Effect of UV-A on endophyte colonisation of Arabidopsis thaliana.

UV-A, an important part of sunlight radiation, is typically absent in experiments on plant-endophyte interactions. We examined the impact of UV-A in the 350-400 nm range (UV-A1 waveband) on the plant interactions with fungal endophytes belonging to different taxonomic groups: Paraphoma chrysanthemicola, Phomopsis columnaris, Diaporthe eres, Mucor sp., and yeast Sporobolomyces ruberrimus. Physiologically relevant levels of UV-A did not substantially affect the colonisation of shoots and roots by endophytes. UV-A upregulated the expression of genes involved in the establishment of symbiosis. Specifically, the expression of PDF1.2 was affected by P. chrysanthemicola and S. ruberrimus only under UV-A conditions. Additionally, UV-A exposure upregulated the mRNA levels of ICS1 and PAL1, genes important for plant responses to stress factors. Inoculation with P. chrysanthemicola and S. ruberrimus led to increased expression of the ICS1 gene. We did not observe significant interactions between the effects of UV-A and the presence of endophytes on other examined plant traits, including plant fresh weight, root system architecture, and expression of plant photoreceptor genes. For these physiological parameters, the effects of the presence of endophytes did not depend on UV-A supplementation. Our findings indicate that while UV-A does not substantially influence plant colonisation by the endophytes, it does trigger the upregulation of plant defence genes and affects the shoot growth of Arabidopsis.