Nitrogen form and mycorrhizal inoculation amount and timing affect flavonol biosynthesis in onion (Allium cepa L.)

Mycorrhizal symbiosis is known to be the most prevalent form of fungal symbiosis with plants. Although some studies focus on the importance of mycorrhizal symbiosis for enhanced flavonoids in the host plants, a comprehensive understanding of the relationship still is lacking. Therefore, we studied the effects of mycorrhizal inoculation of onions (Allium cepa L.) regarding flavonol concentration and the genes involved in flavonol biosynthesis when different forms of nitrogen were supplied. We hypothesized that mycorrhizal inoculation can act as a biotic stress and might lead to an increase in flavonols and expression of related genes. The three main quercetin compounds [quercetin-3,4'-di-O-beta-d-glucoside (QDG), quercetin-4'-O-beta-d-glucoside (QMG), and isorhamnetin-4'-O-beta-d-glucoside (IMG)] of onion bulbs were identified and analyzed after inoculating with increasing amounts of mycorrhizal inocula at two time points and supplying either predominantly NO3 (-) or NH4 (+) nitrogen. We also quantified plant dry mass, nutrient element uptake, chalcone synthase (CHS), flavonol synthase (FLS), and phenyl alanine lyase (PAL) gene expression as key enzymes for flavonol biosynthesis. Inoculation with arbuscular mycorrhizal fungi (highest amount) and colonization at late development stages (bulb growth) increased QDG and QMG concentrations if plants were additionally supplied with predominantly NH4 (+). No differences were observed in the IMG content. RNA accumulation of CHS, FLS, and PAL was affected by the stage of the mycorrhizal symbiosis and the nitrogen form. Accumulation of flavonols was not correlated, however, with either the percentage of myorrhization or the abundance of transcripts of flavonoid biosynthesis genes. We found that in plants at late developmental stages, RNA accumulation as a reflection of a current physiological situation does not necessarily correspond with the content of metabolites that accumulate over a long period. Our findings suggest that nitrogen form can be an important factor determining mycorrhizal development and that both nitrogen form and mycorrhizas interact to influence flavonol biosynthesis.