Auxin conjugates: their role for plant development and in the evolution of land plants

Abstract

Auxin conjugates are thought to play important roles as storage forms for the active plant hormone indole-3-acetic acid (IAA). In its free form, IAA comprises only up to 25% of the total amount of IAA, depending on the tissue and the plant species studied. The major forms of IAA conjugate are low molecular weight ester or amide forms, but there is increasing evidence of the occurrence of peptides and proteins modified by IAA. Since the discovery of genes and enzymes involved in synthesis and hydrolysis of auxin conjugates, much knowledge has been gained on the biochemistry and function of these compounds, but there is still much to discover. For example, recent work has shown that some auxin conjugate hydrolases prefer conjugates with longer-chain auxins such as indole-3-propionic acid and indole-3-butyric acid as substrate. Also, the compartmentation of these reactions in the cell or in tissues has not been resolved in great detail. The function of auxin conjugates has been mainly elucidated by mutant analysis in genes for synthesis or hydrolysis and a possible function for conjugates inferred from these results. In the evolution of land plants auxin conjugates seem to be connected with the development of certain traits such as embryo, shoot, and vasculature. Most likely, the synthesis of auxin conjugates was developed first, since it has been already detected in moss, whereas sequences typical of auxin conjugate hydrolases were found according to database entries first in moss ferns. The implications for the regulation of auxin levels in different species will be discussed.

Comprehensive Tissue-Specific Transcriptome Analysis Reveals Distinct Regulatory Programs during Early Tomato Fruit Development

“…Cluster 5 contained genes expressed in the seed coat, embryo, and endosperm and had an overrepresentation of genes in the hormonal function category, particularly genes related to auxin and GA activity. These included several members of the AUXIN (AUX)/INDOLE-3-ACETIC ACID (IAA) family of transcriptional regulators and several Gretchen Hagen (GH3) family genes, which encode enzymes putatively involved in auxin conjugation (Ludwig-Müller, 2011). Genes in cluster 6 were predominantly expressed in seed tissues but also showed relatively high expression in other fruit tissues, except the pericarp.…”

Endogenous Auxin Profile in the Christmas Rose (Helleborus niger L.) Flower and Fruit: Free and Amide Conjugated IAA

“…Amide conjugates with Val, Phe, Leu, Gly, and Ala were accumulated at a lower rate, mostly in the seeds (from 0.4 to 3 pmol/g FW), and some of them in the pericarps (Table 1). IAA-Asp was the first detected amide conjugate of IAA in pea seedlings (Andreae and Good 1955) and was the predominant form in most dicotyledonous plants (see for review Ludwig-Müller 2011). It has been postulated that IAA conjugated to Asp (and possibly Glu) is an irreversible catabolite.…”

“…Different plant species have distinct IAA conjugate profiles. In general, monocots accumulate mostly ester conjugates, whereas dicots preferably accumulate amide conjugates (see for review Bajguz and Piotrowska 2009;Ludwig-Müller 2011). Studies of the metabolism and the physiological role of auxins in reproductive development have been focused on commercially interesting species such as maize (Jensen and Bandurski 1994), pea (Magnus and others 1997), bean (Bialek and Cohen 1989), tomato (Epstein and others 2002), and wheat (Hess and others 2002).…”

Regulation of Auxin Homeostasis and Gradients in Arabidopsis Roots through the Formation of the Indole-3-Acetic Acid Catabolite 2-Oxindole-3-Acetic Acid

“…However, our understanding of these processes and their relative importance remains rudimentary. It is known that auxin can be conjugated to amino acids and sugars to either storage forms or to irreversible degradation products, and several genes involved in IAA conjugation and deconjugation have been identified (reviewed in Ludwig-Müller, 2011;Korasick et al, 2013). Data from our laboratory and others have demonstrated that oxidation of IAA to 2-oxindole-3-acetic acid (oxIAA) is another important pathway for IAA degradation in plant species such as Arabidopsis (Östin et al, 1998;Kowalczyk and Sandberg 2001;Kai et al, 2007;Novák et al, 2012;Peer et al, 2013), maize (Zea mays; Reinecke and Bandurski, 1983), and Pinus sylvestris (Ernstsen et al, 1987).…”