As sessile organisms, plants spend their entire lives at the site of seed germination. Consequently, they require a suite of strategies to survive very diverse environmental stresses. Part of this plasticity relies on the ability of most plant organs to grow in directions that are dictated by specific cues from the environment, seeking out better conditions to fulfill their primary functions. Typical guidance for the growth of plant organs is provided by gravity, light, touch, gradients in humidity, ions, oxygen, and temperature. Such directional growth, defined by vectorial stimuli, is called a tropism and is believed to significantly contribute to plant survival.
The concept of tropism was introduced 200 years ago, when Knight (1806) postulated that a plant’s perception of gravity might modulate its ability to direct shoots to grow upward and guide roots downward. Eighty years later, Darwin (1880) made seminal contributions to the field by documenting a wide array of tropic responses and identifying regions of the root and shoot specialized for the perception of light and gravity. He also predicted the existence of auxin by proposing the presence of a plant growth regulator (hormone) whose gravity-induced redistribution across the tip of an organ might signal differential growth.
Since these discoveries, our analysis of tropic growth has expanded to include measurements of responses to light, touch, and gradients in humidity, ions, chemicals, and oxygen. However, only recently have the data converged to provide a picture of the hysiological, molecular, and cell biological processes that underlie plant tropisms. Thus, the last few years have witnessed a true renaissance in the analysis of tropic response, mainly driven by the marrying of modern tools and strategies in the fields of forward and reverse genetics, biochemistry, cell biology, expression profiling, and proteomics, to a very careful analysis of the growth process itself.
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