Heading time in bread wheat (Triticum aestivum L.) is determined by three characters: vernalization requirement, photoperiodic sensitivity and narrow-sense earliness, which are involved in the phase transition from vegetative to reproductive growth. We identified and characterized the APETALA1 (AP1)-like MADS box gene in wheat (WAP1) as an activator of phase transition. Its expression starts just before the phase transition and is maintained during the reproductive phase. Inhibition of WAP1 expression in the transgenic plants by co-suppression affected neither vernalization requirement nor photoperiodic sensitivity, but resulted in delayed narrow-sense earliness, indicating that WAP1 accelerates autonomous phase transition. Analyses of the WAP1 expression in the near-isogenic lines (NILs) for spring habit genes (Vrn) revealed that WAP1 transcripts were induced by vernalization strongly in the NILs with Vrn dominant alleles and weakly with the recessive alleles. Furthermore, WAP1 expression was up-regulated by a long photoperiod in both NILs with and those without a photoperiod-insensitive gene (Ppd). These results suggest that WAP1 is a key gene in the regulatory pathway controlling the phase transition from vegetative to reproductive growth in wheat.

Summary: To elucidate the genetic mechanism of flowering in wheat, we performed expression, mutant and transgenic studies of flowering‐time genes. A diurnal expression analysis revealed that a flowering activator VRN1, an APETALA1/FRUITFULL homolog in wheat, was expressed in a rhythmic manner in leaves under both long‐day (LD) and short‐day (SD) conditions. Under LD conditions, the upregulation of VRN1 during the light period was followed by the accumulation of FLOWERING LOCUS T (FT) transcripts. Furthermore, FT was not expressed in a maintained vegetative phase (mvp) mutant of einkorn wheat (Triticum monococcum), which has null alleles of VRN1, and never transits from the vegetative to the reproductive phase. These results suggest that VRN1 is upstream of FT and upregulates the FT expression under LD conditions. The overexpression of FT in a transgenic bread wheat (Triticum aestivum) caused extremely early heading with the upregulation of VRN1 and the downregulation of VRN2, a putative repressor gene of VRN1. These results suggest that in the transgenic plant, FT suppresses VRN2 expression, leading to an increase in VRN1 expression. Based on these results, we present a model for a genetic network of flowering‐time genes in wheat leaves, in which VRN1 is upstream of FT with a positive feedback loop through VRN2. The mvp mutant has a null allele of VRN2, as well as of VRN1, because it was obtained from a spring einkorn wheat strain lacking VRN2. The fact that FT is not expressed in the mvp mutant supports the present model.

Summary: Homeotic transformation of stamens into pistil‐like structures (pistillody) has been observed in a cytoplasmic substitution (alloplasmic) line of wheat (Triticum aestivum L.) cv. Norin 26, which has the cytoplasm of a wild relative species, Aegilops crassa L. On the other hand, an alloplasmic line of wheat cv. Chinese Spring (CS) with Ae. crassa cytoplasm has normal flowers. This is due to the presence in the CS nucleus of a fertility‐restoring gene, Rfd1. Deletion mapping analysis revealed that Rfd1 is located on the middle part of the long arm of chromosome 7B. To investigate the function of the Rfd1 gene by a loss‐of‐function strategy, we produced alloplasmic lines of CS ditelosomic 7BS [(cr)‐CSdt7BS] and CS monotelodisomic 7BS [(cr)‐CSmd7BS] with the Ae. crassa cytoplasm, and characterized their phenotypes. The line (cr)‐CSdt7BS without Rfd1 exhibited pistillody in all florets, and also female sterility. Scanning electron microscopy of the young spikes revealed that the pistillody was induced at an early stage of stamen development. The pistillate stamens often developed incomplete ovule‐like structures with integuments instead of tapetum and pollen grains. It is possible that MADS box genes are associated with the induction of pistillody, because the expression of wheat APETALA3 homologue (WAP3) was reduced in the young spikes of (cr)‐CSdt7BS. In addition, a histological study indicated that the female sterility in (cr)‐CSdt7BS is due to the abnormality of the ovule, which fails to form an inner epidermis and integuments in the chalaza region. The line (cr)‐CSmd7BS, hemizygous for Rfd1, showed partial pistillody (51%) and restored female fertility up to 72%. These results suggest that the induction of both pistillody and ovule deficiency caused by the Ae. crassa cytoplasm is inhibited by the Rfd1 gene in a dose‐dependent manner.

Bread wheat (Triticum aestivum) is a hexaploid species with A, B, and D ancestral genomes. Most bread wheat genes are present in the genome as triplicated homoeologous genes (homoeologs) derived from the ancestral species. Here, we report that both genetic and epigenetic alterations have occurred in the homoeologs of a wheat class E MADS box gene. Two class E genes are identified in wheat, wheat SEPALLATA (WSEP) and wheat LEAFY HULL STERILE1 (WLHS1), which are homologs of Os MADS45 and Os MADS1 in rice (Oryza sativa), respectively. The three wheat homoeologs of WSEP showed similar genomic structures and expression profiles. By contrast, the three homoeologs of WLHS1 showed genetic and epigenetic alterations. The A genome WLHS1 homoeolog (WLHS1-A) had a structural alteration that contained a large novel sequence in place of the K domain sequence. A yeast two-hybrid analysis and a transgenic experiment indicated that the WLHS1-A protein had no apparent function. The B and D genome homoeologs, WLHS1-B and WLHS1-D, respectively, had an intact MADS box gene structure, but WLHS1-B was predominantly silenced by cytosine methylation. Consequently, of the three WLHS1 homoeologs, only WLHS1-D functions in hexaploid wheat. This is a situation where three homoeologs are differentially regulated by genetic and epigenetic mechanisms.

Floral organ formation has been the subject of intensive study for over 20 years, particularly in the model dicot species Arabidopsis thaliana. These studies have led to the establishment of a general model for the development of floral organs in higher plants, the so-called ABCDE model, in which floral whorl-specific combinations of class A, B, C, D, or E genes specify floral organ identity. In Arabidopsis, class A, B, C, D, E genes encode MADS-box transcription factors except for the class A gene APETALA2. Mutation of these genes induces floral organ homeosis. In this review, I focus on the roles of these homeotic genes in bread wheat (Triticum aestivum), particularly with respect to the ABCDE model. Pistillody, the homeotic transformation of stamens into pistil-like structures, occurs in cytoplasmic substitution (alloplasmic) wheat lines that have the cytoplasm of the related wild species Aegilops crassa. This phenomenon is a valuable tool for analysis of the wheat ABCDE model. Using an alloplasmic line, the wheat ortholog of DROOPING LEAF (TaDL), a member of the YABBY gene family, has been shown to regulate pistil specification. Here, I describe the current understanding of the ABCDE model for floral organ formation in wheat.