Systemic Aquired Resistances: Current Research
Experiment
Disease development and aspect of aspects of oxidative metabolism in Xanthomonas
axonopodis pv. citri infected leaves of Kumquat, Grapefruit and Hamlin.
Material and Methods
Plants were pruned 90 days before inoculation and grown in environmental growth
chambers with temperatures 23°C to 32°C and relative humidity 65 to 95 %, 75 to
100 % fully expanded leaves were used for all bacterial inoculations.
Bacterial strain and inoculation
The primary source of inoculum was strain (Xac-NK08) isolated from a foliar lesion
on a Valencia tree located in a commercial citrus grove near Immokalee, FL.
Leaves were inoculated using a tuberculin syringe (1 cc) without needle. Xanthomonas
axonopodis pv. citri inoculum was slowly infiltrated in to the abaxial
leaf surface on both sides of the mid vein.
Disease development in kumquat and grapefruit
Fig. 1. Disease development in Kumquat inoculated with Xanthomonas axonopodis
pv citri. The number at the bottom of each leaf indicates days after inoculation
(dai). Disease development in Kumquat leaves following infiltration produced localized
yellowing or necrosis restricted to the inoculation site at 2 to 12 dai (Fig. 1)
and leaf abscission occurred from 7 to 12 dai.
Fig. 2. Disease development in Grapefruit inoculated with Xanthomonas axonopodis
pv citri. The number at the bottom of each leaf indicates days after inoculation
(dai). Disease development in Kumquat leaves following infiltration produced localized
yellowing or necrosis restricted to the inoculation site at 2 to 21 dai (Fig. 1).
Antioxidant enzymes
Isoforms of hydrogen peroxide degrading enzymes (Catalase (CAT), peroxidase (POD),
and Ascorbate peroxidase (APOD) were detected in control (CON) and canker (XAC)
infected leaves of Grapefruit. Higher activity and more number of catalase isoforms
were detected in infected leaves. Similarly the activity of peroxidase and ascorbate
peroxidase was also higher in infected leaves to maintain lower levels of hydrogen
peroxide. Salicylic acid (SA) is known to inhibit the activity of CAT and APOD (Vlot
et al., 2009) to maintain higher levels of hydrogen peroxide which can directly
kills the bacteria or act as a signaling molecule for SAR (Vlot et al., 2009
and Durrant and Dong, 2004). It seems levels of SA are not sufficient in grapefruit
to suppress the activity of CAT and APOD to impart a negative effect on canker.
External application of SA may be promising.
The number at each lane indicates days after inoculation.
References
- Vlot et al., 2009. Salicylic acid a multifaceted, hormone to combat disease.
Annual Review of Phytopathology, 47: 177-206.
- Durrant and Dong, 2004. Systemic acquired resistance Annual Review of Phytopathology,
42: 185-209. Physiol. Mol. Plant Pathol. 71: 240-250.