Understanding genetic determinants of bacterial adaptation to plant niches: Toward more robust bioinoculant design.

Plant-associated bacteria offer a sustainable alternative to chemical inputs in agriculture, yet the field performance of microbial inoculants remains inconsistent. A central but frequently overlooked cause is that bioprospecting pipelines still select strains on the basis of plant-growth-promoting activities, while the bacterial genetic traits that determine whether a strain can reach, colonise and persist in a given root compartment are rarely assessed. Previous reviews have largely catalogued growth-promoting functions or described root microbiome assembly from the plant's perspective, here we take the bacterium's. We trace the journey of a bacterium from bulk soil to its final niche and, at each stage, identify the genes and pathways (genetic determinants) that encode key bacterial traits involved, such as chemotaxis and motility, nutrient acquisition and metabolic independence, competitive and defensive arsenals, attachment and biofilm formation, evasion of plant immunity, and endophytic entry and persistence. Throughout, we use evidence from comparative genomics and metagenomics, (meta)transcriptomics, targeted mutagenesis and genome-wide screens such as Tn-seq, obtained in systems ranging from binary interactions to synthetic communities and natural soils. For each stage we examine the current state of knowledge, identify critical gaps, and propose avenues to deepen our mechanistic understanding of the bacterial traits involved in plant colonisation. We argue that this mechanistic understanding is the missing foundation for developing inoculants with more consistent and robust performance across diverse agroecosystems.