New puzzle-piece shows how growth hormones work in plants

Stanford, California. Both plant and animal growth is controlled by steroid hormones–signaling molecules that tell specific genes in cells to begin the physiological process of increasing cell size. Although these molecular managers operate similarly in plants and animals, the chain of events in regulating cellular functions appears to be very different in the two kingdoms. In animals, hormone reception begins in the nucleus of the cell. In plants, a steroid hormone family called brassinosteroids (BRs) start to work on the surface of the cell. A bucket-brigade of activity then wins its way into the cell’s nucleus to activate specific genes that tell the cell to grow. “We found a key component in this complex chain reaction in the cell nucleus that promotes cell growth,” stated co-author Zhi-Yong Wang, of Carnegie’s Department of Plant Biology in Stanford, CA (zywang24@stanford.edu) . The research has important implications for the possibility of understanding how to manipulate the signaling machinery to increase plant growth and yield. The paper is published in the January 27, 2005, Science Express.

As Wang explained: “We’ve known for some time what happens at the cell’s surface, so understanding what is happening in the nucleus is very important for unraveling this mystery of plant growth. We found that in the model plant Arabidopsis, a protein in the cell nucleus called BZR1, which is activated when the BR hormone is present, has a previously unknown segment where molecular binding occurs. Instead of stimulating an activity, the protein binds to a DNA sequence (named brassinosteriod-response element or BRRE), which stops the process of transcription–the transfer of genetic information from the DNA template molecule to messenger RNA–for a gene named CPD. Because the CPD gene orders the production of an enzyme needed for BR synthesis, this suppression stops the production of BR conferring a feedback regulation of BR production. When the BR steroid level is high, BZR1 is activated and BR synthesis is reduced. When the level is low, the synthesis is high. This feedback regulation is critical for maintaining an optimal steroid level for plant growth.

Using sophisticated microarray analyses of mutant Arabidopsis plants, which produce a more active version of BZR1, the scientist also found that in addition to coordinating equilibrium in plants, BZR1 also promotes growth, thus playing a dual role. The researchers further identified other genes controlled by BZR1, and studies of these genes will enhance the understanding of how steroid hormones regulate plant growth.