Histopathology of Seed-Borne Infections - Applied Research Center ...

170 Histopathology of Seed-Borne Infections6.1.1 INVASION OF PLANT PARTSBacterial diseases of seedling and plant may be established from seed-borne infection(Neergaard, 1979) or may take place through soil, air, water, insects, and nematodes.The entry of phytopathogenic bacteria may be passive and occur through naturalopenings (stomata, lenticels, hydathodes, or nectariferous surfaces [nectarthodes]),through wounds including scars left by dropping of hairs, cracks, or by means ofenzymes. Entry through noncutinized surface of hairs, stigma, and anthers has alsobeen suggested. The enzymatic penetration of the surface depends on maceratingand digesting enzymes that degrade the barrier presented by the complex plant cellwall. Enzymatic degradation of plant tissue is important in pathogenesis, andenzymes involved in soft rotting by bacteria are polygalacturonase, pectate lyases,cellulase, proteases, and nucleases (Klement, Rudolph, and Sands, 1990). Only afew phytopathogenic bacteria can degrade cellulose at a rate comparable to that ofmany cellulolytic fungi (Kelman, 1979). Although bacterial degradation of cell wallcomponents, including lignin, has been reported in wood following extensive exposureto high moisture, conclusive evidence for lignolytic capabilities of the specificbacteria involved has not been demonstrated (Liese, 1970). Furthermore, no bacterialplant pathogens that degrade lignin are known (Kirk and Connors, 1977). This isimportant since the seed coat in the majority of true seeds and the pericarp in oneseededindehiscent fruits possess lignified cells.The stomata on the leaf and stem are the common passagex for entry of bacteria.Zaumeyer (1930) found that Xanthomonas axonopodis pv. phaseoli (Bacteriumphaseoli) enters the leaves, stems, and pods of the bean plant through the stomata(Figure 6.1A to E). The entry of bacteria through the stomata seems to be commonand has been reported for X. axonopodis pv. malvacearum in cotton (Thiers andBlank, 1951); Pseudomonas syringae pv. pisi in pea (Skoric, 1927); P. s. pv. lachrymansin cucumber (Wiles and Walker, 1951); Burkholderia plantari, B. glumae,and Pantoea agglomerans (Erwinia herbicola) in the lemma and palea of rice grains(Azegami, Tabei, and Fukuda, 1988; Tabei et al., 1988, 1989); Clavibacter michiganensissubsp. michiganensis in tomato (Layne, 1967); and Curtobacterium flaccumfacienspv. flaccumfaciens in beans (Schuster and Sayre, 1967). Fukuda,Azegami, and Tabei (1990) found that P. s. pv. syringae (P. s. pv. japonica) invadedthe leaf blade, leaf sheath, lemma, and palea through the stomata in barley andwheat. Azegami, Tabei, and Fukuda (1988) also observed that B. plantari andB. glumae entered young rice seedlings through the stomata present in the surfacesof coleoptiles and leaf sheaths. Tabei (1967) found that nonpathogenic bacteria andXanthomonas oryzae pv. oryzae (X. c. pv. oryzae) entered through the stomata inrice, and pointed out that this seems to occur easily since some stomata in coleoptileand leaf sheaths are always open. Ramos and Valin (1987), while examining therole of stomatal opening and frequency on infection of Lycopersicon species byXanthomonas vesicatoria, noted positive correlation between stomatal frequency onadaxial and abaxial leaf surfaces and the number of spots and lesions produced onartificial inoculation. Bacterial spots also were considerably reduced when the stomatalclosure was physiologically induced or chemically suppressed by abscissicacid or phenylmercuric acetate before inoculation with X. vesicatoria.