Chapter 5. Effect of pathogen on plant physiology

Transcription

1 Chapter 5 Effect of pathogen on plant physiology

2 Effect of pathogens on host physiology Photosynthesis Respiration Permeability of cell membranes Translocation of water and nutrients Transcription and translation

3 Photosynthesis The influence of virus infection Reduction of chloroplast numbers Reduction in chlorophyll content Chloroplast abnormalities Reduction in photochemical activity Stimulating CO 2 incorporation at early stage of infection, but declined after virus infection for several days. Reduction in sucrose content

4 Photosynthesis The influence of bacterial infection Decrease in chloroplast stroma (chloroplast content) Disorientation of chloroplasts Destroy of chloroplast integrity - HR Suppression of CO 2 fixation

5 Hypersensitive reaction (HR, 過敏性反應 ) E. C. Stakman (1915) is generally credited with the use of the term, hypersensitive reaction (HR). HR involves the extremely rapid death of only a few host cells which limits the progression of the infection.

6 Characteristics of hypersensitive response Cessation of cytoplasmic streaming Membrane damage Burst of reactive oxygen species Protoplast (vacuoles) collapse Release of second metabolites fluorescent compounds Browning of cells

7 Appearance of HR 42 hpi 120 hpi Oat Rodney (Pg-2) infected with incompatible isolates of Puccinia graminis f.sp. avenae Pga-1H

8 Appearance of HR 42 h 60 h Oat Rodney (Pg-2) infected with incompatible isolates of Puccinia graminis f.sp. avenae Pga-1H

9 Photosynthesis The influence of fungal infection Reduce chloroplast RNA content Loss of chlorophyll Inhibit photophosphorylation coupling mechanism Inhibit electron transport (continued)

10 Photosynthesis Suppress CO 2 fixation The CO 2 fixation is usually suppressed in fungus-infected plants. A biphasic inhibition of CO 2 fixation is observed in barley infected by powdery mildew fungi. The first phase of inhibition occurs prior to or during early sporulation, when no chlorosis is seen on leaves. The second phase of inhibition occur about six days after infection of barley when chlorotic symptoms are visible. Rust disease are exception, in that CO 2 fixation is enhanced in the early stage of disease, but inhibited in the later stage of disease.

11 Photosynthesis Stimulation of CO 2 fixation in uncolonized leaves A stimulation of CO 2 fixation in the light is characteristic of the noninfected leaves of heavily rusted bean plants. Altered translocation of organic compounds In disease caused by obligate parasites, translocation is enhanced from the uninfected tissues (source) to the infected tissues (sink), while the export from infected leaves is greatly suppressed. => Green island In the late blight disease of potato, which is caused by a nonobligate parasite (Phytophthora infestans), there was no evidence for movement of photosynthetic products from uninfected to infected leaf tissues.

12 Photosynthesis Starch accumulation Obligate parasites seem to cause starch accumulation in the diseased host chloroplasts as well as in host cytoplasm of nongreen tissue.

13 Green island on wheat infected with wheat powdery mildew

14 Green island Occurred in plant infected by obligate parasite, such as powdery mildew or rust fungi. It usually occurred in later stage of disease. Active starch accumulation and chlorophyll synthesis.

15 Green island Green island has been supposed to be due to juvenility effect of cytokinins and their action on nutrient metabolism. Cytokinin produced at the infection sites exerts a juvenility effect on tissues and directly govern short- and long-distance movement of nutrients. The cytokinins increase in rust-infected bean leaves is of host origin.

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20 Respiration Pathological respiration induced by viruses Respiration rate Nonhypersensitive hosts (systemic hosts) Slightly increase in respiration rate of inoculated leaves Hypersensitive hosts A much more pronounced increase in respiration activity than systemic hosts.

21 Respiration Pathological respiration induced by bacteria In pepper Xanthomonas vesicatoria interaction, an immediate increase in O 2 consumption was detected in resistant tissue, whereas susceptible tissue did not reflect an increase in respiration until about 30h after inoculation. In incompatible interaction, the bacteria usually caused great increase in respiratory rate.

22 Respiration Pathological respiration induced by fungi Respiration rate is usually increased in diseased plants. In the early stage of disease, synthetic processes induce high rates of respiration, whereas in the late stages injury and decomposition of tissues lead to increase respiration. Pathological respiration in resistant hosts O 2 consumption increases more rapidly in resistant plants infected either by obligate or facultative parasites at the early stage of disease; later, the respiration rate gradually decreases.

23 Effect of pathogens on permeability of host cell membranes Pathogens can change the permeability of host cell membranes by mechanical injury, enzymatic degradation, or toxins.

24 Translocation of water and nutrients Plants infected by viruses Although there are exceptions, virus infection generally leads to a reduction in transpiration rate, which is often correlated with a reduced leaf stomatal aperture. Accumulation of carbohydrates in leaf tissue is a characteristic of severe virus diseases. It is usually accompanied by phloem necrosis and/or gummosis, particularly in the later stage of disease.

25 Translocation of water and nutrients Plants infected by bacteria Bacteria can enter the vascular systems, both xylem and phloem, through wounds. Production of EPS might clot the vascular system and cause wilting.

26 Translocation of water and nutrients Plants infected by fungi Absorption of water by diseased roots is usually inhibited. In vascular wilt diseases, water flow through the vessels of diseased stems is reduced. In plants with vascular disease, the transpiration is significantly less than in healthy plants. This lower transpiration is closely related to the plugging of vessels and the resultant shortage of water in the leaves. Polysaccharides (ex., tyloses) produced by Fusarium may also be involved in the obstruction of normal water translocation.

27 Translocation of water and nutrients Powdery mildew strongly inhibits stomatal opening, thereby reducing transpiration rate. However, when epidermis is ruptured, enhanced transpiration is observed.

28 Transcription and translation Transcription and translation of host cells usually increase in response to pathogen infection, but with a higher level for resistant plants, since the resistant plants need to proceed the defense reactions.

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