During postembryonic plant development, cell division is coupled to cell growth. There is a stringent requirement to couple these processes in shoot and root meristems. As cells pass through meristems, they transit through zones with high rates of cell growth and proliferation during organogenesis. This transition implies a need for coordinate regulation of genes underpinning these two fundamental cell functions. Here, we report a mechanism for coregulation of cell division control genes and cell growth effectors. We identified a GCCCR motif necessary and sufficient for high-level cyclin CYCB1;1 expression at G 2/M. This motif is overrepresented in many ribosomal protein gene promoters and is required for high-level expression of the S27 and L24 ribosomal subunit genes we examined. p33 TCP20 , encoded by the Arabidopsis TCP20 gene, binds to the GCCCR element in the promoters of cyclin CYCB1;1 and ribosomal protein genes in vitro and in vivo. We propose a model in which organ growth rates, and possibly shape in aerial organs, are regulated by the balance of positively and negatively acting teosinte-branched, cycloidea, PCNA factor (TCP) genes in the distal meristem boundary zone where cells become mitotically quiescent before expansion and differentiation.

Phytochromes are dimeric proteins that function as red and far-red light sensors influencing nearly every phase of the plant life cycle. Of the three major phytochrome families found in flowering plants, PHYTOCHROME C (PHYC) is the least understood. In Arabidopsis and rice, PHYC is unstable and functionally inactive unless it heterodimerizes with another phytochrome. However, when expressed in an Arabidopsis phy-null mutant, wheat PHYC forms signaling active homodimers that translocate into the nucleus in red light to mediate photomorphogenic responses. Tetraploid wheat plants homozygous for loss-of-function mutations in all PHYC copies (phyC AB ) flower on average 108 d later than wild-type plants under long days but only 19 d later under short days, indicating a strong interaction between PHYC and photoperiod. This interaction is further supported by the drastic down-regulation in the phyC AB mutant of the central photoperiod gene PHOTOPERIOD 1 (PPD1) and its downstream target FLOWERING LOCUS T1, which are required for the promotion of flowering under long days. These results implicate light-dependent, PHYC-mediated activation of PPD1 expression in the acceleration of wheat flowering under inductive long days. Plants homozygous for the phyC AB mutations also show altered profiles of circadian clock and clock-output genes, which may also contribute to the observed differences in heading time. Our results highlight important differences in the photoperiod pathways of the temperate grasses with those of well-studied model plant species.

SummaryA precise regulation of flowering time is central to plant species survival. Therefore, mechanisms have evolved in plants to integrate various environmental cues to optimize flowering time. In this study, we show that the product of the wheat gene TaFT, which integrates photoperiod and vernalization signals promoting flowering, interacts with bZIP proteins TaFDL2 and TaFDL6. We also show that TaFDL2 can interact in vitro with five ACGT elements in the promoter of the meristem identity gene VRN1, suggesting that TaFDL2 is a functional homologue of Arabidopsis FD. No direct interactions between the TaFT protein and the VRN1 promoter were detected. Transgenic wheat plants over-expressing TaFT showed parallel increases in VRN1 transcripts, suggesting that TaFT provides transcriptional activation of VRN1, possibly through interactions with the TaFDL2 protein. The same transgenic plants also showed increased transcript levels of TaFT2 (a TaFT paralogue), indicating that TaFT2 acts downstream of TaFT. The fact that TaFT2 interacts with another bZIP protein TaFDL13, which lacks the ability to interact with the VRN1 promoter, suggests that TaFT and TaFT2 have different gene targets. This observation agrees with the functional divergence observed for the TaFT and TaFT2 orthologous genes in rice. The temperate cereals analyzed so far show VRN1 transcripts in the leaves, a characteristic not observed in Arabidopsis or rice. The high levels of TaFDL2 transcripts observed in wheat leaves provide a simple explanation for this difference. We present a hypothesis to explain the conservation of VRN1 expression in the leaves of temperate cereals.

Winter wheat (Triticum spp.) varieties require long exposures to low temperatures to flower, a process called vernalization. The VRN2 locus includes two completely linked zinc finger-CCT domain genes (ZCCT1 and ZCCT2) that act as flowering repressors down-regulated during vernalization. Deletions or mutations in these two genes result in the elimination of the vernalization requirement in diploid wheat (Triticum monococcum). However, natural allelic variation in these genes has not been described so far in polyploid wheat (tetraploid Triticum turgidum and hexaploid Triticum aestivum). A tetraploid wheat population segregating for both VRN-A2 and VRN-B2 loci facilitated the characterization of different alleles. Comparisons between functional and nonfunctional alleles revealed that both ZCCT1 and ZCCT2 genes are able to confer vernalization requirement and that different ZCCT genes are functional in different genomes. ZCCT1 and ZCCT2 proteins from nonfunctional vrn2 alleles have mutations at arginine amino acids at position 16, 35, or 39 of the CCT domain. These positions are conserved between CCT and HEME ACTIVATOR PROTEIN2 (HAP2) proteins, supporting a model in which the action of CCT domains is mediated by their interactions with HAP2/HAP3/HAP5 complexes. This study also revealed natural variation in gene copy number, including a duplication of the functional ZCCT-B2 gene and deletions or duplications of the complete VRN-B2 locus. Allelic variation at the VRN-B2 locus was associated with a partially dominant effect, which suggests that variation in the number of functional ZCCT genes can be used to expand allelic diversity for heading time in polyploid wheat and, hopefully, improve its adaptation to different environments.Wheat (Triticum aestivum) is one of the major crop species and occupies a wide range of environments from 65°N to 45°S (Lantican et al., 2005). This wide adaptability is favored by diverse growth habits, which include winter and spring forms. Winter wheats are sown in autumn and require long exposures to cold temperatures (vernalization) to accelerate flowering. The vernalization requirement prevents flower development during winter, protecting sensitive floral organs from freezing temperatures. Spring wheats are planted in the spring or in the fall (in regions with mild winters) and do not have a vernalization requirement.The three major genes responsible for natural variation in vernalization requirement in wheat (and also in barley [Hordeum vulgare]) are VRN1, VRN2, and VRN3. VRN1 is a homolog of the Arabidopsis (Arabidopsis thaliana) meristem identity gene APETALA1, which determines the transition between the production of leaves and flowers at the shoot apical meristem (Danyluk et al., 2003;Trevaskis et al., 2003;Yan et al., 2003). Mutagenized plants of diploid wheat (Triticum monococcum; 2n 5 14; A m genome similar to the A genome of polyploid wheat) with complete deletions of the VRN1 gene fail to flower (Shitsukawa et al., 2007), indicating that VRN1 is essential for the initiation of...