Thiamin addition to soil increases potato tuber thiamin content and has no effect on yield

Plants synthesize thiamin, also known as vitamin B₁, in green tissues to support various central metabolic pathways, such as glycolysis and the tricarboxylic acid cycle. Some soil microorganisms also produce thiamin and may release it to the soil where it is acquired by plant roots. Thiamin is essential for human health, but humans do not synthesize thiamin and must consume it through regular dietary intake. Exogenously supplied thiamin may be beneficial for plant growth, but this is controversial and may depend on plant species. In this study, we investigated the effect of soil thiamin amendment on yield and tissue thiamin content of potato varieties, Clearwater Russet, Russet Norkotah, and Snowden, grown in a greenhouse. Different amounts of thiamin (up to 108 mg kg^-1 soil) were applied on a weekly basis from emergence until harvest. Tuber yield was not affected by thiamin supplementation regardless of soil type. Thiamin quantification showed that roots, tubers and stems of plants grown in soil supplemented with thiamin accumulated up to 58, 6 and 3 times, respectively, the amount of thiamin found in control plants, while leaf thiamin content did not change. Expression of the thiamin biosynthesis genes THI1 and THIC in leaves of Clearwater Russet and Snowden did not differ between plants grown in soil supplemented with thiamin and control plants. Metagenome sequencing of samples from control and thiamin-treated soils suggested that thiamin supplementation altered the soil microbial community. Our results indicate that increasing soil thiamin concentration is not likely to increase potato yield. However, thiamin supplementation of soils should be considered as a tuber thiamin biofortification strategy.