Brassinosteroids control AtEXPA5 gene expression in Arabidopsis thaliana

To elucidate the spatial and temporal roles of EXPANSIN A5 (AtEXPA5) in growth and development of Arabidopsis thaliana, phenotypic alterations in loss-of-function mutants were observed. Seedlings of the null mutant, expA5-1, had shorter roots and hypocotyls than those of wild-type plants under both light and dark conditions. Compared to wild-type plants, the mutants had smaller rosette leaves. AtEXPA5 was dominantly expressed in aerial parts of A. thaliana, especially in the inflorescence stems and flowers. Expression of AtEXPA5 was enhanced by exogenously applied brassinosteroids. AtEXPA5 expression was reduced in a brassinosteroid-deficient mutant (det2) and a signaling mutant (bri1-301), while it was increased in bzr1-1D, a dominant mutant of a brassinosteroid transcription factor. A double mutant, bzr1-1DXexpA5-1, showed reduced growth compared to the bzr1-1D mutant. In addition, the brassinazole resistance of bzr1-1D was impaired in the double mutant. These findings indicate that AtEXPA5 is a growth-regulating gene whose expression is controlled by brassinosteroid signaling downstream of BZR1 in A. thaliana.