Cover image

Cover Gram-negative bacteria Xanthomonas campestris pv. campestris (Xcc) and its host plants. During the co-evolutionary process with hosts, the X. campestris species differentiated into over 141 pathovars (pv.), each with a specific host range and unique strategy of infection. Among them, Xcc (yellow bacteria), the causative agent of black rot disease in crucifers, is one of the model organisms for studying plant-microbe interactions. Differing from bacterial pathogens with narrow-spectrum host ranges, Xcc can invade most cruciferous plants (background montage), including cabbage, broccoli, cauliflower, and the model plant Arabidopsis thaliana, resulting in serious agricultural yield loss worldwide. Toward understanding the molecular mechanism of Xcc pathogenesis, Qian and colleagues employed an integrative study strategy, including whole genome sequencing, intra-species comparative genomic analysis, and large-scale transposon mutagenesis, to systematically explore the essential genetic steps required for full virulence of a well-documented strain, Xcc 8004. The results revealed genetic variations, such as single nucleotide polymorphisms, strain-specific genes, and cross-genome rearrangements that may contribute to the evolution of virulence and host specificity. In addition, they identified a profile of functionally diverse genes from pathogenicity-deficient mutants expected to facilitate further biochemical and physiological studies. (Cover art by Wei Qian. [For details, see Qian et al., pp. 757–767.])