From Seed to Savior: Understanding the Biocontrol Abilities of Seed-Derived Streptomyces Strains against Seed borne Bean Bacterial Diseases

Phaseolus vulgaris L. (Common bean) is the most important legume for direct consumption worldwide and a leading food used to fight global hunger. The seed-borne bacterial pathogens, Xanthomonas phaseoli pv. phaseoli ( Xpp ) and Curtobacterium flaccumfaciens pv. flaccumfaciens ( Cff ) are considered an important constraint in crop production. Certain strains of Streptomyces exhibit the ability to inhibit pathogenic bacteria, attributed to their production of various antimicrobial compounds. The Streptomyces FS2 and FS123 strains that exhibited high similarity with Streptomyces murinus and Streptomyces collinus based on 16S rRNA gene sequences were isolated from bean seeds, and antibacterial activities against the Xpp and Cff and also their effects on seedling growth index were investigated in this study. Both FS2 and FS123 strains successfully inhibit the growth of the Xpp and Cff in the zone of inhibition test. Assessments under greenhouse conditions exhibited strain FS123 with a dual behavior increase in disease severity and the area under the disease progress curve (AUDPC) of common bacterial blight disease (CBB) and a significant decrease of bacterial wilt (BW) disease. Our experiments showed that treating bean seeds with FS2 strain protects against both seed-borne diseases. We found that the pathogens population is affected due to plant treatment with the Streptomyces strains, especially in leaf tissues and endophytic situations. Also, the FS2 strain demoted plant growth despite the disease suppression. The total length of plantlets decreased by 68.52% and 17.89%, and total weights decreased by 44.79% and 10%, respectively, in FS2 +  Xpp and FS2 +  Cff treatment. Our results demonstrate the interesting biocontrol potential of the Streptomyces strains in bean protection against Xpp and Cff pathogens and open up promising perspectives for controlling these seed-borne diseases. However, attention to the damaging effect of the Streptomyces strains towards plant growth is crucial before introducing biocontrol materials.