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They can exert strong effects even when they are present in low concentrations. They tend to be small molecules. They exert their effects on the same cells that produce them. They only function early in development.
It tends to lose a proton and become negatively charged, because the cell interior has a high pH. It activates expansin proteins. It stimulates an increase in proton pumping. It tends to gain a proton and become uncharged, because the cell interior has a low pH.
Myb proteins must be activated and alpha-amylase transcription increased. Water must flow out of the cell, and the cell wall must become more extensible. Water must flow into the cell, and the cell wall must become more extensible. The auxin influx and efflux carriers must both be activated.
Light and gravity increase polar transport of auxin. Light and gravity receptors increase auxin concentrations throughout the plant. Light destroys auxin. Light and gravity receptors cause auxin to move laterally in the tips of shoots and roots.
ABA induces a transcription activator, while GA induces a trancription repressor. ABA induces a transcription repressor, while GA induces a transcription activator. Both hormones can bind directly to the gene's promoter. ABA decreases translation of the alpha-amylase mRNA; GA increases it.
If roots are dry, ABA concentrations in leaves increase. If roots are dry, ABA concentrations drop drastically. If roots are given sufficient water, guard cells close anyway. If roots are dry, guard cells begin to close---even though leaves are not experiencing water stress
