The Agents That Shape the Floral Traits of Sunflowers

Flowers come in a wide array of shapes, sizes, colors, and scents. Their diversity is downright astounding. Each individual species of flowering plant has its own lengthy story to tell detailing how it came to look and act the way it does. This is its evolutionary history. Unraveling this history is a nearly insurmountable task, but one that scientists continue to chip away at piece by piece.

In the case of floral traits – particularly for flowers that rely on pollinators to produce seeds – it is safe to say that millennia of interactions with floral visitors have helped shape not only the way the flower looks, but also the nature of its nectar and pollen. However, flowers are “expensive” to make and maintain, so even though they are necessary for reproduction, plants must find a balance between that and allocating resources for defense – against both herbivory and disease – and growth. This balance can differ depending on a plant’s life history – whether it is annual or perennial. An annual plant has one shot at reproduction, so it can afford to funnel much of its energy there. If a perennial is unsuccessful at reproduction one year, there is always next year, as long as it has allocated sufficient resources towards staying alive.

Where a plant exists in the world also influences how it looks. Abiotic factors like temperature, soil type, nutrient availability, sun exposure, and precipitation patterns help shape, through natural selection, many aspects of a plant’s anatomy and physiology, including the structure and composition of its flowers. Additional biotic agents like nectar robbersflorivores, and pathogens can also influence certain floral traits.

This is the background that researchers from the University of Central Florida and University of Georgia drew from when they set out to investigate the reasons for the diverse floral morphologies in the genus Helianthus. Commonly known as sunflowers, Helianthus is a familiar genus consisting of more than 50 species, most of which are found across North America. The genus includes both annuals and perennials, and all but one species rely on cross-pollination to produce viable seeds. Pollination is mainly carried out by generalist bees.

Maximilian sunflower (Helianthus maximiliani)

Helianthus species are found in diverse habitats, including deserts, wetlands, prairies, rock outcrops, and sand dunes. Their inflorescences – characteristic of plants in the family Asteraceae – consist of a collection of small disc florets surrounded by a series of ray florets, which as a unit are casually referred to as a single flower. In Helianthus, ray florets are completely sterile and serve only to attract pollinators. Producing large and numerous ray florets takes resources away from the production of fertile disc florets, and sunflower species vary in the amount of resources they allocate for each floret form.

In a paper published in the July 2017 issue of Plant Ecology and Evolution, researchers selected 27 Helianthus species and one Phoebanthus species (a closely related genus) to investigate “the evolution of floral trait variation” by examining “the role of environmental variation, plant life history, and flowering phenology.” Seeds from multiple populations of each species were obtained, with populations being carefully selected so that there would be representations of each species from across their geographic ranges. The seeds were then grown out in a controlled environment, and a series of morphological and physiological data were recorded for the flowers of each plant. Climate data and soil characteristics were obtained for each of the population sites, and flowering period for each species was collected from various sources.

The researchers found “all floral traits” of the sunflower species to be “highly evolutionarily labile.” Flower size was found to be larger in regions with greater soil fertility, consistent with the resource-cost hypothesis which “predicts that larger and more conspicuous flowers should be selected against in resource-poor environments.” However, larger flower size had also repeatedly evolved in drier environments, which goes against this prediction. Apart from producing smaller flowers in dry habitats, flowering plants have other strategies to conserve water such as opening their flowers at night or flowering for a short period of time. Sunflowers do neither of these things. As the researchers state, “this inconsistency warrants consideration.”

The researchers speculate that “the evolution of larger flowers in drier environments” may be a result of fewer pollinators in these habitats “strongly favoring larger display sizes in self-incompatible species.” The flowers are big because they have to attract a limited number of pollinating insects. Conversely, flowers may be smaller in wetter environments because there is greater risk of pests and diseases. This is supported by the enemy-escape hypothesis – smaller flowers are predicted in places where there is increased potential for florivory and pathogens. Researchers found that lower disc water content had also evolved in wetter environments, which supports the idea that the plants may be defending themselves against flower-eating pests.

Seed heads of Maximilian sunflower (Helianthus maximiliani)

Another interesting finding is that, unlike other genera, annual and perennial sunflower species allocate a similar amount of resources towards reproduction. On average, flower size was not found to be different between annual and perennial species. Perhaps annuals instead produce more flowers compared to perennials, or maybe they flower for longer periods. This is something the researchers did not investigate.

