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The expression of the Arabidopsis ATHB-2 gene is light-regulated both in seedlings and in adult plants. The gene is expressed at high levels in rapidly elongating etiolated seedlings and is down-regulated by a pulse of red light (R) through the action of a phytochrome other than phytochrome A or B, or by a pulse of far-red light (FR) through the action of phytochrome A. In green plants, the expression of the ATHB-2 gene is rapidly and strongly enhanced by lowering the R:FR ratio perceived by a phytochrome other than A or B. Returning the plant to a high R:FR ratio results in an equally rapid decrease of the ATHB-2 mRNA. Consistently, plants overproducing ATHB-2 show developmental phenotypes characteristic of plants grown in low R:FR: elongated petioles, reduced leaf area, early flowering, and reduced number of rosette leaves. Taken together, the data strongly suggest a direct involvement of ATHB-2 in light-regulated growth phenomena throughout Arabidopsis development. [ABSTRACT FROM AUTHOR]