Abstract
Roots typically grow downward into the soil where they anchor the plant and take up water and nutrients necessary for plant growth and development. While the primary roots usually grow vertically downward, laterals often follow a gravity set point angle that allows them to explore the surrounding environment. These responses can be modified by developmental and environmental cues. This review discusses the molecular mechanisms that govern root gravitropism in flowering plant roots. In this system, the primary site of gravity sensing within the root cap is physically separated from the site of curvature response at the elongation zone. Gravity sensing involves the sedimentation of starch-filled plastids (statoliths) within the columella cells of the root cap (the statocytes), which triggers a relocalization of plasma membrane-associated PIN auxin efflux facilitators to the lower side of the cell. This process is associated with the recruitment of RLD regulators of vesicular trafficking to the lower membrane by LAZY proteins. PIN relocalization leads to the formation of a lateral gradient of auxin across the root cap. Upon transmission to the elongation zone, this auxin gradient triggers a downward curvature. We review the molecular mechanisms that control this process in primary roots and discuss recent insights into the regulation of oblique growth in lateral roots and its impact on root-system architecture, soil exploration and plant adaptation to stressful environments.
Highlights
As sessile organisms, plants have to use directional cues within their environment to grow towards areas that allow them to best perform their primary functions, which include gas exchange, photosynthesis, and reproduction for shoots, and anchorage, water and nutrients uptake for roots.Examples of such cues include gravity, direction of incident light, gradients in water, ions, oxygen, chemicals, and temperature, and impedance to growth, to cite only a few.Gravity is a uniform directional cue on Earth, and acquisition by higher plant roots of rapid mechanisms allowing its detection and use as a growth guide contributed to their successful colonization of land ecosystems [1]
We will briefly summarize what we know about the molecular mechanisms that contribute to auxin transport in angiosperm roots
Recent advances in our understanding of the molecular mechanisms that modulate the various phases of root gravitropism have resulted from careful genetic and cell biological investigations of the process in multiple model organisms
Summary
Plants have to use directional cues within their environment to grow towards areas that allow them to best perform their primary functions, which include gas exchange, photosynthesis, and reproduction for shoots, and anchorage, water and nutrients uptake for roots. Accumulation of nutrients such as phosphorus in upper layers of the soil, will promote an increase in lateral root GSA leading to a radial expansion of the root system and allowing better exploration of the upper layers of the soil These amazing root-system behaviors are important in agriculture as they allow plants to better adapt to the rapid changes that typically occur in their environments, taking maximal advantage of the resources available to them in close proximity. This brief and necessarily incomplete description of plant gravitropic responses illustrates the multiplicity of plant organ responses to this directional cue, emphasizing the importance of developmental and/or environmental parameters in guiding organ interpretation of the gravity signal. We briefly summarize our understanding of the mechanisms that govern signal transmission to the EZ and curvature response
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