Systemic induction of bean isoflavones by a plant growth promoting Rhizobacteriaconsortium against the leaf pathogen Xanthomonas campestris pv. phaseoli

Abstract: Plant growth promoting rhizobacteria are non-pathogenic bacteria able to triggerplant’s defensive metabolism. In some plant species as legumes, isoflavones (IF) are secondarymetabolites relevant for human health and also play a role in plant defense. Although amonglegumes, only soybeans are known for the high IF contents, beans (Phaseolus vulgaris) mayrepresent a considerable input of IF in the diets since they are by far, a lot more popular in theMediterranean area than soybean. Increasing bean productivity according to environmentallyfriendly agricultural practices is a challenge that increases its attractiveness if the product has anadded value such as a high IF content. This may be achieved with biofertilizers which aim toimprove plant nutrition at the same time that the plant’s defensive metabolism is elicited. SincePGPR may use different mechanisms to achieve these goals, our rationale was to evaluate if acombination of PGPR with different mechanisms would result in better effects than upon simpleapplications, evaluating changes in growth and IF contents in early stages of bean developmentand protection against leaf pathogen.Two different experiments were carried out to address these goals. A short experiment inwhich the consortium and the individual PGPR were inoculated on two-day old pre-germinatedseeds sown on sterile pots filled with vermiculite. Six days after inoculation, photosynthesis wasmeasured and seedlings were harvested. Weight and height of shoots, cotyledons and roots wereregistered and IF in shoots (free of cotyledons) and roots were analyzed by HPLC. On the secondexperiment, pre-germinated seeds were transferred to 500ml pots and inoculated twice with theconsortium or the individual bacteria, one upon and the second 12 days after; six days aftertransplant the second inoculation, plants were pathogen challenged and one week after, diseasesymptoms were recorded. All strains were able to prime the plant since all decreased plant heightin the shirt experiment, indicating that plants detour C metabolism to defensive metabolismcompromising growth (Conrath et al., 2002). Despite these changes in total IF were nonsignificant under any treatment, including the consortium, this was not correlated to protectionachieved on long experiments. Individual strains performed a lot better than the consortium, thateven increased the disease symptoms; interestingly BB1 significantly increased daidzin levelscoupled to a decrease on its aglycon and these plants showed the lowest disease incidence (85%protection). Therefore, based on these data, it may be concluded that the consortium does notseem to provide any advantage to the use of individual strains when considering biofertilizersformulation, and systemic protection may be associated to other metabolites different from IF.