Genetica 94: 225-234, 1994. 225 9 1994 KluwerAcademic Publishers. Printed in the Netherlands. Light inducibility and tissue specificity of the R gene family in maize Chiara Tonelli, Silvana Dolfini, Angela Ronchi, Gabriella Consonni & Giuseppe Gavazzi Dipartimento di Genetica e di Biologia dei microrganismi, Via Celoria 26, 20133 Milano, Italia Receivedand accepted15 August1994 Key words: anthocyanin, maize, helix-loop-helix, tissue specificity, paramutation Abstract The red and purple anthocyanin pigments of plants are visible genetic markers and their spatial and temporal accumulation is strictly regulated. Anthocyanin biosynthesis is also modulated by environmental factors such as light and temperature. Thus this process represents an appealing model system for the study of gene regulation, as well as for studying developmental biology. In maize, the pattern of pigmentation of the plant is controlled by the R, Sn and B genes, a small family of HLH transcription factors. Here we report the pattern of light induction and tissue specific expression of the regulatory and structural genes involved in this biosynthesis. Two Sn alleles differing in their light response have been considered and analyzed by Northern and in situ hybridization experiments. An unusual phenomenon of interaction between the homologous R and Sn genes leading to a partial gene silencing is reported. We hypothesize a model in which silencing is achieved through methylation of specific sites. Introduction Most of the red and purple pigments present in high- er plants are anthocyanins. Since the beginning of the century, the anthocyanin pathway has been a favourite object of study by plant geneticists because the mutant phenotypes are viable and the range, of expression is large and easily scorable. As a result, a wealth of information on the structure and function of the genes involved in this pathway has been gathered in the past (reviewed in Dooner, Robbins & Jorgensen, 1991; Martin & Gerats, 1993). Pigmentation within the plant is controlled by numerous genes that affect the type, amount and distribution of pigments syn- thesized. The structural genes encoding the enzymes involved in the biosynthesis are coordinated in their activity by genes that regulate the spatial and tempo- ral deposition of the pigments among the various tis- sues. Anthocyanin biosynthesis is also modulated by environmental stimuli such as light, temperature, phy- topathogens, wounding and nutrient supply. Therefore, this process presents features that make it an appealing model system for the analysis of gene regulation as well as for studies in developmental biology. In maize the structural genes C2, A1, A2, Bzl and Bz2 are coordinately controlled by a number of regu- latory genes (C1, Pl, R, B, Lc and Sn). Genetic and biochemical studies have indicatdd that the level of the biosynthetic enzymes is affected by mutations in the regulatory genes (Dooner & Nelson, 1977; Doon- er, 1983; Gavazzi et aI., 1985b). The genes involved in this pathway have been cloned. Evidence has been obtained that the expression of the structural genes is controlled by the products of the regulatory genes at the transcriptional level. Genetic and molecular analysis indicates that the regulatory genes can be grouped in two families: the R gene family, encoding related proteins with homology to the DNA binding- protein domain of Myc oncoproteins (Chandler et al., 1989; Ludwig et al., 1989; Tonelli et al., 1991; Consonni et al., 1992, 1993), and the C1/Pl family, encoding related proteins with homology to the DNA binding-domain of Myb oncoproteins (Cone, Burr & Burr, 1986; Paz-Ares et al., 1987; Cone et al., 1993). The presence of a functional allele of both of these families is required for the transcriptional activation of the biosynthetic genes in a specific tissue of the plant.