Sweet tooth

Like humans, plants, too, have a penchant for sugar

Last Updated: Saturday 04 July 2015

-- according to an article published in the science journal Biologist by Matthieu D Barrett and Steven M Smith of the University of Edinburgh, uk, when plants and microorganisms -- including bacteria and yeast -- come in contact with sugar present in the soil their sugar 'sensing' systems activate the pathways for sugar catabolism.

On the practical level, manipulation of this sugar-sensing system has enormous potential benefits for agriculture. A spoonful of sugar has often been advocated to get a rich yield of vegetables or make flowers bloom to prize-winning conditions. For example, Barrett and Smith point out, tricking a potato into keeping its normal photosynthesis rate even in the presence of huge levels of sugar in the soil (a condition in which sugar production through photosynthesis would normally shut down) could lead to more assimilate going into storage. The result might be more numerous or bigger tubers.

Plants produce 160 billion metric tonnes of carbohydrate every year through photosynthesis. This, the Biologist report states, is roughly equivalent to the sum total of the body mass of all the human beings in the world.

Sugar-sensing capabilities of plants have been under intense scrutiny for the past 10 years. When plants come in contact with glucose, genes present in them and responsible for metabolising carbon sources other than glucose are promptly switched off. This response is usually referred to as 'carbon catabolite repression' or 'glucose repression'. But how do plants manufacture sugar? How do they decide on the quantity of sugar to be produced? What do plants do with the sugar produced?

Firstly, sugar-sensing genes allow various plant organs to react appropriately to the available level of sugar in the soil. The article in the Biologist states that plant genes that respond to sugar can be assigned 'feast' or 'famine' functions in different organs.

'Famine' genes are switched on when there is a scarcity of sugar in the soil and switched off when there is plenty. For example, sugar-sensing genes involved in manufacturing sugar through photosynthesis when there is no sugar in the soil are called 'famine' genes. Other 'famine' genes include ms , which allows the conversion of lipid (fat) into sugar. This makes perfect sense as manufacturing or re-mobilising sugar within its cells despite its presence in the soil only constitutes a waste of energy for the plant.

On the other hand, genes with 'feast' functions switch on when sugar is available in plenty in the soil and include those involved in the synthesis of storage materials such as starch and proteins.

Secondly, during times of abundance -- a 'feast' situation -- plants replenish their sugar reserves for times of 'famine'. This is evident in a potato plant, in which the excess sugar is stored in the tubers in the form of starch.

On the other hand, according to Barrett and Smith, when sugar resources in the soil are scarce, plants utilise their reserved sugar, besides manufacturing sugar via photosynthesis. It is important that plants accumulate sufficient sugar for storage to use during periods of famine. One of the plant functions depending on the presence of sugar is defence. Genes involved in the production of protective pigments in response to ultra-violet light, genes for anti-fungal enzymes, genes responsible for the production of cell wall material to repair structural damage and several others are all activated by sugar. Another important aspect is the quantity of sugar produced in a plant.

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