Finally, abiotic factors were not found to have any influence on the relative investment of ray to disc florets or the color of disc florets. Variations in these traits may be influenced instead by pollinators, the “biotic factor” that is considered “the classic driver of floral evolution.” This is something that will require further investigation. As the researchers conclude, “determining the exact drivers of floral trait evolution is a complex endeavor;” however, their study found “reasonable support for the role of aridity and soil fertility in the evolution of floral size and water content.” Yet another important piece to the puzzle as we learn to tell the evolutionary history of sunflowers.

Advertisements

Tomato vs. Dodder, or When Parasitic Plants Attack

At all points in their lives, plants are faced with a variety of potential attackers. Pathogenic organisms like fungi, bacteria, and viruses threaten to infect them with diseases. Herbivores from all walks of life swoop in to devour them. For this reason, plants have developed numerous mechanisms to defend themselves against threats both organismal and environmental. But what if the attacker is a fellow plant? Plants parasitizing other plants? It sounds egregious, but it’s a real thing. And since it’s been going on for thousands of years, certain plants have developed defenses against even this particular threat.

Species of parasitic plants number in the thousands, spanning more than 20 different plant families. One well known group of parasitic plants is in the genus Cuscuta, commonly known as dodder. There are about 200 species of dodder located throughout the world, with the largest concentrations found in tropical and subtropical areas. Dodders generally have thread-like, yellow to orange, leafless stems. They are almost entirely non-photosynthetic and rely on their host plants for water and nutrients. Their tiny seeds can lie dormant in the soil for a decade or more. After germination, dodders have only a few days to find host plants to wrap themselves around, after which their rudimentary roots wither up. Once they find suitable plants, dodders form adventitious roots with haustoria that grow into the stems of their host plants and facilitate uptake of water and nutrients from their vascular tissues.

A mass of dodder (Cuscuta sp.) - photo credit: wikimedia commons

A mass of dodder (Cuscuta sp.) – photo credit: wikimedia commons

Some plants are able to fend off dodder. One such instance is the cultivated tomato (Solanum lycopersicum) and its resistance to the dodder species, Cuscuta reflexa. Researchers in Germany were able to determine one of the mechanisms tomato plants use to deter dodder; their findings were published in a July 2016 issue of Science. The researchers hypothesized that S. lycopersicum was employing a similar tactic to that of a microbial invasion. That is, an immune response is triggered when a specialized protein known as a pattern recognition receptor (PRP) reacts with a molecule produced by the invader known as a microbe-associated molecular pattern (MAMP). A series of experiments led the researchers to determine that this was, in fact, the case.

The MAMP was given the name Cuscuta factor and was found “present in all parts of C. reflexa, including shoot tips, stems, haustoria, and, at lower levels, in flowers.” The PRP in the tomato plant, which was given the name Cuscuta receptor 1 (or CuRe 1), reacts with the Cuscuta factor, triggering a response that prohibits C. reflexa access to its vascular tissues. Starved for nutrients, the dodder perishes. When the gene that codes for CuRe 1 was inserted into the DNA of Solanum pennellii (a wild relative of the cultivated tomato) and Nicotiana benthamiana (a relative of tobacco and a species in the same family as tomato), these plants “exhibited increased resistance to C. reflexa infestation.” Because these transgenic lines did not exhibit full resitance to the dodder attack, the researchers concluded that “immunity against C. reflexa in tomato may be a process with layers additional to CuRe 1.”

photo credit: wikimedia commons

photo credit: wikimedia commons

A slew of crop plants are vulnerable to dodder and other parasitic plants, so determining the mechanisms behind resistance to parasitic plant attacks is important, especially since such infestations are so difficult to control, have the potential to cause great economic damage, and are also a means by which pathogens are spread. It is possible that equivalents to CuRe 1 exist in other plants that exhibit resistance to parasitic plants, along with other yet to be discovered mechanisms involved in such resistance, so further studies are necessary. Discoveries like this not only help us make improvements to the plants we depend on for food, but also give us a greater understanding about plant physiology, evolutionary ecology, and the remarkable ways that plants associate with one another.

Additional Resources:

Article: The Intelligent Plant

The New Yorker’s last issue in 2013 included an article by Michael Pollan called “The Intelligent Plant” in which Pollan explores some of the latest research revealing the ability of plants to sense their environment in ways that are analogous to seeing, hearing, and smelling. In the article Pollan dialogs back and forth between plant scientists who call this line of research “plant neurobiology” and plant scientists who seem to abhor that term. As the article progresses, you learn that the arguments between the two groups are not necessarily about the science itself but about vocabulary. Can plants learn the way we understand the term, to learn? Can we really say that plants are intelligent or conscious? Aren’t those traits reserved for organisms with brains? And regarding brains, plants don’t have them, so why plant neurobiology? Neuroscience is the study of nervous systems, so plant neurobiology must be a misnomer, right?

