Systematic review reveals gaps in standardized protocol to determine efficacy of PGPB use in plants grown under abiotic stress conditions at the field level

Plant growth-promoting bacteria (PGPB) are widely investigated as tools to mitigate the effects of abiotic stresses in crops; however, their effectiveness under field conditions is still highly variable and sometimes controversial. Consequently, systematic reviews represent a valuable tool for addressing scientific and biotechnological questions. Here, we used the PRISMA guidelines and Scopus database to formulate and answer four relevant questions concerning methodologies and modes of action of PGPB on plants under abiotic stresses. Based on a set of 212 PGPB articles published between 2017 and 2023, our results revealed that PGPB are mostly applied single strain liquid inoculants on seeds (particularly cereals) and typically tested under controlled conditions (growth chamber and greenhouse). Our results also revealed that plant parameters (e.g., physiology and photosynthetic pigments) were typically measured as indicators of the effectiveness of PGPB. In contrast, few studies have investigated the application of PGPB in planta (e.g., germination or flowering stage) under field conditions. The most representative genera of PGPB studied are Bacillus, Pseudomonas and Enterobacter; however, a wide diversity of PGPB taxa (similar to 60) were also identified as potential PGPB. The main stress conditions to be alleviated by PGPB include drought, salinity and metal toxicity, using diverse action mechanisms, including 1-aminocyclopropane-1-carboxylic acid deaminase activity, tryptophane-induced auxin production, and phosphorus solubilization. In general, our review points to gaps in the adoption of standardized protocols for testing PGPB, universal indicators of efficiency, and the validation of application of a high diversity of PGPB taxa particularly at field level.