Tag Archives: roots

A 36-hour timelapse of slow gravitropism (left; a fern) and fast gravitropism (flowering plant). Credit: IST Austria, Yuzhou Zhang.

How plants evolved to follow gravity

It’s common sense that plants have root systems that grow downward, following gravity. However, it hasn’t always been like this. Plant-life first evolved in water and only began spreading to land around 500 million years ago. Although the evolutionary origin of the mechanism of gravity-induced root growth — called gravitropism — remains a mystery, scientists have now uncovered new insights that offer a broader view of how and when gravitropism evolved.

A 36-hour timelapse of slow gravitropism (left; a fern) and fast gravitropism (flowering plant). Credit: IST Austria, Yuzhou Zhang.

A 36-hour timelapse of slow gravitropism (left; a fern) and fast gravitropism (flowering plant). Credit: IST Austria, Yuzhou Zhang.

The researchers at the Institute of Science and Technology of Austria analyzed various plant species, representing various lineages from the more primitive mosses, ferns, lycophytes, to the more modern seed plants (gymnosperms and flowering plants). The roots of each type of plant were forced to grow horizontally so that the researchers could observe if and when the roots would bend downwards to follow gravity.

Mosses and other rudimentary types of plants turned out to have a very slow gravity response. However, gymnosperms and flowering plants, which first appeared around 350 million years ago bent downward much faster, thereby exhibiting a more efficient form of gravitropism.

Simplified schematic of plant evolution. Credit: IST Austria.

Simplified schematic of plant evolution. Credit: IST Austria.

By analyzing the distinct phases of gravitropism, the researchers led by Yuzhou Zhang, a postdoc at IST Austria, identified two crucial components. One is represented by amyloplasts — plant organelles filled with starch granules — which function as a sort of gravity sensor. This particular component was particularly evident in gymnosperms and flowering plants which have amyloplasts concentrated in the very bottom of the root tips. In contrast, amyloplasts in ferns, clubmosses, and firmosses are randomly distributed within and above the root tip.

Amyloplast perception is then signaled from cell to cell by the second gravitropism component: the growth hormone auxin. Genetic experiments on Arabidopsis thaliana, a small flowering plant native to Eurasia and Africa and a common model plant used in research, revealed that a transporter molecule called PIN2 directs auxin flow.

Almost all green plants produce PIN proteins, but it’s only in seed plants that PIN 2 molecules gather at the shoot-ward side of the root system. This unique configuration to seed plant plants enables them to transport auxin towards the shoot, allowing the growth hormone to travel from the place of gravity perception to that of growth regulation.

Amyloplasts are filled with strach granules (black dots). The organelles are seen her in the root of a fern (left) and that of a seed and flowering plant (right). In the latter, the amyloplasts gather at the very bottom of the root tip, enabling more efficient gravitropism. Credit: IST Austria.

Amyloplasts are filled with starch granules (black dots). The organelles are seen her in the root of a fern (left) and that of a seed and flowering plant (right). In the latter, the amyloplasts gather at the very bottom of the root tip, enabling more efficient gravitropism. Credit: IST Austria.

Beyond gaining a better understanding of how plants sense and follow gravity for optimal growth, the researchers believe that their findings could also be of practical significance.

“Now that we have started to understand what plants need to grow stable anchorage in order to reach nutrients and water in deep layers of the soil, we may eventually be able to figure out ways to improve the growth of crop and other plants in very arid areas,” Zhang said in a statement, adding that: “Nature is much smarter than we are; there is so much we can learn from plants that can eventually be of benefit to us.”

The findings were reported in the journal Nature Communications

Plant soil.

What is soil? Here’s the inside scoop

Ground beneath your feet — all planets have it (yes even gas giants). But the one on our Earth is a little different from those in other places. So, what exactly is soil, and what makes ours stand out?

Plant soil.

Image via Pixabay.

