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On-Line Glossary: R

Each entry consists of a term (in bold), a definition of the term, and a number in parentheses (0) indicating the source of the definition. Clicking on the speaker icon will give you the pronunciation of the term. (Be sure to close the window for the sound player after each use.) The pencil icon will give you a drawing, and the camera icon will give you a photograph.


race. A subspecies group of pathogens that infect a given set of plant varieties. (2)

rachis. In fungi: A conidiophore elongating to one side of a terminally produced spore, often resulting in a zig-zag-shaped structure. (22)

range. Of a plant pathogen: The geographical region or regions in which it is known to occur. (20)

recombinant DNA. DNA molecules in which sequences, not normally contiguous, have been placed next to each other by in vitro methods. (10)

rectum. In nematodes: Posterior gut of the female. A narrow, dorsoventrally flattened tube that is lined with cuticle and separated from the intestine by a sphincter muscle. (14)

reflexed. Bent back. (14)

REMI. Restriction enzyme mediated integration. A method of transformation that generates tagged mutations. (19)

reniform. Kidney-shaped. (14)

replicon. Any DNA sequence or molecule that possesses a replication origin and is therefore potentially capable of being replicated in a suitable cell. (16)

resistance. The ability of an organism to exclude or overcome, completely or in some degree, the effect of a pathogen or other damaging factor. (2)

resistant. Possessing resitance. (3)

response. The change produced in an organism by a stimulus. (20)

resting spore. A thick-walled spore, particularly one formed by a sexual process, germinating only after an extended period of dormancy (e.g., an overwintering teliospore). (16)

restriction endonuclease. An endonuclease that binds to double-stranded DNA at a specific nucleotide sequence and then, if both strands of the DNA lack appropriate modification at that sequence, cleaves the DNA either at the recognition sequence or at another site in the DNA molecule. (16)

reticulate. Having net-like markings. (14)

retrorse. In a backward or downward direction. (14)

revolute. Of the margin of the pileus: rolled back or up. (17)

RFLP. Restriction fragment length polymorphism. Inherited differences in sites for restriction enzymes (e.g., caused by base changes in the target site) that result in differences in the lengths of the fragments produced by cleavage with the relevant restriction enzyme. RFLPs are used for genetic mapping to link the genome directly to a conventional genetic marker. (13)

rhabdions. In nematodes: Plates in the cuticular lining of the stoma that make up the walls of the various divisions of the stoma. (14)

rhizoid. A root-like structure forming part of the thallus in certain algae and fungi; it may anchor the organism to the substratum and/or act as an absorptive organ. (16)

rhizomorph. A macroscopic, typically rope-like strand of compacted tissue formed by certain higher fungi. Rhizomorphs often are enduring structures that can remain dormant under adverse conditions. (22)

rhizomycelium. Branching, anucleate or sparsely nucleate, rhizoidal filaments of variable width forming part of the thallus in some fungi. (Pl. rhizomycelia.) (16)

rhizoplane. The root surface. (16)

rhizosphere. An environment regarded, variously, as (a) that region of the soil modified as a result of the uptake and deposition of substances by a growing root, (b) the root itself, together with that volume of soil which it influences, (c) the root surface together with that region of the surrounding soil in which the microbial population is affected by the presence of a root. (16)

Rickettsiae. Microorganisms similar to bacteria in most respects but generally capable of multiplying only inside living host cells; parasitic or symbiotic. (2)

ringspot. A type of local lesion consisting of single or concentric rings of discoloration or necrosis, the regions between the concentric rings being green. The center of the lesion may be chlorotic or necrotic. (16)

RNA blotting. A technique for transferring RNA from an agarose gel to a nitrocellulose filter on which it can be hybridized to a complementary DNA. (13)

roestelium. In the Uredinales an aecium with a cornute peridium, thin at the sides, usually rupturing by longitudinal slits, and made up of characteristically marked and imbricated cells. The name is taken from the form-genus Roestelia. (Pl. roestelia.) (17)

rogue. A variation from the standard varietal type; also, to remove such undesirable plants (especially those infected with viruses) from the growing crop. (20)

roguing. The removal of diseased plants from a crop in order to prevent the spread of the disease. (16)

rosette. An abnormal condition in which the leaves form a radial cluster on the stem. (16)

rot. The softening, discoloration, and often disintegration of a succulent plant tissue as a result of fungal or bacterial infection. (2)

roundworm. Nematode. (20)

rugose. Wrinkled. (14)

russet. Brownish, roughened areas on the skin of fruit as a result of cork formation. (2)

rust. 1. Fungus of the class Urediniomycetes. 2. Any of various plant diseases caused by members of the Urediniomycetes or by species of Albugo. The diseases are called "rusts" because many of the causal agents form rust-colored spores on affected plants. (16)

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