Plants’ Sense of Self

Plants need more than gravity to grow straight

Plants’ Sense of Self

Plants, we know, are capable of modest movements, despite being fixed where they stand. They may lean toward a sunny window, and generally grow up, away from the force of gravity. However, a new study has shown that gravity is not the only factor plants take into consideration as they grow: they are also aware of the curve of their own branches and stems, and maintaining correct posture is achieved by balancing the two. This relationship is a quantifiable trait that could have practical applications for agriculture, forestry, and even a changing climate.

Plants, we know, are capable of modest movements, despite being fixed where they stand. They may lean toward a sunny window, and generally grow up, away from the force of gravity. However, a new study has shown that gravity is not the only factor plants take into consideration as they grow: they are also aware of the curve of their own branches and stems, and maintaining correct posture is achieved by balancing the two. This relationship is a quantifiable trait that could have practical applications for agriculture, forestry, and even a changing climate.

 

A time-lapse video of a plant growing, from a tiny green shoot pushing up through the moist, dark soil, to the bursting open of a blossom, is always a small joy to behold. Seeing something we generally view as static suddenly take on such frenetic movement is quite wondrous. But plants do move, in more ways than just up, and they’re doing it all the time. Good posture is as important for plants as it is for people, allowing them to reach the light they need and ensuring a stable footing, wherever they may grow. And, although too slow for us to see, plants are in constant, active motion, fighting against forces like wind or gravity and correcting their position to stay straight.

A study by Bastien et al., which was selected for the cover of the January 8 issue of the journal PNAS, has helped to reveal how, exactly, this movement is controlled. The authors, a multidisciplinary group of French researchers, have expanded on the traditional view that sensing the direction of gravity is sufficient for plants to send their roots toward it and their stems away from it, yielding happily upright geraniums, tomatoes or oaks. Although its own roots can be traced back to the 19th century, the “sine law”, a mathematical formula describing plants’ gravity-sensing ability, fails to represent correctly all conditions observed in experiments. The new work reveals that plants must also sense themselves, their own curvature, and take both factors into account in equal measure – a finding that could have important applications in agriculture and the forestry industry.

Successive images of a plant placed horizontally curving to grow upright.
Successive images of a growing Arabidopsis thaliana plant
straightening out after being placed horizontally.
©INRA / R. Bastien; S. Douady; B. Moulia
 

As it grows, a plant can change its shape – the angle or curve of a stem, say – in response to some external stimulus, like light. These movements are called tropisms. The study of gravitropism, movement in response to gravity, was pioneered by Charles Darwin and his son Francis, among others. Over time, certain cellular and molecular processes behind this phenomenon have been identified: statocysts, for example, are cells that use the complex motion of intercellular structures called statoliths to detect gravity. But on the level of a whole organ, it remained unclear how the tropic movement of plants was coordinated.

Researchers at France’s National Institute for Agronomic Research (INRA) and the University of Paris-Diderot, worked with 11 species of flowering plants, from tiny wheat sprouts to poplar trunks, representing a broad range of types, habitats and uses. The plants were grown to a given developmental stage before being placed in a dark room, tilted, clamped in a horizontal position and left to their own devices under these strange, new conditions.

Through detailed analysis of time-lapse photos and measurements of the curving and decurving of plants as they adjusted their growth to their environment, the team arrived at a mathematical model that very accurately describes plant growth across two orders of magnitude. What’s special about this new model is the way it reveals that, in addition to gravity, plants also use something long considered the reserve of humans and other animals: a sense of self. Proprioception, as it is called, allows plants to sense their own curvature and modify it as needed. The relationship between proprioception and graviception is fine-tuned by a plant, according to the size of the growing organ in question. The ratio of the former to the latter gives what the authors call the bending number, B, which determines the dynamics of a plant’s movement and predicts both the trajectory and the final steady-state it will eventually reach.

What molecular methods plants employ to control this balance between sense of self and gravity will open up a new area of investigation. A very practical benefit of the B number is that it represents a quantitative genetic trait. From now on, simple measurements will make it possible to quickly analyze large numbers of variants within a species. The considerable genetic variability already observed for the value of B could be seized upon, in the hope of improving harvests or producing straighter trunks of higher quality wood. Even predicting the consequences of climate change could find an application for this new understanding of plant senses, as more extreme weather may bring higher winds that challenge our crops and forests even more. An array of applications exists, stemming from the knowledge that plants, in fact, know themselves.