Abstract
Ralstonia solanacearum species complex (RSSC) posses extremely abundant type III effectors (T3Es) that are translocated into plant cells via a syringe-like apparatus assembled by a type III secretion system (T3SS) to subvert host defense initiated by innate immunity. More than 100 T3Es are predicted among different RSSC strains, with an average of about 70 T3Es in each strain. Among them, 32 T3Es are found to be conserved among the RSSC and hence called the core T3Es. Here, we genetically characterized contribution of abundant T3Es to virulence of a Japanese RSSC strain OE1-1 toward host plants. While all the T3Es members of AWR family contributed slightly to virulence, those of the GALA, HLK, and SKWP families did not influence full virulence of OE1-1. Mutant OE1-1D21E (with deletion of all 21 T3Es members of four families) exhibited slightly impaired virulence, while mutant OE1-1D36E (deleting all 21 T3Es of 4 families and 15 core T3Es) exhibited substantially reduced virulence. Mutant OE1-1D42E (deleting all 21 T3Es of 4 families, 15 core T3Es and 6 extended core T3Es) failed to cause any disease on tobacco plants with leaf infiltration but retained faint virulence on tobacco plants with petiole inoculation. The proliferation of mutant OE1-1D42E in tobacco stems was substantially impaired with about three orders of magnitude less than that of OE1-1, while no impact in tobacco leaves if directly infiltrated into leaves. On the contrary, the OE1-1D42E mutant retained faint virulence on eggplants with leaf infiltration but completely lost virulence on eggplants with root-cutting inoculation. The proliferation of OE1-1D42E mutant both in eggplant leaves and stems was substantially impaired. Intriguingly, mutant OE1-1D42E still caused necrotic lesions in tobacco and eggplant leaves, indicating that some other than the 42 removed effectors are involved in expansion of necrotic lesions in host leaves. All taken together, we here genetically demonstrated that all the core and extended core T3Es are nearly crucial for virulence of OE1-1 toward host plants and provided currently a kind of T3Es-free strain that enables primary functional studies of individual T3Es in host cells.
Highlights
Ralstonia solanacearum is a soil-borne Gram-negative plant vascular bacterium that is widely distributed and can cause lethal bacterial wilt on more than 450 plant species belonging to 50 botanical families worldwide (Mansfield et al, 2012; Jiang et al, 2017)
All the T3Es members of 4 families contribute together, but partially, to full virulence of OE1-1 toward tobacco plants. These results indicated that some other T3Es besides these four families might be required for full virulence of OE1-1
We provided genetic evidence to demonstrate that all the core and extended core T3Es are together crucial for virulence of OE1-1 on host tobacco plants
Summary
Ralstonia solanacearum is a soil-borne Gram-negative plant vascular bacterium that is widely distributed and can cause lethal bacterial wilt on more than 450 plant species belonging to 50 botanical families worldwide (Mansfield et al, 2012; Jiang et al, 2017). Different from the T3SS, T3Es are hugely variable among different RSSC strains (Mukaihara et al, 2010; Peeters et al, 2013; Sabbagh et al, 2019). There are more than 100 T3Es predicated among different RSSC strains with an average of about 70 T3Es per strain that is significantly more abundant than those of any other plant pathogenic bacteria (less than 30 T3Es in each plant pathogenic bacterium, such as Pseudomonas syringae or the Xanthomonads) (Sébastien et al, 2011; Genin and Denny, 2012; Peeters et al, 2013). Abundant studies have demonstrated that highly variable repertoires of T3Es might be responsible for the host range of different RSSC strains (Genin and Denny, 2012; Coll and Valls, 2013). Deletion of T3Es of both RipAA and RipP1 enables GMI1000 to cause wilt symptoms on tobacco plants (Poueymiro et al, 2009), while removal of RipB enables RS1000 to cause wilt symptoms on tobacco plants, and RipAA and RipP1 contribute in part to avirulence of RS1000 on tobacco plants (Nakano and Mukaihara, 2019), indicating that different effector repertories are recognized by different plants, which leads to virulence or avirulence of the pathogen
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