Calendula officinalis—A Great Source of Plant Growth Promoting Endophytic Bacteria (PGPEB) and Biological Control Agents (BCA)

The application of beneficial bacteria may present an alternative approach to chemical plant protection and fertilization products as they enhance growth and resistance to biotic and abiotic stresses. Plant growth-promoting bacteria are found in the rhizosphere, epiphytically or endophytically (Plant Growth Promoting Endophytic Bacteria, PGPEB). In the present study, 36 out of 119 isolated endophytic bacterial strains from roots, leaves and flowers of the pharmaceutical plant Calendula officinalis were further identified and classified into Bacillus, Pseudomonas, Pantoea, Stenotrophomonas and Rhizobium genera. Selected endophytes were evaluated depending on positive reaction to different plant growth promoting (PGP) traits, motility, survival rate and inhibition of phytopathogenic fungi in vitro and ex vivo (tomato fruit). Bacteria were further assessed for their plant growth effect on Arabidopsis thaliana seedlings and on seed bio-primed tomato plantlets, in vitro. Our results indicated that many bacterial endophytes increased seed germination, promoted plant growth and changed root structure by increasing lateral root density and length and root hair formation. The most promising antagonistic PGPEB strains (Cal.r.29, Cal.l.30, Cal.f.4, Cal.l.11, Cal.f.2.1, Cal.r.19 and Cal.r.11) are indicated as effective biological control agents (BCA) against Botrytis cinerea on detached tomato fruits. Results underlie the utility of beneficial endophytic bacteria for sustainable and efficient crop production and disease control.