Gravitropism in Lateral Roots

Ryan brought up a very good point when we were looking at root caps on Thursday -- does the root cap on a lateral root contain statoliths? Remember, statoliths are those starch-filled organelles thought to be involved in gravitropism, or the plant's ability to sense the earth's gravitational pull and respond accordingly (ie, roots go down, shoots go up). Statoliths are found in cells called statocytes.

In the germination videos we saw earlier, we see that lateral roots don't start growing straight down -- they tend to grow out away from the primary root. This makes sense, right? If all the roots responded in a similarly strong way to the pull of gravity, we'd have our whole root system growing straight down in a mass of parallel roots! Then the plant would be missing out on exploiting all those nutrient-rich pockets of soil that are located all around it -- it would only be taking advantage of the nutrients held in the soil directly below. So it would behoove a plant to direct its root growth in response to a more complex function of gravity + nutrient availability + water + avoidance of obstacles, not just gravity. (As always, it's more complicated than it seems at first glance!) And this is what we see in more detailed studies of root system development, though the primary root does seem to be mainly influenced by gravitropism. But the seemingly different growth of lateral roots has received less attention. In this paper, researchers examined the development of gravisensitivity in lateral roots of Arabidopsis.

While gravitropism has been extensively studied in the primary root, the response of lateral roots to gravitropic stimuli has poorly been described. It has been assumed that the molecular mechanisms are similar to what occurs in primary roots. However, this response is species-dependent: in some species lateral roots are unresponsive to gravity [15] while in others lateral roots grow at a set angle relative to the gravity vector, a response known as plagiogravitropism [16]. This indicates that lateral roots have an endogenous (genetic) programme for gravitropism that has been selected to optimize root system architecture depending on each species' ecological niche and lifecycle. (Guyomarc'h et al. 2012)

Gravisensitivity has been recorded in primary roots even prior to germination. What this study found was that, yes, lateral root caps do have statocytes and functioning statoliths, but the gravitropic response in lateral roots seems to be delayed when compared to primary roots, with the lateral roots seeing reorientation after ~12-24 hours.

An earlier study found that in Arabidopsis, some lateral roots are just "programmed" to grow horizontally rather than vertically. These lateral roots still showed gravitropism -- if you tilted the plant, the lateral roots would reorient to their original plane of growth -- but they grew at what researchers call a gravitropic set-point angle which was not straight down. You can think of this like a compass responding to the Earth's magnetic field -- the primary root grows south, while a lateral root might grow south-east. Going in either direction would require "evaluating" the earth's magnetic field, but the responses to that evaluation would be different. In plants, they're responding to gravity instead of magnetism, and not sending all of their roots in the same direction. Which makes sense -- if we're performing a search-and-rescue, we wouldn't send everyone out due north, would we? We'd send groups of searchers in a number of directions to increase our search area. Plants are doing the same thing, but searching for water and nutrients, and avoiding competitors and obstacles.

Pull up any plant and you'll see that root system development is dependent on a number of factors beyond simply up vs down. Here's a good article reviewing root system architecture.