A research group including Naruto University of Education has found that the angiosperm Drosera spatulata reinforces its defenses when its flowers are targeted by insect herbivores after its leaves have previously been eaten. The study suggests that plants can "remember" previous herbivore attacks and adjust the intensity of their defensive responses, even without a brain.
Photo provided by Associate Professor Kazuki Tagawa, Naruto University of Education
Despite its name, D. spatulata is not a moss. It is a flowering plant belonging to the genus Drosera. The plant opens its pink flowers for about half a day, during which pollen from the stamens is deposited on the stigma, leading to seed production. It is a carnivorous plant that captures insects with the sticky secretions produced by red hairs on its spoon-shaped leaves. However, its leaves and flowers are eaten by the larvae of the sundew plume moth (Buckleria paludum).
Photo courtesy of Associate Professor Kazuki Tagawa, Naruto University of Education
In 2018, Associate Professor Kazuki Tagawa of Naruto University of Education, an evolutionary ecologist, discovered that stimulating the base of flowers in the genus Drosera causes them to close within about 2 to 10 minutes. Flower closure helps protect the ovules, which develop into seeds, from being eaten by the larvae of the sundew plume moth.
However, if flowers closed too readily in response to harmless stimuli such as wind, rain, or non-threatening organisms, they could lose opportunities for cross-pollination, which promotes genetic diversity, and instead be forced to rely on self-pollination to produce seeds. Tagawa and his colleagues hypothesized that Drosera spatulata plants that had previously experienced leaf herbivory might use that "memory" to trigger a stronger flower-closing response to subsequent stimuli.
Photo provided by Associate Professor Kazuki Tagawa, Naruto University of Education
Tagawa and his colleagues conducted experiments using indoor-grown D. spatulata plants one day before flowering. They prepared 15 plants whose leaves were removed to simulate herbivory ("herbivory-experienced" plants) and 17 untreated plants as controls. To mimic a sundew plume moth larva climbing the stem and approaching a flower, they pinched the calyx tube at the base of the flower with tweezers once per second for a total of 10 times. When the degree of flower closure was measured 15 minutes later, the herbivory-experienced plants had closed their flowers to a significantly smaller angle than the control plants.
The researchers also examined how responses varied depending on the time of stimulation. In the control plants, flower closure was relatively weak in the early morning shortly after the flowers opened and became progressively stronger later in the day. In contrast, the herbivory-experienced plants consistently exhibited strong flower-closing responses regardless of when the stimulus was applied.
Photo provided by Associate Professor Kazuki Tagawa, Naruto University of Education
The findings show that plants can adjust the strength of their defenses in one organ based on herbivory experienced by another organ. Tagawa said, "Going forward, we hope to clarify the molecular mechanisms involved, including the chemical substances responsible for this response, and to investigate its evolutionary background through comparisons among different species."
The study was conducted in collaboration with the Museum of Nature and Human Activities, Hyogo, and Aichi University of Education, and was published in the international journal Ethology on July 2.
Original article was provided by the Science Portal and has been translated by Science Japan.