2 - Auxins

Abstract

The plant hormone auxin is involved in the regulation of plant growth and development. Indole-3-acetic acid (IAA) was the first plant hormone identified and is the predominant form of auxin. Auxin is a small organic acid yet influences cell expansion and division, cell elongation and differentiation, and a variety of physiological responses, thus significantly affecting the final shape and function of cells and tissues in plants. Extensive genetic studies have revealed the pathways of auxin production, auxin transport, and auxin signaling. Auxin is mainly derived from the same biosynthetic pathway as tryptophan, and also from a tryptophan-independent pathway. It is produced particularly in shoot and root meristems and is transported long distance in the vasculature to other parts of the plant. Of particular significance is the role of auxin transporters known as PIN (PINOID) proteins that create auxin gradients within and between cells and help to establish long-distance polar transport pathways. Auxin signaling involves binding to F-box protein TIR1/AFB (TRANSPORT INHIBITOR RESPONSE/AUXIN SIGNALING F-BOX) together with a regulatory protein of the AUX/IAA family, forming a coreceptor complex. This leads to ubiquitination and degradation of the AUX/IAA protein with the release of ARF (AUXIN TRANSCRIPTION FACTOR) transcription factors, resulting in consequent changes in expression of target genes. These processes influence almost every aspect of plant development and growth. A comprehensive understanding of auxin regulation will facilitate the development of effective agricultural practices using auxin. In this chapter, we focus on auxin metabolism, auxin transport, and auxin signaling.