Changes in PIN (green) localization during organ formation (left to right) redirects auxin flow.

Friml/Elsevier

Unlike we mammals, plants develop new organs throughout their lifetime. In the face of environmental change, this ability is important for their survival, as plants must “adapt developmentally rather than grow legs and run away,” according to Jirí Friml (Universität Tübingen, Germany). To adapt, plants take advantage of cells that can change their fate. If nutrient content in the soil is favorable, for example, growth of the main root slows, and some of its cells proliferate and differentiate into the multiple cell types that make up lateral roots. At the tip of shoots, groups of plant stem cells called meristems initiate leaves or shoots depending on environmental conditions. Reproductive organs are also formed postembryonically.

Now, Friml, Eva Benková, and colleagues show that initiation of all of these varied organs is due to the same plant hormone. “We're now starting to understand how auxin can do all these things,” says Friml. “You can get a flower or a leaf or a root with the same auxin and the same transport system.”

The authors show that auxin concentrates at sites where these new organs will form. The auxin gradient is established by a family of polarly localized proteins, known as PINs, that are involved in cell-to-cell auxin transport. Auxin is normally transported from the tip of a plant to its roots to maintain the apical–basal growth axis. But the authors see that, during organ formation, the polarity of PIN localization changes to redirect some of the auxin flow. And where new auxin pools accumulate, new organs form. PIN mutants that disturb auxin flow are deficient in organ initiation.

What the totipotent cells become in response to auxin is influenced by the regulatory genes they express before auxin arrives. Thus, the developmental or environmental cues need only change PIN localization to spur whatever growth or adaptive response is needed. ▪

Reference:

Benková, E., et al.
2003
.
Cell.
115
:
591
–602.