Hormonal interactions are crucial for plant development. In Arabidopsis, cytokinins inhibit root growth through effects on cell proliferation and cell elongation. Here, we define key mechanistic elements in a regulatory network by which cytokinin inhibits root cell elongation in concert with the hormones auxin and ethylene. The auxin importer AUX1 functions as a positive regulator of cytokinin responses in the root; mutation of AUX1 specifically affects the ability of cytokinin to inhibit cell elongation but not cell proliferation. AUX1 is required for cytokinin-dependent changes of auxin activity in the lateral root cap associated with the control of cell elongation. Cytokinin regulates root cell elongation through ethylene-dependent and -independent mechanisms, both hormonal signals converging on AUX1 as a regulatory hub. An autoregulatory circuit is identified involving the control of ARR10 and AUX1 expression by cytokinin and auxin, this circuit potentially functioning as an oscillator to integrate the effects of these two hormones. Taken together, our results uncover several regulatory circuits controlling interactions of cytokinin with auxin and ethylene, and support a model in which cytokinin regulates shootward auxin transport to control cell elongation and root growth.

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Phytohormones, such as auxin and cytokinin, are known to be involved in the regulation of plant responses to salinity stress and counteract the adverse effect of stress conditions. This work investigated the effects of the exogenous spraying of indole-3-acetic acid (IAA) and kinetin (KIN) during the reproductive phase on grain yield by examining the 1000-grain weight and filledgrain percentage as well as the changes in starch, total soluble sugars, sucrose, glucose and fructose concentrations in the grains of two rice cultivars under salt stress. The results indicated that the applied IAA and KIN led to an increased grain yield, 1000-grain weight and filled-grain percentage for both rice cultivars under salt stress. The storage starch content in the grain of the salt-sensitive cultivar was more than that in the salt-tolerant cultivar under IAA application compared with KIN, whereas a decrease in the total soluble sugar content was observed with both IAA and KIN treatments, in comparison to the non-hormone treatment. Interestingly, this study showed that IAA led to a much higher increase in the sucrose content in grain, as compared to the KIN. Furthermore, this experiment suggests that glucose and fructose may play important roles during salt stress because there were clearly higher concentrations of these sugars in the grain of the stressed cultivars under IAA and KIN application: it appears that their accumulation was the earliest response detected during the grain-filling period in rice. Finally, this work indicated that an increase in the rice grain yield, 1000-grain weight and filled-grain percentage are associated with an increase in the contents of starch, sucrose, glucose and fructose in grain caused by the application of IAA and KIN.

Plants such as wheat and barley that are strategically important crops need to be considered to develop a comprehensive toxicity profile for nanoparticles (NPs). The present study was aimed to investigate the effects of chitosan and SiO2 NPs on wheat and barley plants. Two factorial experiments (seeds priming and direct exposure) were performed based on a completely randomized design in four replications. Results showed that the seeds priming with the NPs had not significant effect on germination parameters such as Germination Percentage (GP), Germination Rate (GR), Germination Value (GV), Mean Germination Time (MGT), Pick Value (PV) and Mean Daily Germination (MDG). In contrast, exposure of the seeds to the NPs had significant effects on these parameters. In both experiments, treatments had significant effects on shoot, seedling, root length, fresh and dry weight, as well as vigor indexes as compared to the control. In most traits, the best concentration of NPs was 30 ppm, whereas applications of the NPs with 90 ppm displayed adverse effects on majority of the studied traits. According to these results, selectivity in applications of NPs with suitable concentration and method is essential for different plant species.