Evolution is based on diversity, and sexual reproduction is key to creating a diverse population that secures competitiveness in nature. Plants as largely immobile organisms had to solve a problem: they needed to find ways to spread their genetic material beyond individual flowers. To make sure that flying pollinators such as insects, birds and bats come to the flowers to pick up pollen, plants evolved special organs, the nectaries, to attract and reward the animals.
Scientists from the Max Planck Institute for Chemical Ecology in Jena (Germany) and their colleagues from Stanford and Duluth (USA) have identified the sugar transporter that plays a key role in plants' nectar production. SWEET9 transports sugar into extracellular areas of the nectaries where nectar is secreted. Thus, SWEET9 may have been crucial for the evolution of flowering plants that attract and reward pollinators with sweet nectar.
Despite the obvious importance of nectar, the process by which plants manufacture and secrete it has remained a mystery. New research from a team led by Wolf Frommer, director of the Plant Biology Department, Carnegie Institution for Science in Stanford, in collaboration with the Carter lab in Minnesota and the Baldwin lab at the Max Planck Institute for Chemical Ecology in Jena, Germany, now identified key components of the sugar synthesis and secretion mechanisms. Their work also suggests that the components were recruited for this purpose early during the evolution of flowering plants. Their work is published by Nature.
The team used advanced techniques to search for transporters that could be involved in sugar transport and were present in nectaries. They identified SWEET9 as a key player in three diverse flowering plant species, thale cress Arabidopsis thaliana, turnip Brassica rapa and coyote tobacca Nicotiana attenuata, and demonstrated that it is essential for nectar production.
In specially engineered plants lacking SWEET9, the team found that nectar secretion did not occur but sugars rather accumulated in the stems. They also identified genes necessary for the production of sucrose, which turn out to be also essential for nectar secretion. Taken together, their work shows that sucrose is manufactured in the nectary and then transported into the extracellular space of nectaries by SWEET9. In this interstitial area the sugar is converted into a mixture of sucrose and other sugars, namely glucose and fructose. In the plants tested these three sugars comprise the majority of solutes in the nectar, a prerequisite for collection by bees for honey production.
"SWEETs are key transporters for transporting photosynthates from leaves to seeds and we believe that the nectarial SWEET9 sugar transporter evolved around the time of the formation of the first floral nectaries, and that this process may have been a major step in attracting and rewarding pollinators and thus increasing the genetic diversity of plants," Frommer said.
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