Mushrooms And Auxin: What's The Connection?

Mushrooms, which are grown on plant residues, are assumed to be influenced by plant growth hormones. Auxin, a hormone produced by plants, is known to regulate cell elongation and the differentiation of the meristem into vascular tissue, thereby promoting leaf development and arrangement. It is also involved in the promotion of root development and growth in tomatoes and maize. Additionally, it has been observed that mushrooms produced without the use of hormones tend to have higher total biomass compared to those grown with hormones. While the role of auxin in mushrooms is not fully understood, studies have shown that arbuscular mycorrhizal fungi can increase auxin levels in plants, promoting their growth and development.

Auxin is a plant growth hormone

Oyster mushrooms, or Pleurotus ostreatus, are grown on plant residues as substrates, so it is logical to assume that plant growth hormones affect their growth. Experiments have been conducted to test the effects of various hormonal-treated plant substrates on the development and yield of oyster mushrooms. The results showed that mushrooms produced on substrates that were not treated with hormones had the highest total biomass compared to those grown on hormonally treated substrates. This may be due to increased mitotic cell division creating nutrient competition between the many individual pinnings and fruiting bodies.

Hormone auxins in plants are responsible for cell elongation and control the differentiation of the meristem into vascular tissue, promoting leaf development and arrangement. The interaction-promoting role of auxins in symbiotic interactions of plants with mycorrhizal fungi, nodulating bacteria, or plant-growth-promoting rhizobacteria may rely on the defense-modulating effects of auxin. Various plant-associated microbes produce and secrete auxins themselves, while others manipulate their distribution within the plant body to shape morphogenesis.

Auxin biosynthesis genes have been found in smut genomes that do not cause galls during biotrophy, indicating a function of auxin beyond gall induction. Some smut fungi species show greater similarity to Ustilago maydis auxin biosynthesis genes even though they are distantly related when comparing whole genomes. All four genes (Tam1, Tam2, Iad1, and Iad2) are conserved among smut fungi, indicating that these auxin biosynthesis genes possibly have a supporting function in gall induction.

Fungi produce auxin

Various microorganisms, including fungi, produce auxins. For example, the phytopathogenic fungus Leptosphaeria maculans biosynthesizes auxins, producing indole-3-acetic acid (IAA) as the predominant auxin metabolite. The production of IAA can be stimulated by supplying precursors such as tryptophan and tryptamine.

Smut fungi, such as Ustilago maydis, also produce IAA, which is associated with elevated auxin levels during their biotrophy in maize. The increase in host IAA levels upon U. maydis infection suggests that fungal IAA production contributes to elevated IAA levels in infected tissue.

Arbuscular mycorrhizal fungi (AMF) promote nutrient absorption and improve plant tolerance to biotic and abiotic stresses. AMF inoculation has been shown to increase the IAA content in the root systems of plants, promoting growth and development, especially under stress conditions.

Fungi-produced auxins also play a role in plant-fungal interactions and pathogenicity. For example, truffles (Tuber melanosporum and Tuber borchii) produce auxin to manipulate hormonal pathways in their host plants. The production of fungal hormones may favour the invasion of plant tissues and the development of fungi inside these tissues.

Auxin's role in plant-fungus interactions

Auxin is a plant hormone that governs many aspects of normal plant growth and development. It is a master regulator in plants for cell division and cell expansion. It also plays a role in plant-microbe interactions, including interactions between plant hosts and pathogenic microorganisms that cause disease.

Auxin is involved in arbuscular mycorrhizal fungi-promoted tomato growth and NADP-malic enzymes expression in continuous cropping substrates. Arbuscular mycorrhizal fungi (AMF) can promote nutrient absorption and improve plant tolerance to biotic and abiotic stresses. The inoculation of AMF in continuous cropping substrates promoted growth and root development and increased the root and NADP-malic enzyme (NADP-ME) activity of tomato seedlings. Root transcriptome analysis demonstrated that the plant hormone signal transduction pathway was highly enriched, and 109 genes that positively correlated with the AMF-inoculated plant phenotype were obtained by gene set enrichment analysis (GSEA), which identified 9 genes related to indole acetic acid (IAA).

