Pghaxc and Pghbxc, Are Regulated by Type Iii Secretion Regulators Hrpx and Hrpg and Are Required for Virulence

Xanthomonas campestris pv. campestris, the causal agent of black rot disease, produces a suite of extracellular cell-wall degrading enzymes (CWDE) that are involved in bacterial virulence. Polygalacturonase (PG) is an important CWDE and functions to degrade the pectic layers of plant cell walls. Although previous studies have documented the virulence functions of PG in Erwinia and Ralstonia species, the regulation of PG genes still needs to be elucidated. In this study, we identified two novel PG genes (pghAxc and pghBxc) encoding functional PG from X. campestris pv. campestris 8004. The expressions of these two PG genes are regulated by the type III secretion regulators HrpX and HrpG and the global regulator Clp. These PG genes could be efficiently induced in planta and were required for the full virulence of X. campestris pv. campestris to Arabidopsis. In addition, these PG were confirmed to be secreted via the type II secretion system in an Xps-dependent manner. The type II secretion system (T2SS) allows most gram-negative bacteria to deliver extracellular hydrolytic enzymes and toxins to their surroundings and hosts, many of which are responsible for pathogenesis in plants and animals (Sandkvist 2001a). Among the virulence factors secreted by the T2SS of phytopathogens, cell-wall degrading enzymes (CWDE), such as polygalacturonases (PG), cellulases, xylanases, and prote-ases, have been studied due to their clear or potential functions in bacterial pathogenesis, and comprehensive studies on the structures and functions of CWDE have been undertaken. CWDE function to break down the components of host cell walls and may play a crucial role in virulence and bacterial PG are hydrolytic enzymes of CWDE that selectively degrade the pectic polymers, the major components of higher-plant middle lamellae and primary cell walls. PG are classified by activity as endo-PG (EC 3. 2. 1. 15) and exo-PG (EC 3. 2. 1. 67). Endo-PG randomly hydrolyze polygalacturonic acid, releasing oligosaccharidic chains of variable length, whereas exo-PG release either galacturonate monomer, in the case of exoPG (EC 3.2.1.67), or dimmer (digalacturonate), in the case of exo-poly-aD -galacturonosidase (EC 3.2.1.82), starting from the nonreducing end (Huang and Allen 1997). PG are produced by a wide range of fungal and bacterial phytopathogens to overcome the physical barriers composed of pectins and to provide abundant carbon resources inside the plant host. Their activities cause symptoms including leaf spot, soft rot, and wilt (Collmer and Keen 1986; Huang and Allen 1997). Although some aspects of expression regulation and …

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