title: Twilight-zone and canopy shade induction of the Athb-2 homeobox gene in green plants.
abstract: We present evidence that a novel phytochrome (other than phytochromes A and B, PHYA and PHYB) operative in green plants regulates the "twilight-inducible" expression of a plant homeobox gene (Athb-2). Light regulation of the Athb-2 gene is unique in that it is not induced by red (R)-rich daylight or by the light-dark transition but is instead induced by changes in the ratio of R to far-red (FR) light. These changes, which normally occur at dawn and dusk (end-of-day FR), also occur during the daytime under the canopy (shade avoidance). By using pure light sources and phyA/phyB null mutants, we demonstrated that the induction of Athb-2 by changes in the R/FR ratio is mediated for the most part by a novel phytochrome operative in green plants. Furthermore, PHYB plays a negative role in repressing the accumulation of Athb-2 mRNA in the dark and a minor role in the FR response. The strict correlation of Athb-2 expression with FR-induced growth phenomena suggests a role for the Athb-2 gene in mediating cell elongation. This interpretation is supported by the finding that the Athb-2 gene is expressed at high levels in rapidly elongating etiolated seedlings. Furthermore, as either R or FR light inhibits cell elongation in etiolated tissues, they also down-regulate the expression of Athb-2 mRNA. Thus, these data support the notion that changes in light quality perceived by a novel phytochrome regulate plant development through the action of the Athb-2 homeobox gene.
authors.affiliation: Centro di studio per gli Acide Nucleici, Dipartimento e Biologia Molecolare, Università di Roma La Sapienza P.le Aldo Moro 5, Rome, Italy.
authors.forename: M
authors.initials: M
authors.lastname: Carabelli
authors.forename: G
authors.initials: G
authors.lastname: Morelli
authors.forename: G
authors.initials: G
authors.lastname: Whitelam
authors.forename: I
authors.initials: I
authors.lastname: Ruberti