The shifting paradigms of auxin biosynthesis

The Yin-Yang of Hormones: Cytokinin and Auxin Interactions in Plant Development

“…Intriguingly, the YUCCA and TAA genes display discrete, localized expression patterns, suggesting that in at least some cases, auxin biosynthesis is highly localized, which likely helps generate the precise spatial distributions of auxin involved in various developmental processes. There is evidence for other pathways of auxin biosynthesis, in particular a Trpindependent pathway, which may play a prominent role in some tissues (Tivendale et al, 2014). IBA is also a source of auxin as it can be converted to IAA via b-oxidation in the peroxisomes and likely serves as a source of auxin for specific processes (Strader and Bartel, 2011).…”

Auxin Biosynthesis

“…Auxin biosynthesis can be divided into two general categories: de novo auxin biosynthesis and the release from auxin conjugates [see recent reviews (Normanly, 2010;Ludwig-Muller, 2011;Mano and Nemoto, 2012;Brumos et al, 2013;Ljung, 2013;Zhao, 2013;Tivendale et al, 2014)]. Indole-3-acetic acid (IAA), the main natural auxin in plants, exists in both free and conjugated forms.…”

“…Because the other pathways are less well defined and they have been reviewed extensively elsewhere (Woodward and Bartel, 2005;Zhao, 2010;Brumos et al, 2013;Korasick et al, 2013;Tivendale et al, 2014), I will not elaborate further on those alternative pathways. In this chapter, I discuss the genetic and biochemical data that led to the establishment of the first complete Trp-dependent auxin biosynthetic pathway.…”

Tryptophan-independent auxin biosynthesis contributes to early embryogenesis in Arabidopsis

“…Although genetic and metabolic studies have clearly established the importance of the Trp-dependent IAA biosynthetic pathway, IAA has also been proposed to be synthesized from IGP and/or indole through a Trp-independent pathway, which remains to be substantiated from genetic, biochemical, and functional studies (9)(10)(11). In this study, we demonstrate that the cytosol-localized INS is a key component in the long-speculated Trp-independent IAA biosynthetic pathway and contributes significantly to the establishment of the apical-basal pattern.…”

“…The TRYPTOPHAN AMINOTRANSFERASE OF ARABIDOPSIS (TAA)/YUCCA (YUC) linear pathway has been considered as a predominant Trp-dependent auxin biosynthetic pathway (4,(6)(7)(8). However, labeling studies and analyses of Trp auxotrophic mutants have long predicted the existence of a Trp-independent IAA biosynthetic pathway (9)(10)(11)(12)(13). When the Arabidopsis and maize seedlings were fed with isotope-labeled precursor, IAA was more enriched than Trp, and the incorporation of label into IAA from Trp is low, suggesting that IAA can be produced de novo without Trp as an intermediate (11,12).…”