Invisible patterns on hibiscus petals are essential for attracting pollinators, study reveals

Researchers discover that plants define the size of circular spots on petals before visible development, influencing the efficiency of bees in pollination

15.09.2024 | 19:24 (UTC -3)
Cultivar Magazine
Early petal development of . (A) Petal organization in flower buds ranging from stage 0a (S0a) to late stage 2 (S2L). Images capture the abaxial side of the petal. Scale bars, 1 mm. (B) Early stages of development of the adaxial petal epidermis, from S0a to late stage 2 (S2L). Pigmentation emerges on both sides of the petal primordium at stage 1 (S1), as indicated by arrows. Scale bars, 100 μm (S0a and S0b), 1 mm (S0c to S2E), and 5 mm (S2L). (C) Mature flower of (stage 5). (D) Grading criteria for petal primordia of . Total cell counts were not assessed in S2E, as only the central stripe of the petal was imaged at that stage. n = 5 petals for each stage.
Early petal development Hibiscus trionum. (A) Petal organization in flower buds ranging from stage 0a (S0a) to late stage 2 (S2L). Images capture the abaxial side of the petal. Scale bars, 1 mm. (B) Early stages of adaxial petal epidermis development, from S0a to late stage 2 (S2L). Pigmentation emerges on both sides of the petal primordium at stage 1 (S1), as indicated by arrows. Scale bars, 100 μm (S0a and S0b), 1 mm (S0c to S2E), and 5 mm (S2L). (C) Mature flower of H. trionum (stage 5). (D) Classification criteria for petal primordia of H. trionum. Total cell counts were not assessed in S2E, as only the central stripe of the petal was imaged at that stage. n = 5 petals for each stage.

Researchers have discovered that hibiscus flowers use an invisible pattern that is established long before the petals develop. This pattern, known as prepatterning, determines the size of areas called bullseyes – circular spots that play a crucial role in attracting pollinators. The study, led by scientists at the University of Cambridge, revealed that the size of the bullseye directly influences the preferences of bees, which can increase pollination efficiency.

The research revealed that bees prefer larger bullseyes. They also fly 25 percent faster between artificial discs with larger patterns. This suggests that flowers with more prominent bullseyes may be more attractive to pollinators, improving efficiency on both sides – for the bees and the flowers.

Patterns on petals are essential for guiding insects, such as bees, to nectar and pollen in the center of flowers. Despite their importance for plant reproduction, little was known until now about how these patterns form. To fill this gap, scientists combined developmental biology, evolutionary biology, and computational modeling to study the phenomenon. The study focused on the plant Hibiscus trionum, which has a bullseye pattern on the petals.

The researchers observed three varieties of hibiscus with different sized bullseyes: H. richardsonii (with small bullseye), H. trionum (medium size) and a transgenic line of H. trionum (large bullseye). The study revealed that the pattern is set long before it is visible on the growing petals. This prepatterning creates a kind of "canvas" where each part of the petal is already programmed to develop specific colors and textures.

The results indicate that plants have the ability to control and modify these patterns in a variety of ways, either during the prepatterning phase or by adjusting cell growth later on. These adaptations could have evolutionary implications, offering a competitive advantage in attracting pollinators and even influencing plant diversity.

Edwige Moyroud, the study’s leader, explains that this diversity is a result of plants’ ability to generate variations in the patterns of their petals. “By studying how bullseyes change, we are trying to understand how nature generates biodiversity,” Moyroud said. The discovery that these patterns are planned in advance challenges the view that the most important processes occur only in the final stages of flower development.

Lucie Riglet, the study’s lead author, developed a technique to observe the early development of hibiscus petals. She found that the pattern begins to form when the petal is still green and has about 700 cells. Even without visible pigmentation, the cells in a specific area begin to differentiate in size, marking the beginning of the bullseye formation.

One of the big questions raised by the study was how plants maintain the proportions of these patterns during growth, as petals increase in size by up to 100 times. Using computer models, the researchers found that plants can adjust cell expansion and division to ensure the patterns remain proportional.

In addition to the biological implications, the study highlighted the importance of these patterns for pollinator attraction. In experiments with artificial floral discs, bees preferred larger bullseyes and were more efficient at foraging, visiting more flowers in less time. This suggests that bullseye size can directly influence the reproductive success of plants.

These findings pave the way for future research into how petal patterns influence the evolution of plant species. The study suggests that prepatterning in petals may have ancient evolutionary origins and that the mechanisms used to create patterns in flowers may apply to other plant organs, such as leaves.

This research is especially relevant for the conservation of plants such as H. richardsonii, a critically endangered species, while H. trionum, which has a larger bullseye, is widely distributed. Further studies will be needed to understand whether bullseye size contributes to reproductive success and pollinator attraction.

More information can be found at doi.org/10.1126/sciadv.adp5574

Spatiotemporal distribution of cell expansion and cell division events across the adaxial epidermis during early stages of petal morphogenesis. (A) Color map of cell area across the WT adaxial petal epidermis during early stages of development (from S0a to S2E). Scale bars, 100 μm. (B) Distribution of cell area across the PD axis of the petal. Graphs consider only the central strip of cells (20% of the petal width) for readability. Cell positions along the PD axis are relative (0 = petal base; 1 = petal tip). Gray lines correspond to the moving average of cell area across all replicates. Statistical differences were calculated using a Shapiro–Wilk test to assess normality and then a t test; ns, not significant, ***P < 0,01. n = 5 petals for each stage. (C) Distribution of cell division events across the adaxial epidermis of S0a and S0b petals. Newly synthesized DNA is labeled using the fluorescently labeled nucleotide analogue 5-ethynyl-2-deoxyuridine (EdU; green), and plasma membranes are stained with PI (red). Scale bar, 100 μm. (D) Probability density function (PDF) of EdU-labeled nuclei along the PD axis of S0a (left) and S0b (right) petals of H. trionum (stripes corresponding to 20% of the petal width and centered along the PD axis were analyzed, see fig. S1E). n = 5 petals for each stage.
Spatiotemporal distribution of cell expansion and cell division events across the adaxial epidermis during early stages of petal morphogenesis H. trionum. (A) Color map of cell area across the WT adaxial petal epidermis during early developmental stages (from S0a to S2E). Scale bars, 100 μm. (B) Distribution of cell area across the PD axis of the petal H. trionum. Graphs consider only the central band of cells (20% of the petal width) for readability. Cell positions along the PD axis are relative (0 = petal base; 1 = petal tip). Gray lines correspond to the moving average of the cell area of ​​all replicates. Statistical differences were calculated using a Shapiro-Wilk test to assess normality and then a t-test; ns, not significant, ***P < 0,01. n = 5 petals for each stage. (C) Distribution of cell division events across the adaxial epidermis of S0a and S0b petals. Newly synthesized DNA is labeled using the fluorescently labeled nucleotide analogue 5-ethynyl-2-deoxyuridine (EdU; green), and plasma membranes are stained with PI (red). Scale bar, 100 μm. (D) Probability density function (PDF) of EdU-labeled nuclei along the PD axis of S0a (left) and S0b (right) petals of H. trionum (stripes corresponding to 20% of the petal width and centered along the PD axis were analyzed, see fig. S1E). n = 5 petals for each stage.

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