Soils are actually very complex things. They’re a mix of solid material — both organic and inorganic — with fluids such as water and air, with a helping of living organisms thrown in to tie everything together. It’s a very dynamic material, one that responds to factors such as climate, terrain, biology, and human activity to name a few. It’s also continuously developing. Depending on where on Earth you look, and when, soils take on unique properties. It’s so complex, in fact, that soils are treated as ecosystems unto themselves.

However, life as we know it would be impossible without soil. So, everybody, grab your spades and let’s take a look at what exactly soil is.

The most bare-bones definition of soil is that it is a mixture of sand, clay, silt, and humus (not the tasty spread). In more geologically-savvy talk, soil is the unconsolidated mineral and/or organic material that forms on the immediate surface of the Earth. According to the Soil Science Society of America Glossary of Soil Science Terms, soil is also “subjected to and shows effects of genetic and environmental factors of: climate (including water and temperature effects), and macro- and microorganisms, conditioned by relief, acting on parent material over a period of time.” In other words, soils represent a mixture of geology and biology, and they are, quite literally, the substrate that fosters life on dry land.

The inorganic parts

 

Sand.

Sand is not readily carried by water.
Image credits Andrew Martin.

The ‘geology’ part includes the inorganic elements of soil — mainly ground-up, weathered rocks and minerals. Weathering are the mechanical or chemical processes through which rocks are broken down into smaller pieces. These rocks and minerals are referred to as the soil’s “parent material“. This inorganic fraction represents around 90% by volume of all the solid material in the mix.

Soil texture is mainly produced by the ratios of different types of inorganic material in the soil. We generally classify these by the size of their particles. The three main types are sand, silt, and clay.

Sand is the coarsest of the three, with particles ranging in diameter from 0.0625 mm to 2 mm. Soils that contain more than 85% sand-sized particles by weight are called sand. Its exact composition varies from place to place, but generally, sand is made up of silica (quartz).

Particles in sand are pretty loose and coarse which creates a lot of open spaces for water to flow through. This makes sand very good at draining and filtering water. Its lack of organic material and inability to hold water, however, makes it a very poor environment for plants to grow in.

Silty water.

Silt is easily mixed with water because of how light its particles are, and can be carried over great distances.
Image credits Andrew Martin.

Silt is an intermediate texture, with particles ranging between 0.0039 mm and 0.0625 mm in diameter. To give you a rough idea, dry silt feels like coarse flour in your hands. Add water to it, and it would feel smooth, almost silky to the touch, without being sticky. Silt is much better than sand at holding in water. Due to its smaller-sized particles, it can be transported over long distances by wind (forming loess) or rivers. It is composed mainly of quartz and feldspar.

Silt an important component for fertile soils. Because of the size of its particles, it’s good at holding water for plants to absorb through their roots, but also creates enough open spaces for air pockets to form (more on that later). Smaller particles also mean they can be circulated around an environment to provide fresh minerals for plants. The Nile River, for example, brings huge amounts of fresh silt, rich in organic material, to bear on its banks every time it floods; this has single handedly fed Ancient Egypt’s ascent in the middle of an inhospitable desert. It had such a central role to play in the Egyptians’ life that they adopted the black silt on the Nile’s banks as a symbol of rebirth.

Clay.

Clay can absorb a lot of water — and all that water increases its volume. Clay contracts and breaks apart when it dries and sheds those water molecules.

Clay is the finest type of material on the list. Its texture feels like that of eye shadow. While the exact point at which a particle is no longer considered silt varies depending on which discipline you’re asking, geologists usually consider particles under 2 micrometers to be clay. It’s usually made up of metallic oxides, which make clay quite colorful. These oxides are why clay-fired bricks are red, but clay can come in a wide range of colors.

Clays are made of wide, flat particles — which means that they have large surface areas compared to the other inorganic fractions. This makes them more chemically active than silt or sand and thus, better able to hold nutrients onto their surfaces. As such, clays can help make very fertile soils. Too much clay, however, can dry plants out — clay particles are good at capturing water but not very good at letting it go.

Overall, the inorganic components of soil mainly include silica, with iron and aluminum oxides. They mainly create soil’s texture and structure. Overall, it isn’t of much value to plants, although fractions like clay can transport nutrients.

