only recently have discoveries advanced our knowledge of the mechanisms involved in this process. A great breakthrough started with the discovery of the blue light photoreceptors, the cryptochromes. Studies with a mutant defective in cryptochrome (cry1, previously hy4) showed reduced production of anthocyanin (Ahmad et al, 1995) indicating that blue light is involved in the regulation of anthocyanin genes in Arabidopsis. In fact, cryptochromes (Blue/UV-A light receptors) seem to interact with phytochromes (red and far-red light receptors) in mediating anthocyanin regulation. Analysis of a combination of single, double, and triple mutants in cryptochrome (cry1) and phytochromes (PhyA, PhB) demonstrated that PhyB and cry1 modulates anthocyanin biosynthesis in a PhyA dependent manner (Neff and Chory, 1998). It is highly possible that similar mechanisms involving both cryptochromes and phytochromes also occur in other plant species.
Very little is known about the signaling transduction events that cause induction of anthocyanin by light. Genes such as COP1 (Constitutive photomorphogenic1) and CIP (COP1 interaction protein) may play important roles in this process, as previously suggested (Yamamoto et al, 1998). These authors identified CIP, to which induction by light results in anthocyanin production. In darkness, COP represses CIP, which in turn is not capable of activating biosynthesis and anthocyanin accumulation (Yamamoto et al, 1998). Phosphorylation and activation of anion channels may also be involved in the light signaling of anthocyanin (Noh and Spalding, 1998). However, their role in the signaling pathway is still obscure.
There is no doubt that light is essential for anthocyanin biosynthesis in above-ground organs. The modulation of anthocyanin in aerial parts is very important due to the vast array of functions attributed to these compounds. Anthocyanins are found in flowers and fruits to attract pollinators. In leaves, their accumulation may be important to protect the photosynthetic apparatus against excessive radiation. Spatial and temporal regulation of anthocyanin may be partially due to transcriptional regulation of the biosynthetic genes. Evidence for transcription regulation was found in blue light studies. Two different units of cis-elements of CHS showed to be modulated by blue light. Unit1 induces expression in flower and seedling and unit2 directs expression in flower but not seedling (Kaiser, 1995; Feinbaum, 1991). More studies are needed in this line to understand how anthocyanins are regulated in different parts of the plant.
Besides light, other abiotic and biotic stresses are also important in modulating anthocyanin production. Low temperature induces accumulation of CHS and PAL transcripts. However no effects on freezing tolerance were observed in tt mutants (Leyva, 1995). The role of anthocyanins in cold acclimation is still unclear.
Anthocyanins may also play a role in defense against pathogens in Arabidopsis. Studies with bacteria revealed the induction of CHS and PAL genes after elicitation with Pseudomonas (Dong, 1991). Additional evidence for the role of anthocyanins in defense was found using gemini virus. Production of anthocyanin was observed in Symptomatic tissues of Arabidopsis infected with geminivirus (Lee et al, 1994). It is possible that anthocyanins form a physical barrier that limits the spread of pathogens. These compounds may interact with pathogen proteins impairing their functioning. Condensed tannins are other flavonoid derivatives known to function this way.
Most of the studies with anthocyanins focus on the above-ground organs. However, underground production of anthocyanins may also occur. The production of these compounds in roots may be related to nutrient stress. Phosphorus deficiency may decrease the inhibitory effects of PO4 on PAL and CHS, stimulating anthocyanin production (Kakegawas et al, 1995). Iron deficiency also stimulates anthocyanin production (McDonough). Salisbury and Ross (1992) pointed out that accumulation of anthocyanin also follows nitrogen and sulphur stress. Besides nutritional stress, a major role of anthocyanins in underground organs may be associated with defense.
[LINK TO ARABIDOPSIS BIOSYNTHETIC PATHWAY]
REFERENCES
Ahmad, M; Lin, C; Cashmore, AR. 1995. Plant Journal, 8: 653-658.
Dong, X. 1991. Plant Cell, 3 (1): 61-72.
Feinbaum, RL. 1991. Molecular and General Genetics, 226 (3): 449-456.
Kaiser, T. 1995. Plant Molecular Biology, 28 (2): 231-243.
Kakegawa, K; Suda, J; Sugiyama, M; Komamine, A. 1995. Physiologia Plantarum, 94: 661-666.
Lee, S; Stenger, DC; Bisaro, DM; Davis, KR. Plant Journal, 6(4): 525-535.
Leyva, A. 1995. Plant Physiology, 108 (1): 39-46.
Lloyd, AM; Walbot, V; Davis, RW. 1992. Science, 258: 1773-1775.
Neff, MM and Chory, J. 1998. Plant Physiology, 118: 27-36.
Noh, B and Spalding, EP. 1998. Plant Physiology, 116: 503-509.
Salisbury, R and Ross, C. 1992. In: Plant Physiology. 4th edition. (editor: Kauser, RM). Wadsworth Publishing Company, Belmont, CA.
Yamamoto, Y; Matsui, M; Ang, L-H; Deng, X-W. 1998. The Plant Cell,10: 1083-1094.
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