A pseudo-response regulator is misexpressed in the photoperiod insensitive Ppd-D1a mutant of wheat (Triticum aestivum L.).

Ppd-D1 on chromosome 2D is the major photoperiod response locus in hexaploid wheat (Triticum aestivum). A semi-dominant mutation widely used in the "green revolution" converts wheat from a long day (LD) to a photoperiod insensitive (day neutral) plant, providing adaptation to a broad range of environments. Comparative mapping shows Ppd-D1 to be colinear with the Ppd-H1 gene of barley (Hordeum vulgare) which is a member of the pseudo-response regulator (PRR) gene family. To investigate the relationship between wheat and barley photoperiod genes we isolated homologues of Ppd-H1 from a 'Chinese Spring' wheat BAC library and compared them to sequences from other wheat varieties with known Ppd alleles. Varieties with the photoperiod insensitive Ppd-D1a allele which causes early flowering in short (SD) or LDs had a 2 kb deletion upstream of the coding region. This was associated with misexpression of the 2D PRR gene and expression of the key floral regulator FT in SDs, showing that photoperiod insensitivity is due to activation of a known photoperiod pathway irrespective of day length. Five Ppd-D1 alleles were found but only the 2 kb deletion was associated with photoperiod insensitivity. Photoperiod insensitivity can also be conferred by mutation at a homoeologous locus on chromosome 2B (Ppd-B1). No candidate mutation was found in the 2B PRR gene but polymorphism within the 2B PRR gene cosegregated with the Ppd-B1 locus in a doubled haploid population, suggesting that insensitivity on 2B is due to a mutation outside the sequenced region or to a closely linked gene.

The pseudo-response regulator Ppd-H1 provides adaptation to photoperiod in barley.

Plants commonly use photoperiod (day length) to control the timing of flowering during the year, and variation in photoperiod response has been selected in many crops to provide adaptation to different environments and farming practices. Positional cloning identified Ppd-H1, the major determinant of barley photoperiod response, as a pseudo-response regulator, a class of genes involved in circadian clock function. Reduced photoperiod responsiveness of the ppd-H1 mutant, which is highly advantageous in spring-sown varieties, is explained by altered circadian expression of the photoperiod pathway gene CONSTANS and reduced expression of its downstream target, FT, a key regulator of flowering.

Comparative sequence analysis of the phytochrome C gene and its upstream region in allohexaploid wheat reveals new data on the evolution of its three constituent genomes.

Bread wheat is an allohexaploid with genome composition AABBDD. Phytochrome C is a gene involved in photomorphogenesis that has been used extensively for phylogenetic analyses. In wheat, the PhyC genes are single copy in each of the three homoeologous genomes and map to orthologous positions on the long arms of the group 5 chromosomes. Comparative sequence analysis of the three homoeologous copies of the wheat PhyC gene and of some 5 kb of upstream region has demonstrated a high level of conservation of PhyC, but frequent interruption of the upstream regions by the insertion of retroelements and other repeats. One of the repeats in the region under investigation appeared to have inserted before the divergence of the diploid wheat genomes, but was degraded to the extent that similarity between the A and D copies could only be observed at the amino acid level. Evidence was found for the differential presence of a foldback element and a miniature inverted-repeat transposable element (MITE) 5' to PhyC in different wheat cultivars. The latter may represent the first example of an active MITE family in the wheat genome. Several conserved non-coding sequences were also identified that may represent functional regulatory elements. The level of sequence divergence (Ks) between the three wheat PhyC homoeologs suggests that the divergence of the diploid wheat ancestors occurred some 6.9 Mya, which is considerably earlier than the previously estimated 2.5-4.5 Mya. Ka/Ks ratios were <0.15 indicating that all three homoeologs are under purifying selection and presumably represent functional PhyC genes. RT-PCR confirmed expression of the A, B and D copies. The discrepancy in evolutionary age of the wheat genomes estimated using sequences from different parts of the genome may reflect a mosaic origin of some of the Triticeae genomes.