Abstract
The hormones auxin and cytokinin regulate numerous aspects of plant development and often act as an antagonistic hormone pair. One of the more striking examples of the auxin/cytokinin antagonism involves regulation of the shoot/root growth ratio in which cytokinin promotes shoot and inhibits root growth, whereas auxin does the opposite. Control of the shoot/root growth ratio is essential for the survival of terrestrial plants because it allows growth adaptations to water and mineral nutrient availability in the soil. Because a decrease in shoot growth combined with an increase in root growth leads to survival under drought stress and nutrient limiting conditions, it was not surprising to find that auxin promotes, while cytokinin reduces, drought stress tolerance and nutrient uptake. Recent data show that drought stress and nutrient availability also alter the cytokinin and auxin signaling and biosynthesis pathways and that this stress-induced regulation affects cytokinin and auxin in the opposite manner. These antagonistic effects of cytokinin and auxin suggested that each hormone directly and negatively regulates biosynthesis or signaling of the other. However, a growing body of evidence supports unidirectional regulation, with auxin emerging as the primary regulatory component. This master regulatory role of auxin may not come as a surprise when viewed from an evolutionary perspective.
Highlights
Adaptive regulation of the shoot/root growth ratio is an evolutionarily conserved developmental mechanism in terrestrial plants, which ensures maximal progeny production under fluctuating environmental conditions
This review outlines the antagonistic roles played by cytokinin and auxin in controlling the shoot/root growth ratio and focuses on the link between this antagonism and the adaption to drought stress and nutrient deficiency, two of the main environmental challenges for terrestrial plants
Research on potato stem single-node cuttings revealed that whereas cy-that cytokinin study with the aquatic plants Lemna gibba and Spirodela polyrhiza revealed tokinin treatments increased, auxin treatments decreased the shoot/root growth ratio treatment promoted frond expansion and duplication while suppressing root. These conclusions reached by In analyses of cytokinin-treated and transgenic lines revealed that elongation addition, research on potatoplants stem single-node cuttings with altered cytokinin content were confirmed by cytokinin receptor mutant studies
Summary
Adaptive regulation of the shoot/root growth ratio is an evolutionarily conserved developmental mechanism in terrestrial plants, which ensures maximal progeny production under fluctuating environmental conditions. In growth conditions in which water and mineral nutrients are not limiting, shoot growth is favored as it embodies traits that support reproductive success and survival of the species, and root development is limited to a level sufficient to sustain shoot development without the unnecessary depletion of photosynthates [1]. Under optimal water and nutrient conditions, plants are predicted to have a high shoot/root ratio [1,2]. This review outlines the antagonistic roles played by cytokinin and auxin in controlling the shoot/root growth ratio and focuses on the link between this antagonism and the adaption to drought stress and nutrient deficiency, two of the main environmental challenges for terrestrial plants.
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