agricultural weeds

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Seed Shattering Lost – The Story of Foxtail Millet

For a plant to disperse its seeds, it must first let go of them. Sounds obvious, but it is a key step in the dispersal process and an act that is actually coded in a plant’s DNA. As fruits ripen and seeds mature, an abscission layer is formed that separates the seed-bearing fruits from the plant. No longer attached to their parents, seeds are left to their own devices. If all goes well, they will find themselves in a suitable location where they can germinate and grow into a whole new plant, fully equipped to make seed babies of their own.

The releasing of mature seeds is known as shattering, a term most commonly used in reference to grasses and plants with dehiscent seed pods (i.e. fruits that split open when ripe, such as those in the bean and mustard families). In grasses, seeds form along a central stem called a rachis. As the seeds ripen, they separate from the rachis and drop from the plant. In some cases, the rachis is brittle and a section of it breaks off with each seed that falls.

Seed shattering is not a desirable trait when it comes to food crops. It’s easy to see how yields can be poor if seeds disperse before they are harvested. Thus, an essential step in domesticating certain agricultural crops was selecting plants that lacked this particular trait. Instead of dropping mature seeds, such plants hold on to them, making them easy to collect. A simple and naturally occurring mutation in the genes of these plants allowed early farmers to select varieties that were more ideal for agriculture than their wild progenitors.

Genetic studies of agricultural crops have located genes in a number of species that code for seed shattering, confirming that domestication in many cases involved selecting plants with this gene turned off. A recent study, published in Nature Biotechnology (October 2020), took a different route in locating this gene, looking instead at a weedy, wild relative of a crop that was domesticated at least 8000 years ago. Green foxtail (Setaria viridis) is the wild antecedent of foxtail millet (Setaria italica), a crop that, while still commonly grown for food in parts of Asia, is mostly grown for hay, silage, and bird seed in North America. Recently, interest in foxtail millet and other millets (a term used to refer to the grains of several different species of grasses) is on the rise due to the ability of these crops to tolerate drought and heat.

Illustration of three Setaria species from Selected Weeds of the United States (Agriculture Handbook No. 366) published in 1970

Setaria viridis is an abundant, widespread weed adapted to human disturbance. It’s of Eurasian origin but has been present in North America since the early 1800’s and was likely introduced both intentionally and accidentally. It’s an annual grass with prominent, bristly flowerheads that are easily recognizable and the reason for its common name, green foxtail. A handful of other closely related, similar-looking species are also common weeds in North America. Due to useful traits including its short life cycle, small genome, and self-fertility, S. viridis has been used frequently as a model species to carry out a variety of scientific studies. The study referred to above aimed to further enhance the use of green foxtail, particularly when it comes to crop science.

Researchers traveled across the United States collecting nearly 600 samples of green foxtail in order to better understand its genome. They found that the North American population of green foxtail is composed of multiple introductions and that, as the species has followed humans around, it has quickly adapted to diverse climates found across the continent. In examining the genome, they were able to identify the genetic underpinnings for three traits that have importance to agriculture: response to climate, leaf angle (which is used as a predictor of yield in grain crops), and seed shattering.

foxtail millet (Setaria italica) via wikimedia commons

The seed shattering gene – which the researchers named Less Shattering 1 (SvLes1) – was an especially interesting discovery. When compared to the orthologous gene found in foxtail millet, they found that a frameshift mutation had caused a disruption in the gene, turning it off. Using CRISPR (a gene editing tool) they were able to recreate a similar interruption in green foxtail, which resulted in a loss of seed shattering similar to that of foxtail millet. It became clear that selecting plants with this mutation was an essential step in the domestication of this ancient grain.

