Auxin biology in roots

Auxin regulates almost every aspect of plant growth and development. Its intracellular concentration is controlled by biosynthesis and degradation. In addition, there is an “auxin pool” that consists of the conjugates with sugars, amino acids, and peptides. Some of the conjugates reversely release auxin, enabling alternative methods to regulate auxin concentrations. Auxin concentrations are also affected by transport. Besides the long distance delivery through the phloem, auxin is transported across the cell by influx and efflux carriers, from the shoot to root with maximum concentration at the root tip. At the root tip, the auxin flow reverses, and shootward auxin transport occurs. An auxin gradient formed this way is indispensable for proper development, maintenance of the meristem, and cell identity. The formation of root hairs is auxin-dependent. Auxin controls not only the initiation of root hairs but also regulates their elongation. In Arabidopsis thaliana, auxin accumulates in atrichoblasts and it is supplied to trichoblasts. Before hair initiation, randomization of cortical microtubule arrays is observed in lettuce seedlings. This action is promoted by auxin and is indispensable to hair formation. Furthermore, light promotes CMT randomization and root hair initiation via auxin signaling. Ethylene is another promoter of root hair formation. Ethylene affects auxin signaling and vice versa. Interactions between these hormones are synergistic for root growth inhibition but antagonistic for lateral root formation. Reactive oxygen species also regulate various responses in plants. They play an important role during root hair elongation, although their precise relationship with auxin is yet not clear.