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.
The CO (CONSTANS) gene of Arabidopsis has an important role in the regulation of flowering by photoperiod. CO is part of a gene family with 17 members that are subdivided into three classes, termed Group I to III here. All members of the family have a CCT (CO, CO-like, TOC1) domain near the carboxy terminus. Group I genes, which include CO, have two zinc finger B-boxes near the amino terminus. Group II genes have one B-box, and Group III genes have one B-box and a second diverged zinc finger. Analysis of rice (Oryza sativa) genomic sequence identified 16 genes (OsA-OsP) that were also divided into these three groups, showing that their evolution predates monocot/dicot divergence. Eight Group I genes (HvCO1-HvCO8) were isolated from barley (Hordeum vulgare), of which two (HvCO1 and HvCO2) were highly CO like. HvCO3 and its rice counterpart (OsB) had one B-box that was distantly related to Group II genes and was probably derived by internal deletion of a two B-box Group I gene. Sequence homology and comparative mapping showed that HvCO1 was the counterpart of OsA (Hd1), a major determinant of photoperiod sensitivity in rice. Major genes determining photoperiod response have been mapped in barley and wheat (Triticum aestivum), but none corresponded to CO-like genes. Thus, selection for variation in photoperiod response has affected different genes in rice and temperate cereals. The peptides of HvCO1, HvCO2 (barley), and Hd1 (rice) show significant structural differences from CO, particularly amino acid changes that are predicted to abolish B-box2 function, suggesting an evolutionary trend toward a one-B-box structure in the most CO-like cereal genes.The control of flowering by photoperiod is an important adaptive characteristic in plants. Studies of the model dicot Arabidopsis have shown that the CO (CONSTANS) gene, isolated by Putterill et al. (1995), has an important role in the photoperiod pathway, which is one of four regulatory pathways controlling the timing of flowering (for review, see Mouradov et al., 2002;Simpson and Dean, 2002). CO acts between the circadian clock and genes controlling meristem identity (Samach et al., 2000;Suárez-Ló pez et al., 2001). In Arabidopsis, CO belongs to a family of 17 putative transcription factors defined by two conserved domains (Putterill et al., 1995;Robson et al., 2001). The first is a zinc finger region near the amino terminus that resembles B-boxes, which regulate protein-protein interactions in several animal transcription factors (Borden, 1998;Torok and Elkin, 2000). The second is a region of 43 amino acids near the carboxy terminus termed the CCT (CO, CO-like, TOC1) domain (Strayer et al., 2000;Robson et al., 2001). Studies using green fluorescent protein fusions show that the CCT domain is involved in nuclear localization of the CO protein but must have an additional role because the late-flowering co-7 mutant, which has an altered CCT domain, correctly localizes the protein .Previous analysis of CO-like genes in Arabidopsis showed that the family is subdivided int...
Barley cDNA and genomic clones homologous to the Arabidopsis flowering time regulator GIGANTEA were isolated. Genetic mapping showed that GIGANTEA is present as a single copy gene in barley (3HS) and rice (1S), while two copies are present in maize (3S and 8S) at locations consistent with previous comparative mapping studies. Comparison of the barley peptide with rice and Arabidopsis gave 94% and 79% similarity, respectively. Northern and semi-quantitative RT-PCR analysis of the barley gene (HvGI) showed the presence of a single mRNA species, with a peak of expression between 6 h and 9 h after dawn in short days (8 h light) and a peak 15 h after dawn in long days (16 h light). This behaviour is similar to that seen in Arabidopsis and rice, showing that sequence and expression pattern were well conserved. A lack of correspondence with the map positions of QTL affecting flowering time (heading date) suggests that variation at HvGI does not provide a major source of adaptive variation in photoperiod response.
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