Soil organisms are key to plant growth and ecosystem functions. Earthworms (EWs) enhance soil and indirectly affect plant growth, while their cutaneous excreta (CEx) contain bioactive compounds capable of eliciting plant responses. However, their role in plant immunity is still not well understood. We hypothesized that EWs and their CEx enhance plant defense against foliar pathogens by activating induced resistance. To test this, we evaluated the effect of Eisenia fetida and their CEx on Solanum lycopersicum (tomato), focusing on growth, physiology, and defense response against Botrytis cinerea. Plants were exposed to EWs, CEx, or water (control), followed by B. cinerea infection after two weeks. Gene expression of defense markers was assessed at 24 and 48 h post-inoculation (hpi), while physiological parameters and disease severity were evaluated at 72 hpi. EWs increased shoot biomass compared to CEx, while both treatments reduced root dry weight, suggesting a possible shift in resource allocation. CEx significantly reduced B. cinerea-induced leaf damage and showed a trend for flavonoid accumulation, a known marker of induced resistance. Both treatments, EWs and CEx, activated the jasmonic acid (JA) signaling pathway, with CEx specifically upregulating genes involved in fungal pathogen defense, sustaining their expression over time. The present study offers, for the first time, clear evidence that EW derived CEx can induce resistance by stimulating plant defense responses. Further biochemical, transcriptomic, and metabolomic analyses are needed to confirm indirect results, along with field validation. Nonetheless, the findings underscore the crucial role of soil biodiversity in enhancing crop resilience.

Introduction Botrytis cinerea is one of the pathogenic fungi causing major problems worldwide in crops such as tomato. Some Plant Growth-Promoting Rhizobacteria (PGPR) can activate induced systemic resistance (ISR) pathways in crops, reducing the need for antifungals.Methods Three strains belonging to the species Peribacillus frigoritolerans (CD_FICOS_02), Pseudomonas canadensis (CD_FICOS_03), and Azotobacter chroococcum (CD_FICOS_04), which exhibit outstanding PGPR properties, were evaluated for their ability to protect tomato plants against B. cinerea infection by ISR via soil inoculation.Results The strains CD_FICOS_02 and CD_FICOS_03 reduced B. cinerea incidence and plant oxidative stress. The first strain mainly increased the expression of genes related to the salicylic acid pathway, while the second increased the expression of genes related to the jasmonic acid/ethylene hormonal pathway, indicating preferential ISR activation by each of these pathways. In addition, CD_FICOS_03 was able to increase the root and aerial biomass production of infected plants compared to the control. Interestingly, although the strain CD_FICOS_04 did not reduce the damage caused by B. cinerea, it increased the biomass of infected plants.Discussion Our results suggest that the best strategy for biocontrol of B. cinerea is to combine the ability to promote plant growth with the ability to induce systemic resistance, as demonstrated by strains P. frigoritolerans CD_FICOS_02 and P. canadensis CD_FICOS_03.

Plants have limited resources to allocate to defences against infection and herbivory. While interactions between plant responses to microbial and herbivore attack are complex, it is often the case that the induction of one response will act antagonistically to the other. Recent studies have shown that plant growth promoting rhizobacteria, which improves overall plant health and general stress resistance, can enhance both anti-microbial and anti-herbivore defences. We investigated how soil application of the biofungicide Serenade ASO (Bacillus subtilis strain QST 713), which primes plant defences against fungal and bacterial infection and promotes plant growth, affects anti-herbivore defences by measuring both constitutive and induced defences. We applied Serenade one or two times to the soil of tomato plants and measured the numbers of type IV glandular trichomes on leaves, the weight gain of a generalist caterpillar (beet armyworms; BAW), and the activity of two enzymes associated with defence against insects (polyphenoloxidase and peroxidase). Serenade treated plants grew faster and foliage from treated plants had significantly higher numbers of glandular trichomes and higher polyphenoloxidase and peroxidase activities than untreated plants. However, Serenade treatment did not affect the degree of induction of plant defences when damaged by BAW feeding and did not slow the growth rate of BAW relative to plants not treated with Serenade. Therefore, the biofungicide Serenade increased plant growth and altered the densities of trichomes and the activities of two defensive enzymes in plants, but it did not affect overall susceptibility of the plants to a generalist herbivore.