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OrchideyeTo make complex organisms takes specialized cells.

Animals and flowering plants require specialized cells with distinct abilities in order to accomplish higher order functions – such as vision or flowering.

It’s somewhat like a symphony orchestra. The orchestra integrates different musicians playing different instruments. The characteristic sounds of the instruments can be blended together to achieve higher levels of music.

Much of the excitement regarding human stem cells arises from the fact that these cells are undifferentiated (unspecialized) and can become special cells, such as nerve cells.

Like animal cells, plant cells start out in an undifferentiated state.

The big question is: How do cells know what cell type to become. In other words, what controls cell fate?

One of the holy grails of biological research is finding what indeed determines cell fate.

In life, what matters most is your position.

Your fate, human fate, is often determined by your relative position in society. Likewise, a cell’s fate, in plants at least, may be determined primarily by its position with regard to other plant cells.

For example, cells on the outside of the stem will likely become epidermal cells. Cells on the inside of the stem are more likely to become vascular cells, for example.

But how do plant cells know their relative position?Spiral

One way cells may know their position is by the nature of chemical signals in their vicinity or neighborhood. Such chemical signals may be a plant hormone, such as auxin, or even a combination of several plant hormones. (Sort of like you can tell that you are near a bakery by the smell of fresh baked bread.)

Another way plant cells may determine their relative position to other cells is by the physical stress and strain on the cells.

A cell on the outside of the stem experiences less physical stress and strain, from a mechanical engineering perspective, compared to cells on the inside of the stem. Just as you experience less stress and strain on the edge of a crowd compared to the center of a crowd.

How can physical stress and strain affect a cell’s fate?

The answer to this question involves the cobweb-like structure inside cells called the cytoskeleton. The illustration below left shows the immunofluorescence labeling of cytoskeletal filaments in green. (The cell wall is labeled with red and the nucleus is colored with blue.)

You know that when you gently pull one edge of a spider’s web, the whole web is affected. Likewise, physical stress and strain on a cell will alter the cytoskeleton, albeit in 3D.

CytoskeletonA 2008 report in the journal Science illustrates how physical stress can determine cell fate in developing stems. Using microscopic laser beams researchers were able to selectively remove individual cells from a developing shoot meristem. This selective cellular ablation altered the physical stress/strain on adjacent living cells. Their experiments support the idea that cytoskeletal orientation and cell fate are coupled in developing stems. (Please see Ref. 1 below for more information.)

It’s known that cytoskeletal orientation can affect plant cell wall biosynthesis, and thus the shape and size of plant cells.

How these physical mechanisms are translated into, for example, changes in gene expression, however, is one of the key questions to understanding developmental biology (e.g., see Ref. 2 below).

News Update: In a paper published in the 2 March 2012 issue of Science magazine (see Ref. 3 below), scientists from Switzerland and France provide evidence that “…mechanical signals are not just passive readouts of gene action but feed back on morphogenesis.”

Bottom line: In building a complex organism such as a flowering plant, or even an animal, your fate as a cell may be determined by your neighbors.

References

1. Hamant, O. and J. Traas (2010) “The mechanics behind plant development.” New Phytologist, Vol. 185, pp. 369-385. (Full Text)

2. Kennaway, R., E. Coen, A. Green and A. Bangham. (2011) “Generation of diverse biological forms through combinatorial interactions between tissue polarity and growth.” PLoS Computational Biology, Vol. 7: e1002071. doi:10.1371/journal.pcbi.1002071. (Full Text)

3. Kierzkowski, D., et al. (2012) “Elastic domains regulate growth and organogenesis in the plant shoot apical meristem.” Science, Vol. 335, pp. 1096-1099. (Abstract)

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