Phytochrome A, phytochrome B and other phytochrome(s) regulate ATHB‐2 gene expression in etiolated and green Arabidopsis plants

ABSTRACT 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.