Auxin/Cytokinin Antagonistic Control of the Shoot/Root Growth Ratio and Its Relevance for Adaptation to Drought and Nutrient Deficiency Stresses

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.

Fertilizers and Fertilization Strategies Mitigating Soil Factors Constraining Efficiency of Nitrogen in Plant Production

“…The first variable is inextricably linked with the plant’s life cycle. Generally speaking, the growth of the root system as an integral part of the shoot system, is a result of both organs functional interdependences [ 67 , 68 ]. Maintaining a stable but temporary balance between the supply of water and nutrients to the shoot by the roots and the return supply of assimilates to the roots form the shoots is the basis that determines the plant growth rate, development of yield components, and yield [ 69 ].…”

Possibility of Vegetable Soybean Cultivation in North Europe

“…The longer plants in the Latvian trials developed if grown by direct sowing in the field (on average for all trial years 0.64 m) in comparison to transplanted plants (on average for all trial years 0.56 m). This can be explained by disrupted cytokinin/auxin balance by transplanting, which results in the increase of cytokinin concentration in the shoots, promoting shoot growth in the transplanted variant [38].…”

Killing two birds with a single stone—genetic manipulation of cytokinin oxidase/dehydrogenase (CKX) genes for enhancing crop productivity and amelioration of drought stress response

“…Additionally, the possibility to alter the endogenous cytokinin levels through genetic manipulation of genes associated with cytokinin homeostasis might be useful in minimizing the effects of various environmental stresses and nutrient deficiencies ( Jameson and Song, 2020a ). The cytokinin group of phytohormones is involved in the regulation of multiple aspects of plant growth and development ( Kieber and Schaller, 2018a ), many of which have direct implications for crop improvement, such as the regulation of root-shoot architecture ( Jing and Strader, 2019 ; Kurepa and Smalle, 2022 ), regulation of inflorescence meristem activity and seed yield output ( Ashikari et al, 2005 ; Bartrina et al, 2011a ; Schwarz et al, 2020a ), regulation of leaf senescence and photosynthesis ( Hönig et al, 2018 ), nutrient uptake ( Gao et al, 2019 ; Kurepa and Smalle, 2022 ) and responses to biotic and abiotic stresses ( Cortleven et al, 2019 ; Liu et al, 2020b ; Li et al, 2021b ; Salvi et al, 2021 ). Thus, there are clear implications of endogenous cytokinin level manipulation in altering plant architecture for the development of robust stress-tolerant crop varieties having optimum root-shoot architecture along with high yield output ( Jameson and Song, 2020a ).…”

“…Under optimum environmental conditions having plenty of water and minerals, growth of shoot is promoted and root growth is maintained at a level sufficient to sustain shoot growth while under water and mineral deficient conditions extensive root growth is promoted to trap more water and minerals and shoot growth is restrained to avoid excessive water loss and mineral depletion. This root-shoot architecture is majorly controlled by the antagonistic effects of two phytohormones auxin and cytokinins ( Kurepa and Smalle, 2022 ). Cytokinins directly repress the formation and development of lateral root primordium and negatively regulate the transcription of genes encoding PIN transporter proteins, effectively disrupting PIN-dependent auxin maxima required for lateral root development ( Jing and Strader, 2019 ).…”

“…Cytokinins are also known to negatively regulate nutrient uptake by repressing the expression of micronutrient transporter genes for several minerals such as zinc ( Gao et al, 2019 ), nitrate ( Kiba et al, 2011 ), sulfate ( Maruyama-Nakashita et al, 2004 ), phosphate ( Martín et al, 2000 ) and, iron ( Séguéla et al, 2008 ). Cytokinins also negatively affect plant’s response to water deficiency and drought stress through a plethora of diverse mechanisms including promotion of shoot growth and inhibition of root growth, increased stomata density and transpiration ( Farber et al, 2016 ), direct interference with ABA-mediated stomatal closure responses and repression of ABA-inducible stress-related genes ( Salvi et al, 2021 ; Kurepa and Smalle, 2022 ). Increased cytokinin signaling in type B ARR1 gain of function mutant indicated that cytokinins induce the expression of ribosomal protein genes RPL4A and RPL4D which resulted in an increase in the overall rate of global protein synthesis in Arabidopsis culminating in growth inhibition and hypersensitivity to osmotic stress which can be reversed by ABA treatment ( Karunadasa et al, 2020 ).…”