The introduction of the Reduced height (Rht)-B1b and Rht-D1b semidwarfing genes led to impressive increases in wheat (Triticum aestivum) yields during the Green Revolution. The reduction in stem elongation in varieties containing these alleles is caused by a limited response to the phytohormone gibberellin (GA), resulting in improved resistance to stem lodging and yield benefits through an increase in grain number. Rht-B1 and Rht-D1 encode DELLA proteins, which act to repress GA-responsive growth, and their mutant alleles Rht-B1b and Rht-D1b are thought to confer dwarfism by producing more active forms of these growth repressors. While no semidwarfing alleles of Rht-A1 have been identified, we show that this gene is expressed at comparable levels to the other homeologs and represents a potential target for producing novel dwarfing alleles. In this study, we have characterized additional dwarfing mutations in Rht-B1 and Rht-D1. We show that the severe dwarfism conferred by Rht-B1c is caused by an intragenic insertion, which results in an in-frame 90-bp insertion in the transcript and a predicted 30-amino acid insertion within the highly conserved amino-terminal DELLA domain. In contrast, the extreme dwarfism of Rht-D1c is due to overexpression of the semidwarfing Rht-D1b allele, caused by an increase in gene copy number. We show also that the semidwarfing alleles Rht-B1d and Rht-B1e introduce premature stop codons within the amino-terminal coding region. Yeast two-hybrid assays indicate that these newly characterized mutations in Rht-B1 and Rht-D1 confer "GA-insensitive" dwarfism by producing DELLA proteins that do not bind the GA receptor GA INSENSITIVE DWARF1, potentially compromising their targeted degradation.

SUMMARY Globally, virus diseases are common in agricultural crops and have a major agronomic impact. They are countered through the deployment of genetic resistance against the virus, or through the use of a range of farming practices based upon the propagation of virus-free plant material and the exclusion of the virus vectors from the growing crop. We review here the current status of our knowledge of natural virus resistance genes, and consider the future prospects for the deployment of these genes against virus infection.

The Geminiviridae is an extensive family of plant viruses responsible for economically devastating diseases in crops worldwide. Geminiviruses package circular, single-stranded DNA (ssDNA) genomes. The characteristic twinned or "geminate" particles, which consist of two joined, incomplete T = 1 icosahedra, are unique among viruses. We have determined the first structure of a geminivirus particle, the Nigerian strain of Maize streak virus (MSV-N), using cryo-electron microscopy and three-dimensional image reconstruction methods. The particle, of dimensions 220 x 380 A, has an overall 52-point-group symmetry, in which each half particle "head" consists of the coat protein (CP) arranged with quasi-icosahedral symmetry. We have modeled the MSV-N CP as an eight-stranded, antiparallel beta-barrel motif (a structural motif common to all known ssDNA viruses) with an N-terminal alpha-helix. This has produced a model of the geminate particle in which 110 copies of the CP nicely fit into the reconstructed density map. The reconstructed density map and MSV-N pseudo-atomic model demonstrate that the geminate particle has a stable, defined structure.

The nucleotide sequence of the DNA of maize streak virus (MSV) has been determined. The data were accommodated into one DNA circle of 2687 nucleotides, in contrast to previously characterised geminiviruses which have been shown to possess two circles of DNA. Comparison of the nucleotide sequences of the DNA of MSV with those of cassava latent virus (CLV) and tomato golden mosaic virus (TGMV) showed no detectable homology. Analysis of open reading frames revealed seven potential coding regions for proteins of mol. wt. greater than or equal to 10 000, three in the viral (+) sense and four in the complementary (‐) sense. The position of likely transcription signals on the MSV DNA sequence would suggest a bidirectional strategy of transcription as proposed for CLV and TGMV. Nine inverted repeat sequences which have a potential of forming hairpin structures of delta G greater than or equal to ‐14 kcal/mol have been detected. Three of these hairpin structures are in non‐coding regions and could be involved in the regulation of transcription and/or replication.

SUMMARYInsertion and deletion mutagenesis of the two virion-sense genes, V1 and V2, of maize streak virus (MSV) prevents symptomatic infections following Agrobacteriummediated 'agroinoculation' of maize seedlings. These genes code for an Mr 10900 protein and for coat protein, respectively. Mutants containing insertions or deletions in the coat protein gene, V2, were able to replicate to low levels, producing dsDNA although virion ssDNA was not detected and symptoms were not observed. Hence, unlike the bipartite geminiviruses, MSV requires coat protein to produce symptomatic systemic infection. Mutations in gene V1 which considerably shortened the Mr 10900 protein (V1 gen¢) resulted either in low levels of replication, in which all the DNA forms associated with a wild-type infection were produced, or in no infection, in which case coat protein production may also have been affected. A V1 mutant generated in vivo with 11 of the 14 N-terminal amino acids altered, was viable and produced symptoms typical of a wild-type infection. Infectivity, assessed by replication and symptom expression, was restored by co-inoculating constructs containing single mutations in different open reading frames, thus rescue can occur by transcomplementation of gene products. The experiments showed that the mutations did not affect the nucleotide sequence requirements for replication and that in all cases intermolecular recombination eventually resulted in dominant wild-type virus.

The Green Revolution dwarfing genes, Rht-B1b and Rht-D1b, encode mutant forms of DELLA proteins and are present in most modern wheat varieties. DELLA proteins have been implicated in the response to biotic stress in the model plant, Arabidopsis thaliana. Using defined wheat Rht near-isogenic lines and barley Sln1 gain of function (GoF) and loss of function (LoF) lines, the role of DELLA in response to biotic stress was investigated in pathosystems representing contrasting trophic styles (biotrophic, hemibiotrophic, and necrotrophic). GoF mutant alleles in wheat and barley confer a resistance trade-off with increased susceptibility to biotrophic pathogens and increased resistance to necrotrophic pathogens whilst the converse was conferred by a LoF mutant allele. The polyploid nature of the wheat genome buffered the effect of single Rht GoF mutations relative to barley (diploid), particularly in respect of increased susceptibility to biotrophic pathogens. A role for DELLA in controlling cell death responses is proposed. Similar to Arabidopsis, a resistance trade-off to pathogens with contrasting pathogenic lifestyles has been identified in monocotyledonous cereal species. Appreciation of the pleiotropic role of DELLA in biotic stress responses in cereals has implications for plant breeding.

Three RNA transcripts encoded by maize streak virus DNA were detected in polyadenylated RNA from virus-infected maize leaves.Two of the transcripts, a major 0.9kb and a minor 1.05kb RNA, were mapped on the virion (+) sense DNA and the other minor transcript of 1.2kb was mapped on the complementary (-) sense DNA, demonstrating that transcription of MSV DNA was bidirectional.The two virion sense transcripts were 3' coterminal at nucleotide 1114 but had 5' termini at nucleotides 2682 and 163 respectively. Virus-specific polyadenylated RNA translated in vitro to produce a 28,000MW polypeptide, specifically immunoprecipitable by antiserum raised against whole virus. The mRNA for this protein was mapped by hybrid-arrested translation to the long open reading frame in virion sense DNA whose potential amino acid composition, calculated from nucleotide sequence data, closely agreed with that determined experimentally for the coat protein.

Maize streak virus (MSV) coat protein (CP) is