Purine permease 5 contributes to riboflavin distribution in Arabidopsis reproductive organs

Abstract Riboflavin (vitamin B□; RF) and its derivatives flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are indispensable cofactors for redox reactions in plants. While higher plants possess a conserved pathway for de novo riboflavin biosynthesis, how flavins are transported and spatially distributed between tissues remains unresolved. In particular, the molecular identity of plasma membrane transporters involved in flavin transport in plants remains largely unknown. Here, we identify Arabidopsis purine permease 5 (AtPUP5) as a plasma membrane–localized protein associated with riboflavin transport and its distribution in plants. Using a riboflavin-auxotrophic yeast mutant, we show that AtPUP5 enhances intracellular accumulation of RF and, to a lesser extent, FMN, whereas FAD accumulation is inefficient under the conditions tested. In planta, AtPUP5 overexpression increases RF accumulation following external application. In contrast, loss-of-function mutants do not display defects in bulk RF uptake at the whole-plant level, indicating that AtPUP5 is not essential for global riboflavin acquisition. Notably, AtPUP5 deficiency results in RF overaccumulation in reproductive organs, including inflorescences, siliques, and seeds, irrespective of external RF supply. This organ-specific phenotype is fully suppressed by genetic complementation and coincides spatially with strong AtPUP5 promoter activity in reproductive tissues. These findings suggest that AtPUP5 contributes to the regulation of localized riboflavin distribution in reproductive tissues. Together, our study provides a molecular framework for plasma membrane–mediated regulation of riboflavin distribution in plants and highlights spatial regulation as an important component of flavin homeostasis. Competing Interest Statement The authors have declared no competing interest. Footnotes We have (i) improved the clarity of Figure S5, (ii) included a new schematic model (Figure 10) summarizing the main findings of the study, (iii) added quantitative RT-PCR analysis data to support the tissue-specific expression pattern of AtPUP5 (Figure S7), and (iv) expanded the Discussion to better integrate our findings with previous studies on flavin metabolism and gene expression patterns. Abbreviations - FAD - flavin adenine dinucleotide - FMN - flavin mononucleotide - RF - riboflavin - PUP - purine permease