This study aimed at deeply exploring the effects of leonardite treatment on sugar beet (Beta vulgaris L.). To this purpose, wedirectly exploited the effect of foliar application on plants grown in the field at four locations. The approach concerned theevaluation of the community compositional structure of plant endophytic bacteria through a metabarcoding approach, theexpression level of a gene panel related to hormonal metabolism and signaling, and the main sugar beet productivity traits.Results indicated that plants treated with leonardite compared with untreated ones had a significant increase (p<0.05) in (i) theabundance of Oxalicibacterium spp., recognized to be endophyte bacterial genera with plant growth-promoting activity, (ii) theexpression level of LAX2 gene, coding for auxin transport proteins, and (iii) and sugar yield. This study represents a step forwardto advance our understanding of the changes induced by leonardite-based biostimulant in sugar beet.