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

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Seed Infection by Bacteria 177scbembbAbembFIGURE 6.5 Location of Xanthomonas campestris pv. campestris in naturally infected seedsof Brassica campestris. A, Ls part of seed showing bacterial masses in seed coat and in thespaces between endosperm and embryo, and in between the cotyledons. B, A magnified viewof part of seed showing aggregation of bacteria in seed coat, endosperm, and the space outsidethe embryo. (Abbreviations: b, bacteria; emb, embryo; sc, seed coat.) (From Sharma, J.,Agarwal, K., and Singh, D. 1992. Seed Res. 20: 128–133.)Azegami, and Tabei (1990) reported that P. s. pv. syringae invaded the kernel in thewheat and barley caryopsis through the funiculus.B6.1.3 INVASION OF THRESHED AND DISSEMINATED SEEDSAfter harvest and threshing or dissemination seeds find an entirely different environmentfrom the environment during their development inside the fruit. Such seedsare directly exposed to the surroundings and have a well-formed cuticule; wax
178 Histopathology of Seed-Borne Infectionsdeposits, when present; a seed coat with protective thick-walled and lignified layers;a hilum; micropyles, closed or open to various extents; raphe, when present,; andpits or micropores and cracks on the surface. Since seeds are stored under dryconditions, no free moisture is available. The soil environments of seeds are highlyvariable and range from dry to water-logged conditions.Field bacteria in general are not expected to cause internal infection in dry seedsduring storage; however, seed contamination is possible during threshing. Underfavorable conditions of humidity and temperature, in true seeds, the avenues forinfection are (1) the seed surface through the cuticle, natural openings such as thestomata as in cotton, micropores, cracks, or through injuries caused during threshing;(2) the hilum, which is covered by cuticle, but usually has fissures; (3) micropyles,particularly the open type; and (4) accessory structures, e.g., hairs, wings, aril, andcaruncle.One-seeded fruits such as caryopsis, cypsils, achenes, cremocarp, and mericarpmay also become infected or contaminated during the postharvest period. Contaminationis common during threshing as well as during storage and in soil, but infectionwill depend on favorable environmental conditions. Avenues for infection in suchseeds could be the pericarp and adhering bract surface, separation scar (analogousto hilum), remnants or scars caused by the style and stigma, and persistent accessorystructures. Information on penetration and establishment of bacterial infection ofseeds (true seeds as well as one-seeded fruits) during the postharvest period eitherin storage or in soil is meager. Grogan and Kimble (1967) have reported that inhalo-blight of beans caused by P. s. pv. phaseolicola the hilum and cracks in theseed coat during threshing or from wetting provide sites at which the bacteriumgains entry.6.2 HISTOPATHOLOGY OF INFECTED SEEDSThe important bacterial species that cause seed infection and have been investigatedhistologically are listed in Table 6.1. These belong to the genera Xanthomonas,Pseudomonas, Acidovorax, Burkholderia, Rathayibacter, Clavibacter, Curtobacterium,and Pantoea. The nomenclature followed is as given in Young et al.’s (1996)article “Names of Plant Pathogenic Bacteria, 1854–1995.”6.2.1 XANTHOMONASXanthomonas species cause some of the serious bacterial diseases in crucifers, beans,cotton, and rice (Table 6.1). Xanthomonas c. pv. campestris is a serious pathogenthat causes black rot in crucifers. Cook, Larson, and Walker (1952) reported systemicinvasion by this bacterium through the vascular system of pedicels and pods (silicula).Further invasion through the xylem of the funiculus was recorded and infectionof the seed coat was suspected. Walker (1950) observed that X. c. pv. campestrisreaches the seed coat through the vascular system in the funiculus. Sharma, Agarwal,and Singh (1992) have described the location of X. c. pv. campestris in rape andmustard seeds. The infected seeds were asymptomatic and symptomatic and werecategorized into bold-symptomless, bold-discolored, and ) shriveled-discolored. The
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Histopathology ofSeed-Borne Infecti
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PrefaceThe book deals with only one
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The AuthorsDalbir Singh, Ph.D., for
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ContentsChapter 1 Introduction ....
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4.3.2 Routes for Infection from Ova
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8.4 Survival in Seed ..............
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2 Histopathology of Seed-Borne Infe
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4 Histopathology of Seed-Borne Infe
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2Reproductive Structuresand Seed Fo
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Reproductive Structures and Seed Fo
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14 Histopathology of Seed-Borne Inf
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3Structure of SeedsThe seed habit i
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Structure of Seeds 49(1971) of usin
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Structure of Seeds 51is found in as
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Structure of Seeds 53dry seed of Ly
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Structure of Seeds 553.3.2 SEED COA
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Structure of Seeds 57Endosperm: One
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Structure of Seeds 59scendembepsmuA
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Structure of Seeds 61Endosperm: Sca
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Structure of Seeds 63Chalaza simple
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Structure of Seeds 65stystypmerA Bc
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Structure of Seeds 67epsent endCADB
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Structure of Seeds 69perhscendembpe
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Structure of Seeds 71eposgepi′epo
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Structure of Seeds 73(Zea and Sorgh
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Structure of Seeds 75Baker, D.M. an
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Structure of Seeds 77Maheshwari Dev
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Structure of Seeds 79Vries, M.A. de
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Page 232 and 233: Seed Infection by Viruses 2177.4.2
Page 234 and 235: Seed Infection by Viruses 219seed(s
Page 236 and 237: Seed Infection by Viruses 221AMV fo
Page 238 and 239: Seed Infection by Viruses 223REFERE
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Seed Infection by Viruses 227Wolf,
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