[The many ways flowers send signals to pollinators].

Abstract

The evolutionary success of angiosperms, which make up more than 95 percent of the world's terrestrial flora, is largely based on their interactions with animal pollinators. Indeed, it is estimated that, on average, 87.5 percent of flowering plants are pollinated by animals. The majority are pollinated by insects, but birds, bats, rodents and even lizards can play a significant role in the pollination of some species. Pollinators visit flowers in search of nutritional resources such as nectar, which contains high concentrations of sugars to meet their energy needs, and the pollen itself, which is usually produced in large quantities and is their main source of amino acids, lipids and vitamins. The efficiency of pollination, i.e. the transfer of pollen from one flower to another of the same species, is based on specific signals emitted by the flowers of a given species, which pollinators learn to associate with the presence of floral rewards produced by that species. Animals locate flowers at relatively long distances thanks to global coloured patches, called floral display, produced by more or less grouped flowers or inflorescences. Once near or on the flower, pollinators use species-specific colour signals to identify exactly which flowers contain the resources they are looking for and where they are located. The almost infinite variety of these visual signals, mostly emitted by the petals, is based mainly on the numerous flavonoid pigments anthocyanins and carotenoids. Pollinators also learn from the complex olfactory signals produced by the mixture of volatile organic compounds. These molecules belong to three main chemical classes. Terpenoids are the most abundant, phenylpropanoids the second, and fatty acid-derived molecules the third, but some sulphurs, amines or aliphatic acids are also emitted by some species. Blends of volatiles are emitted from specialised tissues, named osmophores, located on the surface of floral organs as petals or nectaries. Some angiosperm species attract pollinating insects with deceptive flowers that don't produce nectar. Some of these species mimic the colour, shape and scent of rewarding flowers that bloom sympatrically. Others, mainly in the orchid family, specifically attract the males of a bee species by mimicking the pheromone mixture of females in sexually deceptive flowers, resulting in pseudocopulation behaviour of pollinators. Brood site olfactive mimicry is another common strategy of deceptive flowers attracting more or less specifically pollinating flies and beetles. Thanks to these deceptive signals, deceptive flowers save the energy allocated to the production of floral rewards. The visual and olfactory signals emitted by flowers have been described and studied for many years, but other signals involved in the interaction between plants and their pollinators have been highlighted recently. For example, the echo reflected from floral organs or plant structures associated with flowers allows nectar-feeding bats to echolocate the floral resources of particular plants. Electrostatic interactions generated by differences in polarity between a positive insect, such as a bumblebee or a hoverfly, and a negative flower lead to movements of mechanosensory thoracic hairs and provide a signal indicating the absence of a previous visit to that flower and the putative presence of nectar in the flower. This electrical signal appeared to play a significant role in insect learning and foraging efficiency.