Well, despite the arguments over language, the research is pretty compelling. Plants are proving to be more aware of their surroundings and their actions seem to be more calculated than we originally assumed. They are not simply sessile organisms being acted upon, but they are doing some acting – lots of it, in fact. It is a remarkable field of study (whether you choose to refer to it as plant neurobiology or something else), and it will be exciting to see where it takes us.

Pollan’s article is worth a read if you can find the time (be warned, it’s lengthy), and it’s getting some coverage. Pollan recently appeared on Science Friday with Ira Flatow where he talked about his experience researching the article. And Pollan, of course, isn’t the only one talking about this stuff, Wired featured an article about it last month as well.

Check out this video associated with Pollan’s article (narrated by Pollan) of bean plants that appear to be deliberately reaching out to grab onto a pole.

sensitive plant

sensitive plant – Mimosa pudica

photo credit: Wikimedia Commons

Related Posts

Book Review: What a Plant Knows

Documentary: What Plants Talk About

Book Review: What a Plant Knows

What a Plant Knows: A Field Guide to the Senses
by Daniel Chamovitz

Humans commonly anthropomorphize non-humans. It just seems easier, for example, to say that a plant “likes” a particular type of soil, even though we know that a plant doesn’t “like” anything because a plant does not experience emotion. What we really mean to say is that a plant is adapted to and therefore performs best in a particular type of soil. However, knowing this, is it plausible at all to say that a plant can see, smell, feel, hear, sense its location, or remember things? Daniel Chamovitz argues that it is, and he has plenty of credible research to support his thesis.

In short, plants have senses very similar to human senses and are far more aware than we might initially think. To be clear though, Chamovitz states early on in his book that his “use of the word ‘know’ is unorthodox. Plants don’t have a central nervous system; a plant doesn’t have a brain that coordinates information for its entire body.” Nor do they have noses or ears or eyes. Instead, when Chamovitz uses words like “see,” “smell,” “hear,” and “know,” he is referring to various chemical reactions and physiological phenomena that occur in plants which produce reactions that are analogous to human senses. When a willow tree is damaged by tent caterpillars, a neighboring willow tree becomes unpalatable to the caterpillars and thereby resists a similar fate. Why? Because the damaged willow tree releases a gaseous substance that nearby willow trees can sense (or “smell”). This is a signal for them to protect themselves by building up toxic chemicals in their leaves.

Another example offered by Chamovitz involves the ability of some plants to remember winter. Cherry blossoms appear in the spring because winter has passed. A certain period of cold temperatures is what induces this response. If the trees bloom too early, the blossoms will freeze. If they bloom too late, the fruits would not have time to mature before cold temperatures returned. The seeds of winter wheat are planted in the fall and germinate in the spring. They also require a period of cold temperatures in order to germinate. This process is called vernalization, and it involves a specific gene in the plant called flowering locus C (FLC). After vernalization, this gene is turned off which signals the plant to flower (provided that other environmental conditions, such as light and soil temperature, are conducive to flowering, etc.).

A common myth is that plants grow better when people say nice things to them or play relaxing music for them. Chamovitz thoroughly debunks this myth and concludes that no evidence has been found for plants being able to hear. Plants do however possess many of the same genes that humans possess, including several genes that when not functioning properly can result in deafness in humans. These genes encode proteins called myosins. Myosins in humans help form the hair cells in our inner ears which are essential for hearing. Myosins in plants help form root hairs which are essential for absorbing water from the soil. While the functions of these proteins are quite different in humans and plants, mutations in the genes code for these proteins can have drastic results for both.

All this talk about chemistry, genetics, and physiology may sound a bit intimidating…but don’t worry. While Chamovitz endeavours to tell the science accurately and in detail, he does so in a very approachable manner, making this an easy read for anyone with a basic understanding of biology. Even if you don’t fully comprehend the technical stuff, the anecdotes are well told and captivating, and after you finish reading this, you are certain to have a greater appreciation for plants and all of the fascinating things that they can do. While we should be careful to be too anthropocentric, this book makes it clear that plants are a lot like us…or should I say, we are a lot like plants? Either way, we have many things in common (including much of our DNA), which is all the more reason to appreciate plants for the amazing organisms that they are.

This is a video (recommended by Chamovitz) of a dodder plant sensing the location of a tomato plant. Dodder is unable to photosynthesize, so after attaching itself to the tomato plant it will feed on the nutrients that the tomato plant produces.

Related Post:

– Excerpt from What a Plant Knows