Bio-inoculants and sustainability: exploring the functional role of compost-derived bacteria and their impact on tomato plant growth

In recent decades, modern agricultural practices such as the intensive use of chemical fertilisers and increasingly impactful soil management techniques have contributed to soil degradation and the progressive loss of soil biodiversity, leading to a reduction in the availability of microbial strains capable of positively interacting with plants. Among the sustainable strategies available, compost stands out as a promising solution, as it is extensively used in agriculture to improve soil fertility by providing organic matter and nutrients. However, its microbiota and its potential role in promoting plant growth and health require further studies. The aim of this thesis is to characterize bacterial strains isolated from compost and to assess their beneficial role as bio-inoculants in tomato plants (Solanum lycopersicum cv. Micro-Tom), chosen as a model organism for crop species. The first part of this work focusses on identifying the presence of the enzyme ACC deaminase, which is recognised to degrade the ethylene precursor ACC and thereby to enhance plant resilience under abiotic and biotic stresses. This ability was investigated through both qualitative and quantitative assays. Moreover, the study aims to evaluate the impact of single-strain inoculation with K. rhizophila, selected as the most promising candidate based on its previously demonstrated plant growth–promoting properties. Tomato plants were grown under controlled conditions with different soil treatments, with and without compost and each condition was tested with and without bacterial inoculum. Plant performance was monitored through the measurement of biomass, photosynthetic efficiency, pigment content, and metabolic parameters, providing insights into the role of microbial inoculants in sustainable crop management. The results showed that none of the isolated strains produced ACC deaminase, and this finding was further confirmed by molecular data. However, inoculation with K. rhizophila appeared to promote plant growth. Further studies involving other bacterial strains and bacterial consortia are needed to better understand the potential of compost-derived bacteria as biofertilisers in sustainable agriculture.