A new research published in the recent issue of the American Journal of Botany tries to explain how plants sense gravity, and how they direct or signal their cells to grow in response to it.
This is a longitudinal view of an Arabidopsis primary root. Gravity sensing in roots occurs in the central columella cells of the root cap (inset), which contain dense, starch-filled amyloplasts. Scale bar = 100 μm (Blancaflor Elison B)
Gravity affects the ecology and evolution of every living organism. In plants, the general response to gravity is well known: their roots respond positively, growing down, into the soil, and their stems respond negatively, growing upward, to reach the sunlight
Although botanists understand a great deal about how this works, the study reviews what we know so far, from mechanical to genetic approaches; it reveals that there are still substantial gaps in our knowledge of the molecular details and highlights new ideas for potential regulating mechanisms.
“Although the process of gravitropism appears to be a simple plant response that we observe here on earth, the biological processes that control it are rather complex,” says study author Prof Elison Blancaflor of the Samuel Roberts Noble Foundation, Oklahoma.
In plants, the area where gravity is sensed is often spatially distinct from the area of growth. So how do these two discrete areas communicate with each other and direct the plant to where it should grow?
To date, gravity sensing in plants has been explained by the starch-statolith hypothesis. For example, in roots, gravity-sensing cells at the tip of the root contain dense, starch-filled organelles known as amyloplasts. Amyloplasts settle to the bottom of the cells in response to gravity, which then triggers the hormone auxin to move to another, distinct, area of cells and causes them to elongate and bend toward gravity. However, the molecular details of exactly how the physical movement and settling of amyloplasts in one set of cells triggers the accumulation of auxin in another, physically distant, set of cells in a plant remains a mystery.
The most prevalent hypothesis is that the cytoskeleton, or cellular scaffolding, plays a major role in this gravity-sensing, intercellular communication; the cytoskeleton is made up of filaments, consisting of the proteins actin or tubulin, that allow movement of materials along strands, such as is seen in meiosis or mitosis. However, there is a major controversy in the field regarding the role of actin in gravitropism primarily due to contradictory outcomes in studies where actin was inhibited.
Primary root of Medicago truncatula illustrating the process of gravity sensing and differential growth. In a vertically growing root, amyloplasts settle to the bottom part of the columella cell. Scale bar = 300 μm (Blancaflor Elison B)
The current study tackles this controversy by reviewing what we know regarding how amyloplasts work, what affects actin, and how recent genetic studies have discovered that proteins may regulate actin and therefore auxin distribution. For example, recent genetic work using the model plant, Arabidopsis, reveals potential mechanisms as to how the actin cytoskeleton connects the gravity sensing cells to auxin in the growing cells.
Although Prof Blancaflor specifically discusses, based on years of research, how one component of the plant cell, namely the cytoskeleton, controls the process of plant gravitropism, he notes that understanding gravitropism has important implications for agriculture as well.
“Information from basic studies on the cytoskeleton and how plants respond to gravity can inform and provide strategies for genetically engineering crop plants with improved root systems or overall plant architecture,” Prof Blancaflor concludes.
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Bibliographic information: Blancaflor Elison B. 2013. Regulation of plant gravity sensing and signaling by the actin cytoskeleton. American Journal of Botany 100(1): 143-152; doi: 10.3732/ajb.1200283
