The translucens group of Xanthomonas translucens: Complicated and important pathogens causing bacterial leaf streak on cereals

Abstract

Xanthomonas translucens is a group of gram‐negative bacteria that can cause important diseases in cereal crops and forage grasses. Different pathovars have been defined according to their host ranges, and molecular and biochemical characteristics. Pathovars have been placed into two major groups: translucens and graminis. The translucens group contains the pathovars causing bacterial leaf streak (BLS) on cereal crops such as wheat, barley, triticale, rye, and oat. In recent years, BLS has re‐emerged as a major problem for many wheat‐ and barley‐producing areas worldwide. The biology of the pathogens and the host–pathogen interactions in cereal BLS diseases were poorly understood. However, recent genome sequence data have provided an insight into the bacterial phylogeny and identification and pathogenicity/virulence. Furthermore, identification of sources of resistance to BLS and mapping of the resistance genes have been initiated. Taxonomy Kingdom Bacteria; Phylum Proteobacteria; Class Gammaproteobacteria; Order Xanthomonadales; Family Xanthomonadaceae; Genus Xanthomonas; Species X. translucens; translucens group pathovars: undulosa, translucens, cerealis, hordei, and secalis; graminis group pathovars: arrhenatheri, graminis, poae, phlei; newly established pathovar: pistaciae. Host range X. translucens mainly infects plant species in the Poaceae with the translucens group on cereal crop species and the graminis group on forage grass species. However, some strains have been isolated from, and are able to infect, ornamental asparagus and pistachio trees. Most pathovars have a narrow host range, while a few can infect a broad range of hosts. Genome The complete genome sequence is available for two X. translucens pv. undulosa strains and one pv. translucens strain. A draft genome sequence is also available for at least one strain from each pathovar. The X. translucens pv. undulosa strain Xt4699 was the first to have its complete genome sequenced, which consists of 4,561,137 bp with total GC content approximately at 68% and 3,528 predicted genes. Virulence mechanisms Like most xanthomonads, X. translucens utilizes a type III secretion system (T3SS) to deliver a suite of T3SS effectors (T3Es) inside plant cells. Transcription activator‐like effectors, a special group of T3Es, have been identified in most of the X. translucens genomes, some of which have been implicated in virulence. Genetic factors determining host range virulence have also been identified.

Molecular Analysis of Bacterial Isolates From Necrotic Wheat Leaf Lesions Caused by Xanthomonas translucens, and Description of Three Putative Novel Species, Sphingomonas albertensis sp. nov., Pseudomonas triticumensis sp. nov. and Pseudomonas foliumensis sp. nov.

“…Members of the Xanthomonas translucens group are gramnegative plant pathogenic bacteria that can cause serious diseases to important cereal crops as well as forage grasses (Hersemann et al, 2017;Curland et al, 2018;Sapkota et al, 2020). The group is subdivided into 10 different pathovars based on biochemical and molecular characteristics and host ranges (van den Mooter et al, 1987;Sapkota et al, 2020) and are taxonomically placed into the translucens and graminis subgroups (Vauterin et al, 1992;Langlois et al, 2017;Peng et al, 2019). Pathovars undulosa, translucens, hordei, cerealis, secalis and phleipratensis belong to the translucens subgroup and cause bacterial leaf streak (BLS) on wheat, barley, oat, triticale, and rye (Duveiller, 1994;Vauterin et al, 1995;Sapkota et al, 2020).…”

“…The group is subdivided into 10 different pathovars based on biochemical and molecular characteristics and host ranges (van den Mooter et al, 1987;Sapkota et al, 2020) and are taxonomically placed into the translucens and graminis subgroups (Vauterin et al, 1992;Langlois et al, 2017;Peng et al, 2019). Pathovars undulosa, translucens, hordei, cerealis, secalis and phleipratensis belong to the translucens subgroup and cause bacterial leaf streak (BLS) on wheat, barley, oat, triticale, and rye (Duveiller, 1994;Vauterin et al, 1995;Sapkota et al, 2020). The "graminis" subgroup comprises of four pathovars (arrhenatheri, graminis, poae, and phlei) that cause bacterial wilt diseases on forage grasses (Vauterin et al, 1995;Sapkota et al, 2020).…”

“…Pathovars undulosa, translucens, hordei, cerealis, secalis and phleipratensis belong to the translucens subgroup and cause bacterial leaf streak (BLS) on wheat, barley, oat, triticale, and rye (Duveiller, 1994;Vauterin et al, 1995;Sapkota et al, 2020). The "graminis" subgroup comprises of four pathovars (arrhenatheri, graminis, poae, and phlei) that cause bacterial wilt diseases on forage grasses (Vauterin et al, 1995;Sapkota et al, 2020). Giblot-Ducray et al (2009) published a new pathovar, pistaciae, that is pathogenic to pistachio and has not yet been classified to either subgroups.…”

“…In the last decade, the incidence of BLS in wheat has significantly increased in the Midwestern United States, the major wheat-growing regions ( McMullen and Adhikari, 2011 ; Adhikari et al, 2012 ), and wheat-belt of western Canada 1 . The typical symptoms of BLS start with water-soaked streaks that subsequently become translucent necrotic leaf lesions ( Smith et al, 1919 ; Duveiller et al, 1997 ; Sapkota et al, 2020 ), and under high disease pressure, the entire wheat leaf might be severely affected. Bacterial ooze can be seen on the leaf surface under humid and warm weather conditions.…”

Detection of Xanthomonas translucens pv. undulosa from wheat by quantitative PCR

“…Xanthomonas translucens is a species of gram-negative bacteria causing bacterial leaf streak (BLS), also known as bacterial leaf stripe and black chaff, on cereals (Sapkota et al 2020). The disease was first reported on barley (Jones et al 1917) and wheat (Smith et al 1919) in the USA and the pathogens were named Bacterium translucens and B. translucens var.…”