Bacterial Blight Resistance in Rice - Translational Genomics for Crop Breeding: Biotic Stress - page 12

Xoo is a gram-negative bacterium that is
rod-shaped, round-ended, motile, and slime-
producing with a polar flagellum. The length and
width of individual cells are approximately 0.7
to 2.0 μm and 0.4 to 0.7 μm, respectively. Bac-
terial colonies on nutrient solid media are yel-
low, round, and convex (Webster and Gunnell
1992) (Figure 2.1C). Xoo is aerobic, catalase-
positive, able to produce acids from carbohy-
drates, and unable to use nitrate. The optimal
temperature range for Xoo growth is 25 ◦ Cto
30 ◦ C (Bradbury 1984). Identification and clas-
sification of the bacterial pathotypes of Xoo are
helpful for resistance breeding and disease con-
trol of BB. However, the morphological, phys-
iological, and biochemical characters of differ-
ent pathotypes are identical (Reddy and Reddy
1990). Based on the infection responses elicited
in rice lines, Japanese Xoo strains have been clas-
sified into 6 virulence groups (I to VI), Philip-
pines Xoo strains have been classified into 10
virulence groups (race 1 to 10), Chinese Xoo
strains include 7 virulence groups (C1 to C7),
and Indian Xoo strains can be classified into 13
clusters and 5 broad groups (Ezuka and Horino
1974; Vera Cruz 1984; Fang 1990; Nayak et al.
2008).
The genomes of three Xoo strains, includ-
ing Japanese strain MAFF311018, Korean strain

The Disease and Pathogen

BB, also called “kersek” at early growth stage of
the plant, is caused by Xanthomonas oryzae pv.
oryzae ( Xoo ) and is one of the oldest known crop
diseases. It was first reported by the farmers of
Fukuoka (Japan) in 1884 (Yamanuki et al. 1962).
Subsequently, it was found in various parts of
Asian countries, Australia, African countries,
and the United States. BB occurs in both temper-
ate and tropical regions, but outbreaks are more
frequent in irrigated and rainfed lowland areas.
Severe epidemics often occur with strong winds
and continuous heavy rains (Ou 1985). Xoo may
be seed-borne and can be spread by irrigation
water, but this is disputed (Mizukami 1961;
Premalatha and Devadath 1983). The pathogen
may survive on infected cultivated rice plants or
other hosts (wild rice and gramineous weeds)
over winter (Ou 1985). Under favorable condi-
tions, Xoo invades rice leaves through hydath-
odes or wounds, multiplies in the intercellular
space of the underlying epithem, and spreads
into the plant through the xylem vessels, result-
ing in yellow lesions with wavy margins along
the veins that may systemically extend to the
sheath (Figures 2.1A, 2.1B). BB is observed on
both seedlings and adult plants and peaks at the
flowering stage.

Fig. 2.1. Bacterial blight disease of rice. (A) Rice cultivar infected by Xoo . (B) Infected rice leaves after
artificial inoculation of Xoo .(C) Xoo colonies. For a color version of this figure, please refer to the color plate.