Maize

By: Jae Hak Kim

Enzymes involved in the anthocyanin production in maize are (starting from naringenin):

- flavanone 3-hydroxylase (encoded by F3H)

- flavonoid 3'-hydroxylase (PR)

- dihydroflavonol 4-reductase (A1)

- dioxygenase? (A2)

- UDP glucose-flavonol glucosyltransferase (Bz1)

- glutathione S-transferase (Bz2)

The enzymological roles of these enzymes are known only for A1 and Bz1 (Christie et al, 1994), and A2 may be involved in more than two steps to produce anthocyanindins from leucoanthocyanidins (cf. Fig.10.14 in Dey and Harborne, 1997).

In maize two types of anthocyanins, cyanidin glucosides and pelargonidin glucosides, are found. However, in C1/R-expressing BMS (Blace Mexican Sweet) cells in culture, only cyanidin glucosides are found (Grotewold et al, 1998).

Naringenin, a product from tetrahydroxychalcone by chalcone isomerase, is a branching point leading to many different types of secondary chemicals such as glucosyl flavones, phlobaphenes, 3-deoxy anthocyanins as well as two types of anthocyanins, pelargonidin and cyanindin.

Genetic regulation of anthocyanin biosynthesis pathway in corn

Anthocyanins are purple flavonoid pigments that are synthesized in the aleurone layer of the maize endosperm, in the embryo, and in many vegetative plant organs, including leaf, stem, anthers, glumes of the cob, tassel, and coleoptiles.

The synthesis of anthocyanin pigments in maize plant tissues or seeds requires the products of numerous genes, both structural and regulatory. Among these are four regulatory genes, R, B, C1 and Pl which are specific for anthocyanin synthesis. These regulatory genes have been grouped into two families, R/B and C1/Pl based upon the sequence homology and function (Lesnick and Chandler, 1998; Chandler et al, 1989).

Pigment production in any particular part of the plant requires the interaction of a member of the R/B family and a member of the C1/Pl family. For example, anthocyanin synthesis in the body of the plant requires the B & Pl genes, whereas in the aleurone layer of the endosperm the seed-specific R-S gene and the C1 gene are required (Cocciolone and Cone, 1993). In kernels, no transcripts from Bz1, C2 or A1 are detected in the absence of a functional C1 or R allele (Ludwig et al, 1989).

Four loci, R, B, C1, and Pl, coordinately regulate the activities of at least three of the structural genes in the pathway: C2 encoding chalcone synthase; A1 encoding dehydroflavonol 4-reductase: and Bz1 encoding UDP glucose-flavonol glucosyltransferase (See the introduction in Taylor and Briggs, 1990).

Environmental stresses have an impact on the production of anthocyanins in maize. For example, transcripts accumulated for both the regulatory (R and C1) and structural (A1, A2, Bz1, and Bz2) genes of the anthocyanin pathway by low-temperature stress (Christie et al, 1994). Another example is that the anthocyanin structural genes of maize seedlings are differentially regulated in response to light (Taylor and Briggs, 1990). They observed that light-mediated anthocyanin biosynthesis is regulated solely by R and the contribution of B and Pl are negligible in young seedlings.

[LINK TO CORN BIOSYNTHETIC PATHWAY]

[CONCLUSIONS]

REFERENCES

Chandler, V. L., Radicella. J. P., Robbins, T. P., Chen, J., and Turks, D. 1989. Two regulatory genes of the maize anthocyanin pathway are homologous: isolation of B utilizing R genomic sequences. Plant Cell 1: 1175-1183.

Christie, P. J., Alfenito, M. R., and Walbot, V. 1994. Impact of low-temperature stress on general phenylpropanoid and anthocyanin pathways: Enhancememt of transcript abundance and anthocyanin pigmentation in maize seedlings. Planta 194: 541-549.

Cocciolone, S. M. and Cone, K. C. 1993. Pl-Bh, an anthocyanin regulatory gene of maize that leads variegated pigmentation. Genetics 135: 575-588.

Dey, P. M. and Harborne, J. B. 1997. Plant Biochemistry. Academic press.

Grotewold, E., Chamberlin, M., Snook, M., Siame, B., Butler, L., Swenson, J., Maddock, S., St. Clair, G., and Bowen, B. 1998. Engineering secondary metabolism in maize cells by ectopic expression of transcription factors. Plant Cell 10: 721-740.

Lesnick, M. L. and Chandler, V. L. 1998. Activation of the maize anthocyanin gene a2 is mediated by an element conserved in many anthocyanin promoters. Plant Physiol. 117: 437-445.

Ludwig, S. R., Habera, L. F., Dellaporta, S. L., and Wessler, S. R. 1989. Lc, a member of the maize R gene family responsible for tissue-specific anthocyanin production, encodes a protein similar to transcriptional activators and contains the myc-homology region. Proc. Natl. Acad. Sci. USA 86:7092-7096.

Taylor, L. P. and Briggs, W. R. 1990. Genetic regulation and photocontrol of anthocyanin accumulation in maize seedlings. Plant Cell 2:115-127.