The organic parts

Apple soil.

Image credits Annette Meyer.

Geology alone isn’t enough to produce soil. Mars, too, has a lot of ground-up, weathered rocks and minerals about its surface. So does the moon — but that’s just barren dirt. The components that set ‘soil’ apart from ‘dirt’, the bits that give it its unique pizzaz, are biological. Soil is pretty dense in organic material, which makes up around 10% of the volume of its solid components. The two main types of organic matter in soil are the living and the non-living fractions.

The latter improves soil’s ability to hold water, improves its nutritive value, and helps form stable aggregates (i.e. it ties everything together). It mostly originates from the microbial decomposition of animal or plant material (such as leaves, roots, soft tissues, or feces). This breakdown process recirculates nutrients through the soil to feed plants. Exactly how fast this material breaks down depends on how homey the soil is for bacteria. Higher decomposition rates occur in warm, moist conditions with good aeration, a favorable ratio of nutrients, with a pH (soil acidity) near neutral, and without pollutants or toxins.

Organic matter content in soil depends on how much of it is added versus how much of it is broken down over a given unit of time. Bacteria first munch on the juicier bits, such as sugars, starch, and certain proteins. More resistant compounds — such as the structural components in cell walls — are decomposed relatively slowly. These compounds, such as lignin and tannin, impart a dark color to soils containing a significant organic matter content. In general, the darker the soil, the more organic material it contains — so the better it is for growing plants.

Compost.

Compost is just organic material slowly breaking down into soil.
Image credits Manfred Antranias Zimmer.

Living organic matter includes the aforementioned bacteria, earthworms, and everything in between. Fungi, actinomycetes, algae (yes, there is algae in soil), and protozoa also make an appearance. These organisms all work together to make soil the complex ecosystem it is, but they can both support and hinder plant growth. They can help plants by decomposing organic matter, fixing nitrogen (which plants can’t do themselves), and by improving soil quality through aggregation, aeration, and drainage. On the other end of the spectrum, they can compete with plants for nutrients or munch on their roots.

Soil structure

Soil profile.

Its a layered dirt cake!
Image credits Colin Smith.

Soils have complex structures to go along with their complex makeup. Dig down through some and you’ll see it’s made of layers, which are also called ‘horizons’. Taken together, all of these horizons form a soil’s profile. As a rule of thumb, almost all soils have three major horizons, and most also have an organic horizon. In order, a soil’s profile can include:

  • An organic/humus horizon. This material is mostly made up of organic matter in various stages of decay. If you’ve ever walked through a thick forest in spring, you may have noticed that there’s a brown, wet, mushy layer of something between the fallen leaves of last year and the ground proper. That’s plant matter that is rapidly being turned into soil and is part of this organic horizon. This layer shows great variation between soils: it’s thin in some, thick in others, and completely absent in some cases.
  • Topsoil. This horizon is mostly made up of minerals from the soil’s parent material mixed in with a healthy heaping of organic matter. It’s a very cozy place, and most plants and animals that live in soil call this layer home.
  • The eluviated horizon. This is a blanket of gravel, sand, and silt particles of quartz or other particularly hardy materials left over after the clay, organic matter, and more soluble minerals have been washed away by water draining through the soil. It has to be noted that water does this to the above horizons as well, but those tend to be replenished through organic decomposition. The eluviated horizon isn’t seen in most soils but is quite often found in older soils and forest soils.
  • The subsoil. This part is packed with minerals and nutrients washed down from the above layers that get stuck here like coffee grounds onto filter paper.
  • Parent material. The inorganic ‘foundation’ from which the soil developed.
  • The bedrock. A solid mass of solid rocks (usually granite, basalt, quartzite, limestone, or sandstone). If the bedrock is close enough to the surface to weather, it can serve as the parent material for the above soil. But, make no mistake — this is rock. This bit isn’t considered soil.

