More Than What Meets the Eye

When you think of sunflowers, you’re probably imagining tall stalky plants that are vivid yellow with slight variation in height and petal distribution. Flowers are an important part of the reproduction cycle to attract pollinators and protect the plant from predators and harsh environments, flowers have distinctive colors, shapes, and patterns. When you’re thinking of sunflowers, you’re probably not thinking of ultraviolet light; however, there is so much more going on than what is visible to the human eye.

Many flowers contain various pigments that absorb ultraviolet light to create distinct UV patterns. While these patterns are invisible to us, pollinators can see these patterns and flowers use that to their advantage. These patterns have been shown to influence pollinator visitation and preference through increased floral visibility. Additionally, the pigments absorbing the UV light have been shown to have a role in responding to different biotic and abiotic factors including defense against predators, adaption to changing temperatures, and protection against UV radiation.

There are about 50 species of wild sunflowers across North America which are adapted to various habitats. These 50 species have a remarkable phenotypic and genetic diversity which makes them a good model system for studies of adaption, speciation, and domestication. In sunflowers, the petals accumulate UV absorbing pigments at their base while the tips reflect UV radiation which results in a bullseye like UV pattern. While this pattern has been observed between and within many plant species, we still don’t know what drives this variation.

To figure out the ecological and genetic impact on sunflower patterns, Todesco et al. collected two species of annual sunflowers Helianthus annuus which grows across North America and Helianthus petiolaris which prefers sandier soils. The researchers measured the floral UV patterns over two growing seasons and were surprised by the results. While both species had differing UV patterns, H. annuus had a striking variation. Some flowers had almost no UV pigmentation while others were entirely UV absorbing. The researchers were itching to find out what the genetic basis of this UV patterning is and scanned the genomes of multiple flowers to look for genetic markers that were affecting UV patterning.

In H. annuus they discovered that HaMYB111 controls UV pigment production. This gene is a member of a small family of transcription factors that control expression of genes involved in production of flavanol glycosides which are important to plant survival. Flavanol glycosides have been shown to protect plants against biotic and abiotic stressors, UV radiation, cold, drought, and predation. The absorb strongly in the UV range (300-400 nm) and are the pigments responsible for UV patterning.

Now that researchers knew what gene was responsible for variation in UV patterning, they had a new question. Why does this variation exist in the first place? To figure this out, Todesco et al looked at patterns of pollination based on the size of the UV bullseye in two populations. What they found was that large UV patterns have significantly more visits compared to smaller bullseyes. While this experiment shows a clear pattern of pollination preferences, the sample size was small. So the researchers repeated the experiment except this time they used 1484 individuals from 106 H. annuus populations spanning the entire ecological range of this species. They discovered that the highest visitation rates went to plants with intermediate UV patterns. While small or large UV patterns both had infrequent visitations, plants with small patterns had the lowest number of visitors. Since flavanol glycosides contribute to more plant functions, it is unlikely that visual cues for pollinators are the only reason for pattern variation.

In fact, in other plant species scientists have discovered that floral traits can change to protect the plant from abiotic stressors. The scientists started testing various conditions. First they tested UV radiation because in roses, Argentina anserina, bullseye patterns correlate positivity with intensity of UV radiation. However, this was not the case for sunflowers. Next they tested temperature because in the Potentillae tribe of roses, the UV bullseye size gets larger in lower temperatures.  They found out that the bullseye size in sunflowers also relates to temperatures. In the lower summer temperatures, the sunflowers had larger UV bullseye sizes. When testing humidity levels, they found that lower humidity levels also resulted in larger bullseyes, which indicates that larger bullseyes help the sunflowers retain water.

Their work suggests that flavanol glycosides and the correspond UV bullseye pattern not only attracts pollinators but also helps sunflowers protect themselves against water loss. This study really highlights how important it is to look beyond what is already known. For almost a century scientists thought that UV patterns were due to the behaviors of pollinators. When, in reality, the floral diversity is also an outcome of the plant protecting itself from changing climates. This work is an addition to growing literature that demonstrates that abiotic factors contribute to floral diversity.