Differential proteomics analysis reveals that Azospirillium brasilense (Sp7) promotes virus tolerance in maize and tomato seedlings

Plant growth-promoting rhizobacteria such as Azospirillum brasilense Sp7 can protect plants against viruses but the molecular basis of this phenomenon is unclear. We therefore used differential proteomics to study two pathosystems in the presence and absence of Sp7 during early vegetative growth: tomato (Solanum lycopersicum L. cv. Boludo)/Potato virus X (PVX, KJ631111)/Sp7, and maize (Zea mays cv. B73)/Maize dwarf mosaic virus (MDMV, AM110558)/Sp7). In the maize/MDMV system, PDQuest revealed significant variations in the levels of 19 proteins compared to uninfected controls, including the upregulation of NADP-dependent malic enzyme as a form of host-specific viral anticipation, causing a simultaneous increase in the abundance of proteins related to photosynthesis and plastid functions. However, 42 proteins varied significantly in the maize/MDMV/Sp7 system, including the upregulation of radical-scavenging enzymes and proteins related to methionine metabolism, the glutathione-ascorbate cycle and photosynthesis, increasing the photosynthetic rate. In the tomato/PVX system, we observed significant variations in the levels of 58 proteins reflecting the disruption of the Calvin-Benson cycle, responses to oxidative stress and the inhibition of photosystem II (PSII) activity. We identified 26 proteins that varied in the tomato/PVX/Sp7 system; PSII and plastid proteins transiently declined but partially recovered over time as the Calvin-Benson cycle was induced to compensate. Sp7 therefore triggers induced systemic resistance in both pathosystems without affecting the virus titer, although it does delay the appearance of MDMV. The role of ribulose-1.5-bisphosphate carboxylase/oxygenase small subunit as a host target for viruses is discussed in both pathosystems.