A lot of this column, ideally, is empty air. Fertile, medium-grain soils can reach up to 50% porosity, and all this empty space is crucial for its health. The pores allow air and water to pool and (ideally) circulate through the soil, keeping both its flora and fauna alive.

Pore size (which is dictated by particle size) is important, too, as is the distribution of these pores. If a pore is too small, it traps water too tightly for plants to absorb. That’s why soils with a large clay content aren’t very good for agriculture. We till the land as a way to break it up and temporarily increase its porosity and permeability.

Pores that are too large just let water woosh right through, as we discussed earlier, which is just as bad for plants. Large pores that communicate (i.e. that are connected) with one another allow water and air to move freely through the soil, carrying nutrients around. However, smaller pores are better if you’re dealing with drought — they store water better.

As far as flora is concerned, pores can keep different populations of bacteria separate from one another. This is actually pretty good news since it keeps the strains from competing with one another directly. As such, soil porosity allows for multiple organisms that occupy the same ecological niche to coexist, which improves its resilience to shocks such as pollution or climate change.

It’s easy to take the soil under our feet for granted — after all, it’s just dirt in our soles. But hopefully, I’ve helped you see a glimpse of the living, breathing ecosystem that underpins all of the life on dry land, and most of the life on Earth. So the next time you visit the park, spare a moment to think of all the hard work that’s going on beneath your shoes — we wouldn’t be here without it.

Crops employ “austerity measures” to conserve water in drought conditions

A new study of plant roots found that grasses employ a type of “economic austerity” when confronted with drought conditions: the plants limit their root systems’ growth to preserve water in the soil. The discovery could potentially be used to improve crop yields.

Image credits Chris Devaraj

The world’s population has been growing rapidly over the past few decades, and this trend is not going to stop any time soon (see this and this.) The last thing you would want in this scenario is a shortage of food — which is exactly what scientist expect will happen. Seeing this, researchers from Carnegie Mellon University published a paper aiming to understand how agriculturally valuable plants react to drought.

Plants draw most of their water from soil, through their roots. However, not all plants have the same kinds of roots — the study examined grasses, a family which include key species of plants including maize, sorghum and sugarcane. Grasses rely on crown roots to extract water, a type of root unique to this family, which grow down from the regions of the shoot at soil surface (an area known as the crown, hence the name.) The root system starts to form after sprouting and continues to develop throughout the plant’s life.

Maize seedling with crown roots beginning to grow from the base of the shoot (red arrow).
Image credits Jose Sebastian

“Crown roots are like the lanes of a highway connecting the suburbs to the city. As the plant grows, new lanes are added to this highway to increase the flux of water and nutrients from the soil to the shoot,” explains Jose Sebastian, post-doctoral fellow at the Carnegie Institution for Science, and lead author of the study.

The effect of drought on crown root development was poorly documented up to now, so researchers had no way of estimating how the plants would react to a hotter and drier climate. The team, led by José Dinneny, was able to prove that water shortages causes the grasses to suppress crown root growth.

Their results show that the crown is crucial for sensing water availability in the topsoil. If water is scarce, the development of crown roots is suppressed and the grass plant maintains a more limited root system, the team found.

“We normally think about roots as providing access to water, thus it was initially unclear why a plant would shut down root growth under drought,” Dinneny explained.

“We discovered, however, that this response allows the plant to slow the extraction of water from soil and bank these reserves for the future; sort of like the plant version of economic austerity.”

These “austerity measures” are only employed when water is scarce. If moisture is reintroduced into the soil, crown root growth is quickly resumed, so the plant can take advantage of all available water. The team also determined that this suppression is much less pronounced in domesticated grasses such as maize and millet than in wild varieties.

“This suggests to us that plant breeding has unintentionally affected these crop plants’ abilities to cope with drought,” Dinneny said.

Artificial selection or agricultural plants such as maize or other grassy crops aimed at tailoring crown roots’ response to drought could improve these plants’ productivity and preserve ground-water resources.

The full paper, titled “Grasses suppress shoot-borne roots to conserve water during drought” has been published online in the journal PNAS.