APS and APR gene families in Broussonetia papyrifera: evolutionary conservation, functional divergence, and roles in selenium assimilation and biofortification

Selenium (Se) biofortification in forage tree species offers a sustainable strategy to improve nutritional quality. This study investigated the physiological and molecular responses of Broussonetia papyrifera , a high-biomass woody species, to exogenous selenite (Na 2 SeO 3 ) and selenate (Na 2 SeO 4 ). Low Se concentrations (≤ 0.4 mM) significantly enhanced plant growth, while higher concentrations (0.8 mM), especially Na 2 SeO 4 , inhibited biomass accumulation in a dose-dependent manner. Both forms of Se substantially increased foliar Se content, with Na 2 SeO 4 showing higher uptake efficiency. Genome-wide screening identified four sulfate/Na 2 SeO 4 assimilation-related genes including ATP sulfurylase 1 ( BpAPS1 ), BpAPS2 , adenosine 5′-phosphosulfate reductase ( BpAPR1 ), and BpAPR2 . Phylogenetic analyses confirmed the evolutionary conservation of these proteins, which were localized to the chloroplasts. Tissue-specific expression patterns revealed a positive correlation between transcript levels of BpAPS1 , BpAPR1 , and BpAPR2 and Se content, while BpAPS2 expression was inversely correlated, indicating functional divergence. Compared to wild-type (WT) plants, transgenic Arabidopsis thaliana lines overexpressing these genes accumulated 1.27- to 1.60-fold more Se under Na 2 SeO 4 stress, accompanied by coordinated upregulation of key endogenous Se metabolism genes like AtAPS , AtAPR , S-adenosylmethionine carriers 3 ( AtSAM ), homocysteine S-methyltransferase ( AtHMT ), methylselenol methyltransferase ( AtMMT ), suggesting enhanced Se assimilation and detoxification capacity. Collectively, these findings clarify the functional roles of the APS and APR gene families in Se uptake, assimilation, and detoxification in a woody crop, highlighting potential genetic targets for Se biofortification.