It has been believed that a plant hormone, auxin, plays a central role in major growth response during the plant’s development. It has been known that a steroidal plant hormone, Brassinosteroid (BR) acts synergistically with auxin especially in stem elongation, but the molecular mechanisms remained obscure. This work may be the first challenge to uncover molecular interaction of the two hormones.

A very famous synthetic gene, DR5-GUS, has been widely used as a marker for auxin action, in a number of papers including those which have appeared in top-ranking journals. We reported that the gene expression is not specific to auxin action, but the gene expression is also responsive to BR action. Therefore, I believe that the marker gene is important as a tool to study molecular interaction of the two hormones.

Despite numerous physiological studies addressing the interactions between BRs and auxin, little is known about the underlying molecular mechanisms. We studied the expression of Aux/IAA genes in response to treatment with auxin or BR. Exogenous auxin induced these genes quickly and transiently, whereas exogenous BR induced them gradually and continuously. We also found that a fusion of DR5-GUS gene was induced with similar kinetics to those of the native Aux/IAA genes in response to both hormones. These results suggest that the Aux/IAA genes are induced by BR, at least in part, via the activation of the cis-element included in the DR5. Endogenous auxin levels did not increase when seedlings of Arabidopsis wild-type (WT) or the BR-deficient mutant were treated with BR. Furthermore, the levels of Aux/IAA transcripts were lower in the BR-deficient mutant than in the WT, even though endogenous auxin levels were higher in the BR-deficient mutant than in the WT. In conclusion, the lack of evidence for auxin-mediated activation of early auxin-inducible genes in response to BR suggests that the BR and auxin signaling pathways independently activate the transcriptional system of the Aux/IAA and DR5-GUS genes.

I have been interested in the chemical regulation of plant growth and its biological mechanisms. I became a member of Dr. Shigeo Yoshida’s Lab in RIKEN, and met a number of well-known BR researchers as well as with younger post-docs- or pre-docs. The work was achieved as a joint effort of the research team in the Laboratory for Growth Regulation.