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

6Seed Infectionby BacteriaBacteria are prokaryotes that have a rigid cell wall, cell membrane, and often oneor more flagella. A large number of bacteria are saprophytes, and plant pathogenicbacteria are basically facultative saprophytes. The nonfilamentous phytopathogenicbacteria generally belong to Acidovorax, Agrobacterium, Burkholderia, Erwinia,Pantoea, Pseudomonas, Ralstonia, Xanthomonas, and coryneform plant pathogens.These genera are usually seed-borne. The coryneform phytopathogenic species thatwere placed earlier in Corynebacterium have been transferred to Arthrobacter, Clavibacter,Curtobacterium, Rathayibacter, and Rhodococcus. Some of the Pseudomonasspecies are now placed in Acidovorax and Burkholderia, and Erwinia has been placedin Pantoea (Agrios, 1988; Young et al., 1996).The bacteria may cause seed infestation, i.e., carried on the surface of the seedor seed infection, that occurs in the seed coat and other parts of the seed. Both typesare known to be seed-transmitted, causing failure in seed germination and/or diseasesymptoms in seedlings and plants. The effects of the seed infecting bacteria on hosttissues can be determined in histological preparations. Some examples of theseinfections are included here.6.1 PENETRATIONUnlike fungi, bacteria lack mechanisms for forcing their way physically throughprotective barriers, such as cuticle, epidermis, and bark. Before invasion can takeplace, the bacteria must establish themselves on the plant surface, i.e., they shouldbe able to find a proper niche. There are numerous examples of growth of bacteriaon plant foliage. Bacteria multiply with rapidity, and their significance as pathogensmay primarily depend on the fact that they can produce large numbers of cells in ashort time. The penetration and spread of bacteria in plant tissues have been studiedin natural infections and in infections produced after artificial inoculations (Zaumeyer,1930, 1932; Thiers and Blank, 1951; Wiles and Walker, 1951; Pine, Grogan,and Hewitt,, 1955; Layne, 1967; Getz, Fulbright, and Stephens, 1983; Kritzman andZutra, 1983; Rudolph, 1993).The developing ovules and seeds occupy a specific position in angiosperms, asdiscussed in detail in Chapter 2. In order to reach the ovule or seed on a plant theinfection must find passages for invasion of the ovary or developing fruit. Thissituation was discussed in Chapter 4 with respect to fungi but the details are alsoapplicable to bacteria. Seeds after harvest and threshing are directly exposed to theenvironments in storage and in soil.169