Researchers from the Salk Institute in La Jolla, CA have been studying the power struggle between the master genetic switches within plant cells that determine the orientation for growth - that is which end will put down roots, and which end will grow the shoot.

"In what is arguably the most important decision for a plant, setting up the root/shoot axis, occurs during the early embryonic stages," says the study's lead author Jeffrey A. Long, Ph.D., an assistant professor in the Plant Molecular and Cellular Biology Laboratory. "A tightly controlled balancing act between two groups of transcriptions factors ensures that they stay where they belong and don't get into each other's way."

During plant embryogenesis two stem cell populations form - the shoot meristem, which forms all the 'above ground' organs (leaves, stems, flowers); and the root meristem which gives rise to below ground root system.

"Since plant stem cells ultimately give rise to all edible parts of plants understanding how their fate and function are regulated can be directly applied to modify the architecture of plants and to increase the yields of agriculturally important crops," says Long.

The Salk researchers' findings are published in the Feb. 28, 2010 advance online edition of the journal Nature.

In this study, Zachery R. Smith, a graduate student in Long's laboratory, was investigating why plants with a defective TOPLESS gene were developing a second root system, instead of a stem with leaves. It was discovered that the TOPLESS gene codes for a repressor protein - that turns off two genes that can cause root development in the shoot area of the plant known as PLETHORA 1 and 2.

After figuring out the below ground hierarchy, Smith searched through tens of thousands of mutant plants looking for the master regulator of shoot development - until he hit on a member of the CLASS III HD-ZIP transcription factors, known as PHABULOSA, that fit the bill.

When the Salk researchers forcefully expressed members of the CLASS III HD-ZIP family in the traditional territory of the PLETHORA duo, it transformed the root into a shoot, resulting in a seedling with leaves on both ends. "Although it had been known that HD-ZIPs are involved in many aspects of plant polarity nobody had ever shown that they can transform a root pole into a shoot pole," says Long. "This and other experiments showed that HD-ZIP III genes are master regulators of apical fate in early embryogenesis."

Additional studies by the group revealed the antagonistic relationship between the PLETHORA and HD-ZIP III genes. Both gene sets are under multiple modes of regulation that ensures the proper spatial distribution and apical-basal patterning necessary for proper plant development.

Dr. Jeffrey A. Long's faculty page: http://www.salk.edu/faculty/long.html

Discovered via Eureka Alert: http://www.eurekalert.org/pub_releases/2010-02/si-ros022310.php