Flower power in microRNA pathway

C&I Issue 17, 2009

Future food crops could be less dependent on the seasons thanks to the discovery of a new pathway controlling the onset of plant flowering. The finding of this alternative signaling route, which is not dependent on the usual environmental cues such as day length, sunlight and temperature for flowering, means that it could one day be possible to breed new varieties of plants able to produce fruits and seeds in normally inauspicious environments and seasons.

Researchers at the Max Planck Institute for Developmental Biology in Germany made the discovery while studying a variety of common wall cress called Arabidopsis thaliana. Their study showed how plant flowering is triggered by a reduction in the level of short snippets of RNA made in the plant’s leaves, which regulate gene function by binding to complementary motifs in messenger RNA to inhibit protein production.

In the absence of external stimuli, the researchers showed that levels of a specific microRNA156 declined over time as A. thaliana grows, eventually resulting in the production of so-called SPL proteins in sufficient amounts to trigger flowering. SPL proteins are among a group of regulators involved in the plant flowering process; when a plant is young, high levels of microRNA156 inhibit the production of SPL proteins.

‘One potential significance of our work is that people can shorten or prolong the vegetative phase of crops [the period between seed germination and flowering] by reducing or increasing the level of microRNA156,’ according to Jiawei Wang, one of the Max Planck researchers involved in the study (Cell, doi: 10.1016/j.cell.2009.06.014). This could be important commercially for farmers and flower growers, as a longer vegetative plant would allow the plant to produce more leaves while a shorter vegetative time would encourage the plant to produce flowers and seeds sooner – something that could be especially useful for trees, for example, which typically only flower after several years’ growth.

The new pathway is an in-built survival mechanism that always functions, but only comes to the fore when the external conditions are very unfavourable, Wang explains. ‘Flowering is controlled by many exogenous and endogenous cues. Usually, exogenous cues, such as temperature, day length, have a bigger impact on flowering time. The microRNA156 pathway ensures that plants can eventually flower even without these exogenous cues.’

The group is currently investigating whether the pathway is present in commercially important crops, as microRNA156 and SPL proteins, along with other implicated MADs box genes, are found in most plant species, Wang says. He adds, however, that it is still too early to say whether we could grow crops all year round by exploiting this new pathway since reducing microRNA156 levels ‘cannot compensate for the loss of all these exogenous cues’.

Caroline Dean, associate research director at the John Innes Centre in Norwich, UK, describes the new mechanism as ‘of great interest’ as researchers already understand many of the environmental factors involved in flowering. ‘By adding in this information on how plants eventually trigger flowering even when conditions have not become ideal, we get a fuller picture of the mechanisms controlling developmental changes in plants. This knowledge is especially important in crop plants, as we seek to understand potential impacts of climate change on germination and flowering time.’

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