Abstract
Plants, bacteria and some fungi are known to produce indole-3-acetic acid (IAA) by employing various pathways. Among these pathways, the indole-3-pyruvic acid (IPA) pathway is the best studied in green plants and plant-associated beneficial microbes. While IAA production circuitry in plants has been studied for decades, little is known regarding the IAA biosynthesis pathway in fungal species. Here, we present the first data for IAA-producing genes and the associated biosynthesis pathway in a non-pathogenic fungus, Neurospora crassa. For this purpose, we used a computational approach to determine the genes and outlined the IAA production circuitry in N. crassa. We then validated these data with experimental evidence. Here, we describe the homologous genes that are present in the IPA pathway of IAA production in N. crassa. High-performance liquid chromatography and thin-layer chromatography unambiguously identified IAA, indole-3-lactic acid (ILA) and tryptophol (TOL) from cultures supplemented with tryptophan. Deletion of the gene (cfp) that encodes the enzyme indole-3-pyruvate decarboxylase, which converts IPA to indole-3-acetaldehyde (IAAld), results in an accumulation of higher levels of ILA in the N. crassa culture medium. A double knock-out strain (Δcbs-3;Δahd-2) for the enzyme IAAld dehydrogenase, which converts IAAld to IAA, shows a many fold decrease in IAA production compared with the wild type strain. The Δcbs-3;Δahd-2 strain also displays slower conidiation and produces many fewer conidiospores than the wild type strain.
Highlights
Indole-3-acetic acid (IAA) is considered the main auxin in plants and a major regulator of plant growth and development
The IAM pathway is considered the major route for indole-3-acetic acid (IAA) production, yet IAA synthesis via the indole-3-pyruvic acid (IPA) pathway has been described in a broad range of plant growth-promoting bacterial species [13, 15, 16]
IAA synthesis via the IPA pathway has been described in a broad range of plant growth-promoting bacterial species [13, 16] and in a few fungal species [15, 19, 20]
Summary
Indole-3-acetic acid (IAA) is considered the main auxin in plants and a major regulator of plant growth and development. The physiological role of auxins in plants is well understood and several IAA biosynthesis pathways have been proposed, only a limited number of biosynthesis genes have been characterized to date despite decades of research [1, 2]. Tryptophan is considered the major source of IAA synthesis in plants, but in some cases, other indole derivatives such as indole-3-acetamide and indole-3-ethanol (TOL) are employed [3,4,5]. IAA can either be produced from the precursor molecule tryptophan through several pathways involving different intermediates, or via a tryptophan-independent pathway [6].
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