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

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