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Fine structure of aphid stylet pathways and its use in host plant resistance studies

The detailed analysis of stylet pathways is a useful tool in understanding the methods of phloem finding by aphids and this, in turn, may help in elucidating the mechanisms by which some cultivars of crop plants are resistant to aphid attack. Light microscopy has been used extensively to study these mechanisms and also to investigate aphid probing behaviour with a view to understanding the methods by which aphids locate sieve elements. Most studies have revolved around tracing the pathways taken by stylets in sections of wax-embedded plant material, and have relied on the presence of the aphid salivary sheath which remains in plant tissue after the mouthparts have been withdrawn. Pollard (I 973) cites more than 200 investigations using these methods, and reports that stylet pathways vary between aphid species and have been classified as predominantly intercellular, intracellular or a mixture. However, some investigations of stylet paths using electron microscopy suggest that the picture revealed by the relatively simple and widely used technique of sectioning plant material for light microscopy may be misleading (Evert et al., 1973). Whereas light microscopy suggests largely intercellular pathways, the electron microscope reveals the existence of mixed inter-and intracellular penetration in the same species. An accurate and useful picture of the mode of penetration of aphid stylets into host tissues cannot be obtained using light microscope studies alone. In the course of extensive investigations using the grain aphids Rhopalosiphum padi and Sitobion avenae and the cabbage aphid Brevieoryne brassicae we were unable to determine satisfactorily the stylet path in mesophyll tissue from I2 #m thick sections, in particular, whether stylets passed between or through mesophyll cells (Fig la). This is of critical importance when developing hypotheses for phloem-finding by aphids because the non-random nature of the process implies that physical or chemical cues are perceived and these could be fundamentally different depending on whether cellular and or intercellular contents are encountered. We believe that the limited resolving power of the light microscope places restraints on interpretations of the stylet pathways and therefore decided to compare light microscope studies with results obtained from electron microscope examination of similar material.