Biogenic gold nanoparticles conjugated with rhizobacteria enhance tomato growth and suppress pathogen infection

Erwinia persicina (Ep) is recognized as an emerging pathogen of considerable threat to tomato yields. Although Stenotrophomonas rhizophila (Sr) has long been recognized as a plant growth-promoting rhizobacterium (PGPR), the use of this PGPR has sometimes been constrained by the rhizosphere stress factor in the biocontrol of the pathogen. This study investigated the potential of biogenic gold nanoparticles (AuNPs) to enhance the performance of Sr. through bio-conjugation. Biogenic AuNPs were synthesized and conjugated with S. rhizophila (Sr-AuNPs). The physical and biochemical stability of the conjugate was verified via TEM, DLS, and FTIR. Greenhouse trials were conducted across three independent growing seasons to evaluate the impact of the Sr-AuNP conjugate (T3) on tomato growth, yield, and resistance against E. persicina infection (T4) compared to healthy controls (T1) and non-conjugated bacteria (T2). Bio-conjugation facilitated robust root surface colonization, with FESEM-EDS confirming AuNP persistence (7.1-10.2 wt.%) on the root epidermis. The T3 treatment consistently demonstrated superior disease suppression, maintaining the lowest Disease Severity Index (DSI) values (0.65-0.80) throughout the study. Agronomically, T3 was the only treatment that sustained the highest statistical order (Tukey group 'a') in terms of fruit weight throughout the three seasons, reaching 204.62 g in Season 3, which was a remarkable threefold increase from the infected control T4 (72.52 g). These results indicate that AuNPs act as a supporting scaffold that promotes the longevity and efficacy of PGPR. Interestingly, rather than acting solely as a plant growth promoter, the bio-conjugate functioned chiefly as a potent disease suppressor and plant growth stabilizer under high pathogen pressure. This biologically derived method presents itself as a sustainable solution to the threat presented by emergent phytopathogens in contemporary agriculture.