Generally, plants adjust their growth to fit in with their local environment, light being easily the most important factor - after all, they need light to live. These responses are not controlled by photosynthesis itself, however, but by specialized photoreceptor proteins including phytochrome and phototropin which respond to red and blue light, respectively.
Whereas phototropin somehow steers growth direction in higher plants, phytochrome controls most aspects of plant development including germination, stem extension, greening and even flowering. Not surprisingly, it has been found that very many plant genes are regulated by phytochrome, and indeed most textbooks say that phytochrome works exclusively in this manner.
That cannot be true, however, because some phytochrome effects occur almost instantaneously whereas it takes at least 10 minutes to activate a gene and produce a protein. Moreover, in lower plants it is phytochrome that steers growth direction – that simply can't be done via gene regulation.
However, writing in the prestigious Proceedings of the National Academy of Sciences USA, Katharina Jaedicke and colleagues from the Institute for Plant Physiology of the Justus Liebig University in Giessen now report that phytochrome binds to phototropin at the cell membrane in both lower and higher plants. In moss filaments, the phytochrome probably uses the phototropin to steer growth towards the light source because no reaction occurs if the phototropin is missing. The association of phytochrome with phototropin in higher plants probably has a different function in relation to direction sensing, however.The discovery is particularly remarkable because elegant experiments with polarized light carried out in the 1960's predicted that phytochrome was attached to the cell membrane – but up to now there had been no direct evidence for this. Scientists will now be able to use the new findings to design experiments which might provide vital information about how phytochrome and phototropin work and thus how plants respond to light – phenomena which are fundamental to agriculture and thus our food supply.
Christel Lauterbach | idw
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