Student - December 11, 2008

MECHANICS DETERMINE DIRECTION OF CELL GROWTH

Plant cells generally grow in only one direction. But how do plant cells know the right direction and how do they come to agreement with each other? It’s all a matter of tension: cells stretch themselves across the direction of tension that builds up during the growth process. It is not genetics but mechanics that’s at work here. Wageningen plant cell physicist Professor Bela Mulder draws this conclusion with a note of triumph this week in Science.

‘Triumphantly? No, that’s not really how I meant it’, responds Mulder when asked. ‘But it is the case that a number of important structural processes in plants are of a physical nature, although of course they take place in conjunction with the bio-molecular side.’ In his commentary ‘On Growth and Force’ in Science, Mulder discusses the research presented elsewhere in the journal by Hamant et al. on the role of mechanics in plant growth. According to Mulder, this research shows beautifully how cells are in ‘mechanical dialogue with their surroundings’.

Plant cells derive their strength largely from long threads of cellulose which are deposited in the cell wall, Mulder explains. The direction of the threads determines the direction in which the cell can grow. Microtubules play an important role in this. These dynamic tubular structures form a network in a growing cell that lies perpendicular to the growth direction. The protein complexes that spin the cellulose threads use this network as a sort of railway to guide them to the cell wall. This was observed two years ago in plants.

The most recent research shows convincingly how the growing cells choose the right direction to grow together. Accumulated mechanical tension is the key. ‘Growing cells are a bit like inflated balloons. They are all pushing and pulling each other, and that creates mechanical tension. It has now been shown that the cell grows perpendicular to the direction of the maximum tension.’ This tension cannot be measured, but it can be modelled. ‘The researchers have made mechanical models and calculated how the tension develops. This corresponds with the observed growth direction.’

Mulder is pleased with the results. In his commentary he also refers to the increasing use of model building in biology. ‘Modelling is becoming an essential and integral part of publications in these prestigious journals, which is relatively new in the field of biology.’