Advances in the role of auxin for transcriptional regulation of lignin biosynthesis

Lignin is a natural polymer interlaced with cellulose and hemicellulose in secondary cell walls (SCWs). Auxin acts via its signalling transduction to regulate most of plant physiological processes. Lignification responds to auxin signals likewise and affects the development of anther and secondary xylem in plants. In this review, the research advances of AUXIN RESPONSE FACTOR (ARF)-dependent signalling pathways regulating lignin formation are discussed in detail. In an effort to facilitate the understanding of several key regulators in this process, we present a regulatory framework that comprises protein-protein interactions at the top and protein-gene regulation divided into five tiers. This characterises the regulatory roles of auxin in lignin biosynthesis and links auxin signalling transduction to transcriptional cascade of lignin biosynthesis. Our works further point to several of significant problems that need to be resolved in the future to gain a better understanding of the underlying mechanisms through which auxin regulates lignin biosynthesis.