Auxin biosynthesis genes are also found in smut genomes that do not cause galls during biotrophy, indicating a function of auxin beyond gall induction. The gall-forming corn smut fungus U. maydis has been reported to produce indole acetic acid, which is associated with elevated auxin levels during its biotrophy in maize. Several genes involved in the IAA biosynthesis pathway from U. maydis have been identified and characterized, including two NAD-dependent IAAld dehydrogenases (iad1 and iad2) and two predicted aromatic amino acid aminotransferase genes (tam1 and tam2).

Auxin can act as a microbial signaling molecule that modulates pathogen gene expression. It can also act as a plant hormone that modulates host signaling and physiology to increase host susceptibility. Recent studies have shown that indole-3-acetic acid (IAA), the most well-studied form of auxin, promotes disease in many plant-pathogen interactions.

In summary, auxin plays a significant role in plant-fungus interactions by promoting plant growth and development, regulating plant immunity responses, and modulating pathogen gene expression.

Auxin's role in plant defence responses

Auxin is a phytohormone that regulates nearly every aspect of plant growth and development. It acts as a chemical messenger that influences gene expression through a short nuclear pathway, mediated by a family of specific DNA-binding transcription factors known as Auxin Response Factors (ARFs). ARFs act as effectors of auxin response and translate chemical signals into the regulation of auxin-responsive genes.

Auxin and other growth hormones in plants are modulators of defence responses. Auxin has been reported to act antagonistically to salicylic acid (SA) signalling and vice versa. The interaction-promoting role of auxins in symbiotic interactions of plants with mycorrhizal fungi, nodulating bacteria, or plant-growth-promoting rhizobacteria might also partly rely on the defence-modulating effects of auxin. For example, the gall-forming corn smut fungus U. maydis produces indole acetic acid, which is associated with elevated auxin levels during its biotrophy in maize.

Auxin biosynthesis genes are also found in smut genomes that do not cause galls during biotrophy, indicating a function of auxin beyond gall induction. Various plant-associated microbes produce and secrete auxins themselves, while others manipulate their distribution. Auxin is redistributed by PIN-FORMED, ABC, and AUX/LAX transporters within the plant body to shape morphogenesis.

In plant–pathogen interactions, the role of auxin has been studied in Arabidopsis thaliana plants, which differentially modulate auxin biosynthesis and transport during defence responses to the necrotrophic pathogen Alternaria brassicicola. The results indicate that the auxin pathway plays a positive role in regulating plant resistance to A. brassicicola, and that jasmonic acid (JA) and auxin interact synergistically in regulating plant defence responses to necrotrophic pathogens.

Auxin's role in cell division and elongation

Auxin is a master regulator in plants for cell division and cell expansion. It controls several fundamental aspects of plant development, including cell expansion, cell division, pattern formation, root development, and apical dominance. The phytohormone auxin plays a prominent role in regulating pattern formation in a cell type-dependent manner.

Auxin controls cell division and cell elongation through different pathways. For instance, 1-naphthaleneacetic acid (NAA) stimulates cell elongation at concentrations that are much lower than those required to stimulate cell division. In contrast, 2,4-dichlorophenoxyacetic acid (2,4-D) promotes cell division but not cell elongation. The signaling to cell division is triggered by a low-affinity receptor, which is different from the receptor involved in cell elongation.

The existence of two distinct receptors with different affinities for auxin helps explain the differential auxin sensitivity in root and shoot growth. This idea was first conceived more than 60 years ago when discussing the differential auxin sensitivity of root and shoot growth in relation to their different modes of growth, with division being confined to the root.

Auxin also plays a role in embryo development, starting from the acquisition of zygote polarity and elongation, followed by a series of cell divisions. The separated cells quickly establish an apical-basal axis of polarity, followed by the differentiation of an epidermis and the formation of the shoot and root meristem during subsequent rounds of cell division.

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