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Plant growth hormones: Antagonists cooperate

Date:
June 23, 2010
Source:
Max Planck Institute for Developmental Biology
Summary:
The two most important growth hormones of plants, so far considered antagonists, also work synergistically. The activities of auxin and cytokinin, key molecules for plant growth and the formation of organs, such as leaves and buds, are in fact more closely interwoven than previously assumed. Scientists made this surprising discovery in a series of complex experiments using thale cress (Arabidopsis thaliana), a biological reference organism.
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Growth defects caused by the inactivation of ARR7 and ARR15 genes. Left: control plant. Right: plant after inactivation of the two genes. In the bottom part of the illustration, scanning electron micrographs of the corresponding growth zones are shown. At centre is the stem cell zone, where new buds are being formed on the periphery.
Credit: Illustration by Jan Lohmann, Universität Heidelberg

The two most important growth hormones of plants, so far considered antagonists, also work synergistically. The activities of auxin and cytokinin, key molecules for plant growth and the formation of organs, such as leaves and buds, are in fact more closely interwoven than previously assumed.

Scientists from Heidelberg, Tü­bingen (Germany) and Umea (Sweden) made this surprising discovery in a series of complex experiments using thale cress (Arabidopsis thaliana), a biological reference organism.

The research appears in the journal Nature.

All the above-ground parts of a plant -- leaves, buds, stems and seeds -- ultimately arise from a small tissue at the shoot tip, which contains totipotent stem cells. Since plant stem cells remain active over the entire life of the organism, plants, unlike animals, are able to grow and develop new organs over many decades. On the periphery of the tip, auxin triggers cells to leave the pool of stem cells, differentiate and form organs like leaves and buds. Cytokinin stimulates stem cells to divide and proliferate; it maintains the number of cells and thus the plant's growth potential.

Some of the genetic factors involved in cytokinin's effect on plant growth were already known. In the thale cress experiments, which concentrated on the growth zone at the tip of the shoot, stem cell researcher Jan Lohmann and his team now studied the role of auxin in the interplay of the two hormones. It turns out that auxin directly interferes with a feedback loop involving two genes activated by cytokinin -- ARR7 and ARR15 -- which limit the effect of cytokinin. Auxin suppresses these two genes, thereby boosting the effect of cytokinin.

"Auxin acts to support the pool of stem cells," says Jan Lohmann. "When it triggers cells at the periphery of the growth zone to form organs, it still needs to ensure that enough stem cells are supplied." This keeps the number of stem cells from falling below a critical minimum, which is key for plant growth and survival. "We're gradually beginning to understand how hormonal and genetic factors are interwoven to maintain the activity of the growth zone. We now know that hormones and genes interact in multiple ways, each one affecting the other. There are no solitary factors."


Story Source:

The above story is based on materials provided by Max Planck Institute for Developmental Biology. Note: Materials may be edited for content and length.


Journal Reference:

  1. Zhong Zhao, Stig U. Andersen, Karin Ljung, Karel Dolezal, Andrej Miotk, Sebastian J. Schultheiss, Jan U. Lohmann. Hormonal control of the shoot stem-cell niche. Nature, 2010; 465 (7301): 1089 DOI: 10.1038/nature09126

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Max Planck Institute for Developmental Biology. "Plant growth hormones: Antagonists cooperate." ScienceDaily. ScienceDaily, 23 June 2010. <www.sciencedaily.com/releases/2010/06/100623140913.htm>.
Max Planck Institute for Developmental Biology. (2010, June 23). Plant growth hormones: Antagonists cooperate. ScienceDaily. Retrieved May 24, 2015 from www.sciencedaily.com/releases/2010/06/100623140913.htm
Max Planck Institute for Developmental Biology. "Plant growth hormones: Antagonists cooperate." ScienceDaily. www.sciencedaily.com/releases/2010/06/100623140913.htm (accessed May 24, 2015).

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