Clavibacter michiganensis subsp. capsici subsp. nov., causing bacterial canker disease in pepper.

Clavibacter michiganensis is a Gram-stain-positive bacterium with eight subspecies. One of these subspecies is C. michiganensis subsp. michiganensis, which causes bacterial canker disease in tomato. Bacterial strains showing very similar canker disease symptoms to those of a strain originally classified as C. michiganensis have been isolated from pepper. In this paper, we reclassified strains isolated from pepper. On the basis of phylogenetic analysis with 16S rRNA gene sequences, the strains isolated from pepper were grouped in a separate clade from other subspecies of C. michiganensis. Biochemical, physiological and genetic characteristics of strain PF008T, which is the representative strain of the isolates from pepper, were examined in this study. Based on multi-locus sequence typing and other biochemical and physiological features including colony color, utilization of carbon sources and enzyme activities, strain PF008T was categorically differentiated from eight subspecies of C. michiganensis. Moreover, genome analysis showed that the DNA G+C content of strain PF008T is 73.2 %. These results indicate that PF008T is distinct from other known subspecies of C. michiganensis. Therefore, we propose a novel subspecies, C. michiganensis subsp. capsici, causing bacterial canker disease in pepper, with a type strain of PF008T (=KACC 18448T=LMG 29047T).

Complete genome sequence of the cellulase-producing bacterium Clavibacter michiganensis PF008.

The Gram-positive Actinobacterium Clavibacter michiganensis strain PF008 produces a cellulase of biotechnological interest, which is used for degradation of cellulose, a major component of plant cell walls. Here we report the complete genome sequence of this bacterium for better understanding of cellulase production and its virulence mechanism.

Infection processes of xylem-colonizing pathogenic bacteria: possible explanations for the scarcity of qualitative disease resistance genes against them in crops.

KEY MESSAGE: Disease resistance against xylem-colonizing pathogenic bacteria in crops. Plant pathogenic bacteria cause destructive diseases in many commercially important crops. Among these bacteria, eight pathogens, Ralstonia solanacearum, Xanthomonas oryzae pv. oryzae, X. campestris pv. campestris, Erwinia amylovora, Pantoea stewartii subsp. stewartii, Clavibacter michiganensis subsp. michiganensis, Pseudomonas syringae pv. actinidiae, and Xylella fastidiosa, infect their host plants through different infection sites and paths and eventually colonize the xylem tissues of their host plants, resulting in wilting symptoms by blocking water flow or necrosis of xylem tissues. Noticeably, only a relatively small number of resistant cultivars in major crops against these vascular bacterial pathogens except X. oryzae pv. oryzae have been found or generated so far, although these pathogens threaten productivity of major crops. In this review, we summarize the lifestyles of major xylem-colonizing bacterial pathogens and then discuss the progress of current research on disease resistance controlled by qualitative disease resistance genes or quantitative trait loci against them. Finally, we propose infection processes of xylem-colonizing bacterial pathogens as one of possible reasons for why so few qualitative disease resistance genes against these pathogens have been developed or identified so far in crops.

Genome sequence of the hot pepper provides insights into the evolution of pungency in Capsicum species.

Hot pepper (Capsicum annuum), one of the oldest domesticated crops in the Americas, is the most widely grown spice crop in the world. We report whole-genome sequencing and assembly of the hot pepper (Mexican landrace of Capsicum annuum cv. CM334) at 186.6× coverage. We also report resequencing of two cultivated peppers and de novo sequencing of the wild species Capsicum chinense. The genome size of the hot pepper was approximately fourfold larger than that of its close relative tomato, and the genome showed an accumulation of Gypsy and Caulimoviridae family elements. Integrative genomic and transcriptomic analyses suggested that change in gene expression and neofunctionalization of capsaicin synthase have shaped capsaicinoid biosynthesis. We found differential molecular patterns of ripening regulators and ethylene synthesis in hot pepper and tomato. The reference genome will serve as a platform for improving the nutritional and medicinal values of Capsicum species.

Multiple classes of immune-related proteases associated with the cell death response in pepper plants.

Proteases regulate a large number of biological processes in plants, such as metabolism, physiology, growth, and defense. In this study, we carried out virus-induced gene silencing assays with pepper cDNA clones to elucidate the biological roles of protease superfamilies. A total of 153 representative protease genes from pepper cDNA were selected and cloned into a Tobacco rattle virus-ligation independent cloning vector in a loss-of-function study. Silencing of 61 proteases resulted in altered phenotypes, such as the inhibition of shoot growth, abnormal leaf shape, leaf color change, and lethality. Furthermore, the silencing experiments revealed that multiple proteases play a role in cell death and immune response against avirulent and virulent pathogens. Among these 153 proteases, 34 modulated the hypersensitive cell death response caused by infection with an avirulent pathogen, and 16 proteases affected disease symptom development caused by a virulent pathogen. Specifically, we provide experimental evidence for the roles of multiple protease genes in plant development and immune defense following pathogen infection. With these results, we created a broad sketch of each protease function. This information will provide basic information for further understanding the roles of the protease superfamily in plant growth, development, and defense.

A pepper (Capsicum annuum†L.) metacaspase 9 (Camc9) plays a role in pathogen-induced cell death in plants.

Metacaspases, which belong to the cysteine-type C14 protease family, are most structurally similar to mammalian caspases than any other caspase-like protease in plants. Atmc9 (Arabidopsis thaliana metacaspase 9) has a unique domain structure, and distinct biochemical characteristics, such as Ca²âº binding, pH, redox status, S-nitrosylation and specific protease inhibitors. However, the biological roles of Atmc9 in plant-pathogen interactions remain largely unknown. In this study, a metacaspase gene present as a single copy in the pepper genome, and sharing 54% amino acid sequence identity with Atmc9, was isolated and named Capsicum annuum metacaspase 9 (Camc9). Camc9 encodes a 318-amino-acid polypeptide with an estimated molecular weight of 34.6 kDa, and shares approximately 40% amino acid sequence identity with known type II metacaspases in plants. Quantitative reverse transcription-polymerase chain reaction analyses revealed that the expression of Camc9 was induced by infections of Xanthomonas campestris pv. vesicatoria race 1 and race 3 and treatment with methyl jasmonate. Suppression of Camc9 expression using virus-induced gene silencing enhanced disease resistance and suppressed cell death symptom development following infection with virulent bacterial pathogens. By contrast, overexpression of Camc9 by transient or stable transformation enhanced disease susceptibility and pathogen-induced cell death by regulation of reactive oxygen species production and defence-related gene expression. These results suggest that Camc9 is a possible member of the metacaspase gene family and plays a role as a positive regulator of pathogen-induced cell death in the plant kingdom.