How Do Plants Control the Shoot:Root Ratio

Both Carbon and Nitrogen are essential in plant development. The carbon is the fuel for cell division, growth and a respiratory substance and the Nitrogen enables amino acid production and DNA synthesis. In turn this growth causes the shoots and roots to extend and consequently there is normally a fixed ratio between how the plants allocate these resources.

The intimate association between these to essential compounds is effected by the way the plant acquires them. The Nitrogen enters the plant through the root in the form of Nitrates and the Carbon enters in the form of CO2 and is converted to sucrose: the photoassimilate. The Nitrates move up through the xylem from the, carried by the capillary action of the water. The sucrose, produced by photosynthesis, is exported into the phloem and is then transported down to the roots to enable sufficient metabolism in the light deprived areas of the plant. In general, the source for the Carbon is the sink for the Nitrogen and visa versa.

Experiments in the early 20th century notices that the ratio of root to shoots was always constant. It was defined by the equation:

S = bRk

S = shoot mass

R = root mass

B = growth coefficient

K = allometric constant

This provided a logarithmic ratio between the two areas of the plant, of which the allometric constant is similar within species. Although this outlines what the root: shoot ratio is, it doesn't give any information on how the plant controls it.

During the 1980s, a German botanist proposed the functional equilibrium hypothesis. The basis of this relies on the interdependence of the two parts of the plant. The root growth is limited by the supply of the photoassimilate from the shoots and the shoot growth is in turn limited by the supply of Nitrogen from the roots.

However, this theory falters on two principles. Firstly, organs such as the fruit are sinks for both the Nitrogen and the Sucrose and secondly, the root shoot ratio changes depending upon external stimuli, indicating that there is a genetic control governing the root: shoot ratio.

The more recent theories describe sucrose as the sensor molecule, which ultimately governs the root: shoot ratio by way of altering the genetic architecture. Although sucrose isn't a classical phytohormone, it plays a similar role as the plant cells detect the concentration of the sucrose at the different ends of the phloem providing a mechanism of communication between the source and the sink. The most important piece to this new theory is that genes have been discovered that are expressed during higher concentrations of sucrose in the external medium such as the rbcs genes.

Through a combination of sensing and gene control, the plant is able to allocate the sufficient resources allometrically. This enables the plant to control its relative growth rate depending upon the environment. In turn, the plant doesn't randomly allocate the resource and can partition it depending upon the immediate requirements, whether it is in response to stress or due to the seasonal need.

Article Source: Mark A T Ramotowski

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