Overexpression of Brassica rapa NGATHA1 Gene Confers De-Etiolation Phenotype and Cytokinin Resistance on Arabidopsis thaliana

Brassica rapa NGATHA1 (BrNGA1) encodes a B3-type transcription factor. By analyzing Arabidopsis overexpressors of BrNGA1 (BrNGA1ox), we have previously demonstrated that BrNGA1 may be involved in negative regulation of cell proliferation during lateral organ and root growth. In the present study, we have found that BrNGA1ox seedlings grown in the dark display de-etiolation phenotypes, such as short hypocotyls, open and elongated cotyledons, and developing true leaves. BrNGA1ox seedlings as well as adult plants and calli are also resistant specifically to exogenous cytokinins. These data raise the possibility that the de-etiolation phenotypes of BrNGA1ox seedlings may result from an alteration in cytokinin response. We set out to test whether the de-etiolation phenotype is due to cytokinin overproduction or constitutively activated cytokinin response. First, BrNGA1ox was crossed to the CKX2ox plant, an overexpression line of CYTOKIN OXIDASE 2, which is responsible for degradation of active cytokinins. We found, however, no difference in the de-etiolation and shoot growth phenotypes between BrNGA1ox and BrNGA1ox CKX2ox plants. Next, we measured the transcripts level of ARR5 and ARR7, frequently employed as molecular markers for cytokinin signaling and yet found no difference in their transcripts levels of the wild-type and BrNGA1ox seedlings and shoots. These data indicate that biological role of BrNGA1 involved in de-etiolation seems to be associated with neither cytokinin overproduction nor its altered signaling. Possible molecular mechanisms by which BrNGA1 may interfere with cytokinin responses and etiolation are discussed.