An excerpt about seed shattering from Fruit from the Sands by Robert N. Spengler III: 

In many of the world’s domesticated grains, especially those from the founder crops of southwest Asia (i.e. wheat and barley), the earliest phenotypical trait of domestication that archaeobotanists look for is a tough rachis, the small stem by which an individual grain or small cluster of grains is attached to the ear. In their wild form, most grains are programmed to detach easily after the grain ripens; however, in domesticated cereals, the grains remain attached to the ear throughout the harvesting process. This change is an inadvertent result of human harvesting with sickles: as people reap their harvest, the grains with a brittle rachis are dropped and those with a tough rachis are collected, stored, and replanted for successive harvests.

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Summer of Weeds: Flower of an Hour

Hibiscus trionum is a great example of an ornamental plant becoming a widespread weed. Its common name, flower of an hour, refers to its short-lived blooms. Other common names include Venice mallow, bladder hibiscus, bladderweed, modesty, and shoofly. Native to southern Europe and tropical to subtropical parts of Asia and Africa, it was introduced to America as an attractive addition to annual flower beds. It is now naturalized in many states across the country.

Hibiscus is a huge genus in the family Malvaceae, consisting of species found throughout warmer parts of the world. H. trionum is a warm season annual that grows to around two feet tall and has the habit of a sprawling, decumbent vine; an upright, many-branched mound; or something in-between. Its leaves are alternately arranged and three-lobed with coarsely toothed margins. The flowers are solitary and borne in the axils of leaves. They are creamy white to pale yellow with a purple-brown center, and are both cross- and self-pollinated.

flower of an hour (Hibiscus trionum)

Flowering occurs on sunny days throughout the summer. The ephemeral flowers promptly produce a balloon-shaped seed capsule that is hairy and papery with prominent purple veins. Once mature, the capsules split open at the top to reveal five compartments lined with brown to black, kidney- or heart-shaped seeds. Every part of this plant is attractive and interesting to look at, which is why it is no surprise that it is welcome in many flower beds.

Seed capsule of flower of an hour (Hibiscus trionum)

Sites that are in full sun with fertile soil and regular moisture are sought after by flower of an hour. Less fertile soils are still prone to invasion. As with many weeds, disturbance is key, so it is often found in agricultural fields, rangelands, along roadsides, and in vacant lots and construction sites. Its presence in natural areas is a result of escaping from garden beds, agricultural fields, etc.

When we choose to grow plants that have a history of escaping into natural areas, we should be aware of both our proximity to natural areas and the dispersal mechanisms of the plants. Exotic plants that reproduce reliably and prolifically by seed, such as flower of an hour, should be considered unsuitable for gardens that are adjacent to natural areas.

This is because many popular ornamental plants have become invasive in the wild. Plants that are perfectly welcome in our gardens manage to find suitable habitat in natural areas, potentially threatening the livelihood of native plants and/or altering ecological processes such as fire regimes. An example of this where I live is bachelor’s buttons (Centaurea cyanus), which has escaped from gardens and invaded the Boise Foothills. While the impact of this invasion is not well-studied, the speed at which this plant has spread is disconcerting. Even more disconcerting is the fact that seeds of this and other European and Asian species are commonly found in “wildflower” seed mixes distributed throughout North America.

While I am sympathetic towards weeds, I also see them as one of the best reminders of the impacts that humans can have on the planet. They are clear indicators that every step we take has consequences. We should be mindful of this, and we should continue to have the tough conversations that issues like weeds and their impacts encourage us to have. There are no easy answers, but the dialogue must go on. Because all of us – gardeners and non-gardeners/ecologists and non-ecologists alike – generally have an opinion about weeds, they seem like a pretty good place to start.

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Quote of the Week:

From the book Invasive Plant Medicine by Timothy Lee Scott

The nature of a weed is opportunistic, and we, as humans, have created enormous holes of opportunity for these plants to fill. They have adapted to be at our side, waiting for those favorable times to cover the exposed soils that we continually create. With ever-changing genetics of form, function, and transmutation, weeds have evolved to withstand the punishments that humans unleash upon them.