Infection of the sorghum mesocotyl by Helminthosporium maydis (a nonpathogen) and Colletotrichum graminicola (a pathogen) resulted in the rapid accumulation of a pigment complex by two sorghum cultivars. The components of the complex were fungitoxic. The principal compounds have been identified as the 3-deoxyanthocyanidins apigeninidin and luteolinidin. Apigeninidin accumulated in both sorghum cultivars in response to infection and was the predominant pigment. Luteolinidin accumulated in only one of the cultivars. Because of the speed of synthesis, occurrence only in response to inoculation, and fungitoxicity of the individual components, we propose that synthesis of the pigment complex constitutes a defense response and that the compounds apigeninidin and luteolinidin should be considered as phytoalexins.With the exception of the momilactones from rice (1, 2) and the avenalumins from oat (3, 4), phytoalexins have not been identified in members of the Gramineae. In grain sorghum and other sorghum species the cyanogenic glycoside dhurrin is a potential microbial toxicant but is best considered an insect feeding deterrent and phytotoxin rather than a stress metabolite (5-7). Dhurrin, present in relatively high concentration in uninfected tissue, does not increase following microbial infection (8, 9) and the dhurrin content of tissue decreases with maturation (10).The situation in sorghum is further complicated by the fact that seedling leaves exhibit few if any symptoms when inoculated with various fungi. Probably the most pronounced example of this phenomenon is foliar anthracnose caused by Colletotrichum graminicola. In this disease both resistant and susceptible cultivars are resistant in the seedling stage and plants must be about 5 weeks old before they will exhibit symptoms in response to inoculation (11). Such observations have led to the assumption that seedling resistance is the result of the presence of dhurrin (11). However, Fry and coworkers (8,9,12,13) have demonstrated that fungal pathogens of cyanogenic plants produce formamide hydrolyase, which detoxifies hydrogen cyanide, the toxic breakdown product of dhurrin, by converting it to formamide. Therefore, that dhurrin has a role in seedling resistance, either to pathogens or to nonpathogens, is not yet clear.The purpose of this investigation was to evaluate whether resistance in sorghum is associated with the accumulation of previously undetected toxic host metabolites. We describe the rapid accumulation of phytoalexins in the sorghum mesocotyl as a response to attempted infection by a speciespathogenic and a species-nonpathogenic fungus.
MATERIALS AND METHODSPathogens, Plant Material, and Inoculation. The fungi Helminthosporium maydis Nisik. and Miy., race 0, and Colletotrichum graminicola (Ces.) Wils. were grown and spore suspensions were prepared for inoculum as described (14,15). Inoculum concentrations were 5 x i04 spores per ml for H. maydis and 106 spores per ml for C. graminicola.Tween 20 was used as a wetting agent (100 tkl per 100 ...
cDNA clones representing the ssRNA genome of the NY-RPV isolate of barley yellow dwarf luteovirus (BYDV) were sequenced and 5600 nucleotides of the genome were determined. The deduced genome organization has limited similarity to that of another BYDV isolate, Vic-PAV, but is identical to that of beet western yellows (BWYV) and potato leafroll (PLRV) luteoviruses. NY-RPV has six major positive-sense open reading frames (ORFs) and, by comparison with RNA-dependent RNA polymerase and nucleic acid helicase consensus sequence motifs, it is postulated that NY-RPV ORF2 and ORF3 encode the viral replicase, which is expressed by a translational frameshift mechanism. The region of the NY-RPV genome containing the 22K coat protein ORF, the apparently associated internal apparent VPg ORF and the ORF immediately 3'-proximal (ORF6) to the coat protein ORF are organized as reported for other luteoviruses. Evidence is presented showing that ORF6 is expressed by readthrough of the coat protein gene termination codon, and that this protein is associated with the intact virus as a 65K protein. Although NY-RPV infects graminaceous rather than dicotyledonous plants, the taxonomic relationships between BYDV isolates and other luteoviruses deduced from the genome organization and sequence data strongly suggest that NY-RPV is distinct from the PAV-like isolates of BYDV and is more closely related to BWYV and PLRV.
Barley yellow dwarf virus (BYDV) can be separated into two groups based on, among other criteria, serological relationships that are presumably governed by the viral capsid structure. Nucleotide sequences for the coding regions of coat proteins of approximately 22 K were identified for the MAV-PS 1, P-PAV (group 1) and NY-RPV (group 2) isolates of BYDV, The MAV-PS1 and P-PAV coat protein sequences shared 71% deduced amino acid similarity whereas that of the NY-RPV isolate shared no more than 51% similarity with either the MAV-PSI or the P-PAV sequence. Other comparisons showed that these and other BYDV coat protein sequences examined to date share a high degree of identity with those identified from other luteoviruses. Among luteovirus coat protein sequences in general, several highly conserved domains were identified whereas other domains differentiate MAV-PSI and PAV isolates from NY-RPV and other luteoviruses. Sequence similarities and differences among BYDV coat proteins (approx, 22K) are consistent with the serological relationships exhibited by these viruses. Amino acid sequence comparisons between BYDV isolates that share common aphid vectors indicate that it is unlikely that these coat proteins are involved in aphid specificity.
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