How plants counteract against larger neighbours’ shade

Researchers have discovered how plants that ‘lose the battle’ during competitiveness for light because they are shaded by larger neighbours, counteract.

Previously, it was thought that these plants adapted to such a condition by rapid shoot elongation and stretching their leaves towards the sun.

However, scientists could not clarify till date the molecular basis of this so-called shade avoidance syndrome.

Researchers from the Utrecht University in the Netherlands and the Ruhr University in Bochum discovered that a specific transport protein (PIN3) enabled the accumulation of the plant hormone auxin, which plays an important role during adaptation process, in the outer cell layers of the plants, thus enhancing the growth process.

Plants often grow in very complex ecosystems, implying that they are in danger of being overgrown and thus shaded by adjacent larger neighbours.

Plants have a number of adjustment mechanisms enabling them to register competing neighbours and enhance their competitive reaction.

This ensures flexible reaction. Permanent perception of the light intensity and quality is imperative for this process.

Plant hormone specialist professor Stephan Pollmann said that chlorophyll, the photosynthetic pigment in leaves, absorbs almost all shades of blue and far red, only allowing dark red light to pass through the leaves.

There is a significant change in the red to far-red ratio if a plant is shaded by foliage. If the light receptors in the plants register this change, they initiate a number of adjustment mechanisms in their growth and development programme.

Taken together these constitute the so-called shadow avoidance syndrome. They enhance the growth of shoots and the upward movement of the leaves (i.e. the hyponastic response).

Vascular plants produce an entire series of different small signalling molecules, so-called phytohormones, which regulate growth and differentiation processes.

Auxins have an extremely wide spectrum of activity, and are particularly important. They play a decisive role in almost all plant growth processes, including the shade avoidance reaction.

Till date, the underlying mechanism was however not fully comprehended.

Pollmann said that it had been known that the effect of auxin is based on an interaction of auxin formation, transportation and signal transduction.

A low red to far-red ratio influenced all these processes, but the exact mechanisms were not understood.

A group of research scientists working under the auspices of the ecophysiologist Dr Ronald Pierik at the Utrecht University has now managed to shed light on the matter and further clarify the growth processes in the shoots during the shade avoidance syndrome.

They made an interesting observation, that shoot growth during a low red to far-red ratio is subject to an intact auxin perception mechanism and is dependent on the accumulation of auxin in the shoot.

The auxin transport protein PIN-FORMED 3 (PIN3) is primarily responsible for this accumulation.

The formation of PIN3 is enhanced when the ratio between red to far red is low. It primarily accumulates in the lateral endodermal cell walls.

This distribution of PIN3 leads to an auxin flow towards the epidermal cell layers, which are responsible for the elongation growth of the shoot.

Using state-of-the art mass spectrometry, Pollmann succeeded in quantifying and comparing the auxin content in wild-type and genetically created PIN3 mutants, which are not capable of producing the transport protein.

The shade avoidance syndrome was not present in the genetically altered plants without PIN3.

Pollmann summarized that it is thus possible to deduce the important role of PIN3-controlled auxin accumulation during the shade avoidance reaction.

The study has been published in the current edition of the Proceedings of the National Academy of Science.