While investigating why a defective TOPLESS gene messes with a plant's basic architecturemutant embryos develop into a seedling topped with a second root instead of a stem with leavesLong and his team discovered functional TOPLESS codes for a repressor protein that inactivates genes that otherwise would cause root development in the shoot area of the plant.
In the current study Zachery R. Smith, a graduate student in Long's laboratory, discovered that these fate-transforming genes are actually two familiar characters: the genes PLETHORA 1 and 2 had been known to act as master regulators that determine the identity of the root meristem.
"Without TOPLESS to keep them turned off, however, these two transcription factors are free to impose their will on the top half of the plant embryo causing the development of a second root instead of a shoot," explains Smith.
With the "below-ground" hierarchy worked out, the question of how the identity of the shoot meristem is determined was still unanswered. Trying to unearth the missing master regulators of shoot development, Smith searched trough tens of thousands of mutant plants, till 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."
Further studies revealed an antagonistic relationship between the PLETHORA and HD-ZIP III genes, both of which are under multiple modes of regulation that
|Contact: Gina Kirchweger|