Plant size morphs dramatically as scientists tinker with outer layer

The 'third generation' of biofuels combines genetic impro- vements both on the level of feedstock production as on that of the conversion of biomass into useable fuels. Earlier we reported on how top-researchers have already succeeded in designing a hybrid poplar tree with lower amounts of lignin - the 'glue' that keeps cells together and makes wood so hard -, which makes it easier to break down the cell walls to release the sugars from which fuels can be made (previous post). Public acceptance aside, such transgenic crops, in combination with improved specialty enzymes that break down ligno-cellulosic biomass, are set to transform the future of bioenergy. The same logic holds for the future of the 'bioeconomy', in which virtually all petroleum-based products will be replaced by plant-based alternatives.

Researchers from the Plant Biology Laboratory at the Salk Institute for Biological Studies have now announced another important contribution to the field of fundamental plant biology by discovering which part of a plant both drives and curbs growth. The question has been occupying scientists for over a century. Is the mechanism to be found in a shoot's outer waxy layer? Its inner layer studded with chloroplasts? Or the vascular system that moves nutrients and water?

In the March 8 issue of the journal Nature the scientists provide the answer. They succeeded in making tiny plants big and big plants tiny by controlling growth signals emanating from the plant's outer layer, its epidermis (see picture, click to enlarge).

These findings could eventually be used by agronomists to manipulate plant growth pathways to maximize crop yield, or even reduce leaf size or leaf angle in plants that need to be spaced closely together, says the study's lead author, Joanne Chory, Ph.D., professor and director of the Plant Biology Laboratory and investigator with the Howard Hughes Medical Institute.

Chory and her laboratory team have spent years helping to define how a plant "knows" when to grow and when to stop – which is a "big question in developmental biology," she says. For their experiments, they rely on the model system Arabidopsis thaliana, a small plant related to cabbage and mustard whose genome has been decoded. Over the years, the researchers have built up a whole tool kit, learning how to add and subtract genes in order to determine form and function. Among their discoveries is a class of dwarf plants whose size is about one-tenth the size of a single leaf of the full-sized plant:
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Over the past decade, Chory's laboratory and others have shown that these dwarf plants are defective in making or responding to a steroid hormone called brassinolide. Among the genes identified was the plant steroid receptor, BRI1 ("bry-one") that is activated by the steroid. The dwarfed Arabidopsis doesn't express BRI1 at all, unlike normal Arabidopsis, which expresses BRI1 on both the outer waxy, protective epidermis (covering the whole leaf and shoot), and the inner sub-epidermal layer, which contains the chloroplasts that conduct photosynthesis.

In the current study, first author Sigal Savaldi-Goldstein, Ph.D., a postdoctoral researcher in the Plant Biology Laboratory, and Charles Peto, an electron microscopy specialist in the Laboratory of Neuronal Structure and Function, conducted a series of experiments that addressed an old debated question: what tissues of the leaf drive or restrict growth? The answer was simple: the epidermis is in control.

They found that when they drive the expression of the BRI1 receptor in the epidermis of a dwarf Arabidopsis, while leaving the sub-epidermal layer as it was (without BRI1 receptors), the tiny plant morphed into a full-sized plant. In the second set of experiments, they used an enzyme to break down the steroid hormones in the epidermis, and found that a normal sized plant shrunk into a dwarf. "These are simple experiments, but it took 10 years of work in order for us to be able to ask this question," Chory says.

"A second remarkable finding from the study is that "cells in the outer layer talk to the cells in the inner layers, telling them when to grow or to stop growing. This communication is very important to the life of a plant, which can't move and so must have a coordinated system to respond to a changing environment," explains Savaldi-Goldstein.

More information:
Sigal Savaldi-Goldstein, Charles Peto and Joanne Chory, "The epidermis both drives and restricts plant shoot growth", [*abstract], Nature, 446, 199-202 (8 March 2007).