The jungle telegraph: how plants communicate

How does a soybean plant's leaves communicate with its roots, hidden beneath the ground?

It makes a trunk call. And if you could find the molecule that transmits the message, says Prof Peter Gresshoff, you might be able to redesign the root systems of crop plants to survive drought, or to produce extraordinary yields in fertile, well-watered soil.

In the longer term, it might even be possible to develop crop species that make their own nitrogenous fertiliser, and dispense with the synthetic stuff, which pollutes waterways and groundwater.

Gresshoff, Professor of Botany at the University of Queensland, has dug up a piece of genetic treasure in soybean -- a gene that regulates the production of the plant's nitrogen fixing nodules and lateral roots.

His quest for the gene has its own deep roots. As a young researcher at the Australian National University way back in 1981, Gresshoff and his then-student Bernard Carroll, used a mutagenic chemical to mutate soybean seeds.

Among his mutant plants was a soybean that grew prolific numbers of root nodules -- the small structures in which legumes house and nourish their symbiotic nitrogen-fixing bacteria. The work was published in several key papers in 1985-6.

An attempt to commercalise the supernodulating soybean failed -- it grew many nodules, but too few roots.

While still at ANU in 1986, Gresshoff and colleague Dr Angela Delves experimentally grafted shoots from the supernodulating soybean onto the roots of a normal variety.

The grafted plant supernodulated, confirming that root supernodulation is actually induced, not in the root itself, but by remote control, by some anonymous factor produced in the plant's leaves. Gresshoff's team had produced the first evidence for long-distance signalling in plants.

In 1991, now at the University of Tennessee, Gresshoff published his first molecular mapping [discovery] of the gene that controls supernodulation, called NTS-1. Gresshoff proposed that supernodulation was an alteration of the AON (autoregulation of supernodulation). response in legume plants.

AON was the first gene to be identified as a player in the legume symbiosis by which resident nitrogen-fixing bacteria supply the host plant with 'free' nitrogen fertiliser, in return for a free lunch of energy-rich photosynthesate.

On returning to Australia to work at the University of Queensland, Gresshoff and his former PhD student, Carroll, went hunting for gene that, when inactivated in leaf cells, causes the roots to supernodulate.

Evolution has created a Heath-Robinson machine that allows the infant plant to produce nodules in response to a chemical request from the bacterium for lodgings in its as-yet un-nodulated roots.

A recent paper in Nature described how the plant's root cells react to a secreted chemical signal from the bacterium by sending an as-yet unidentified signalling molecule that Gresshoff calls "Factor Q" to the leaves, to switch off a gene that suppresses branching and nodulation in the legume's root system until the bacterium requests entry.

Gene identified In the October 31 issue of ScienceExpress Gresshoff's team announced it had identified the nodulation-suppressor gene, dubbed GmNARK - G. max Nodulation Auto Regulator Kinase.

It turns out to be closely related to the CLV-1 gene in the geneticist's 'green rat', Arabidopsis. The Arabidopsis gene, directly or indirectly generates a signal that determines whether rapidly growing meristem tissue in shoots will give rise to leaves, or to reproductive tissues -- flowers.

CLV1's signal travels only a few millimetres to the shoot meristem, but the signal generated by the corresponding GmNARK gene travels all the way to the roots.

In Gresshoff's soybeans, GmNARK is mutant -- the signal to suppress nodulation is switched off, so the roots automatically produce nodules.

His team is now searching for the molecule that transmits the 'no nodules' instruction -- he calls the unidentified compound SDI (shoot-derived inhibitor).

Controlling plant architecture

Gresshoff says that identifying the components of the signalling system that controls both nodulation and the production of lateral roots will make it possible to control the architecture of plant roots.

By suppressing lateral root formation, for example, geneticists could develop crop plants with very deep, unbranched tab roots, capable of growing in dry or droughted environments.

At the other extreme, geneticists could develop crops with very extensively branched, shallow roots to achieve maximum yields in environments where water is not limited -- such plants would also be able to exploit soil nutrients more effectively.

In the longer term, says Gresshoff, the blue-sky idea of trying to develop non-legume crop plants that will be able to form their own symbioses with nitrogen-fixing bacteria is now looking a little more down-to-earth.

It will be very difficult for cereals, which are all monocotyledons, with their distinctive internal plumbing systems -- but Gresshoff says dicotyledons like tomatoes and other fruits may be candidates for nitrogen-fixing genetic surgery.

Gresshoff suspects that the long-distance signal between leaves and roots is a peptide and if this is the case, he says, the SDI molecule and the GmNARK gene could become a Rosetta Stone for plant biotechnology.

If it emerged that small peptides are to be the elusive messengers that determine how plants develop, it would become possible to custom-design the development and architecture of crop plants to determine when or how they flower, increase their productivity, or make them easier to harvest.

The UQ research team has applied for funding under the Australian Research Council's Centres of Excellence scheme, so it can set up a project to investigate and exploit the signals that control plant architecture.