Nándor Gulyás-Oldal

1-Naphthaleneacetic acid (NAA) is a synthetic auxin used repeatedly in apple production and routinely in plant propagation, yet its direct effects on microorganisms have not been reviewed. We compiled the observations we could locate and verify of a measured NAA effect on a named fungus, bacterium or microalga, from a screened literature corpus, a taxon-directed sweep, targeted searches and, at revision, Scopus and AGRICOLA, and two authors independently assigned each an evidence tier. The result is a structured evidence map of a defined corpus. Effects were organism-specific. Plant-pathogenic fungi were inhibited by about 40% or more only at 100 mg L⁻¹ and above, at which Trichoderma harzianum lost at most 13% of its radial growth, whereas several cultivated fungi were stimulated at 1–8 mg L⁻¹. NAA altered fungal and bacterial secondary metabolism and bacterial virulence gene expression, a rhizobial inoculant degraded it, and microalgae responded biphasically between 0.037 and 18.6 mg L⁻¹. Among the studies retrieved, one field trial, in tobacco, characterised a soil microbial community after NAA application; none did so in an orchard, and no field study measured a named beneficial microorganism. We outline implications for biocontrol compatibility, inoculant use and phyllosphere exposure.

1-Naphthaleneacetic acid (NAA) is a synthetic auxin used repeatedly in apple production and routinely in plant propagation, yet its direct effects on microorganisms have not been reviewed. We compiled every observation we could locate and verify of a measured NAA effect on a named fungus, bacterium or microalga, drawing on a systematically screened literature corpus, a taxon-directed sweep and targeted database searches, and recorded each with its concentration, replication and evidence tier. The effects were organism-specific. Plant-pathogenic fungi were inhibited at concentrations of 100–400 mg L⁻¹, at which the biocontrol fungus Trichoderma harzianum lost at most 13% of its radial growth. Fungal secondary metabolism was strongly modulated, and inhibition of Saccharomyces cerevisiae occurred in minimal but not in rich medium. Direct bacterial growth inhibition was weak, but the bacterial auxin-catabolism repressor IadR bound NAA with slightly higher affinity than indole-3-acetic acid. Microalgae showed biphasic responses spanning 0.037–18.6 mg L⁻¹. No study measured a microbial community before and after exposure, and no field study measured a beneficial microorganism. We outline implications for biocontrol compatibility, inoculant use in propagation and phyllosphere exposure, and the experiments that would close these gaps.