A wheat homologue of <i>PHYTOCLOCK 1</i> is a candidate gene conferring the early heading phenotype to einkorn wheat

Abstract

An X-ray mutant showing an early flowering phenotype has been identified in einkorn wheat (Triticum monococcum L.), for which a major QTL for heading time was previously mapped in the telomeric region on the long arm of chromosome 3A. Recent advances in Triticeae genomics revealed that the gene order in this region is highly conserved between wheat and barley. Thus, we adopted a hypothetical gene order in barley, the so-called GenomeZipper, to develop DNA markers for fine mapping the target gene in wheat. We identified three genes tightly linked to the early heading phenotype. PCR analysis revealed that early-flowering is associated with the deletion of two genes in the mutant. Of the two deleted genes, one is an ortholog of the LUX ARRHYTHMO (LUX)/PHYTOCLOCK 1 (PCL1) gene found in Arabidopsis, which regulates the circadian clock and flowering time. We found distorted expression patterns of two clock genes (TOC1 and LHY) in the einkorn pcl1 deletion mutant as was reported for the Arabidopsis lux mutant. Transcript accumulation levels of photoperiod-response related genes, a photoperiod sensitivity gene (Ppd-1) and two wheat CONSTANS-like genes (WCO1 and TaHd1), were significantly higher in the einkorn wheat mutant. In addition, transcripts of the wheat florigen gene (WFT) accumulated temporally under shortday conditions in the einkorn wheat mutant. These results suggest that deletion of WPCL1 leads to abnormally higher expression of Ppd-1, resulting in the accumulation of WFT transcripts that triggers flowering even under short-day conditions. Our observations from gene mapping, gene deletions, and expression levels of flowering related genes strongly suggest that WPCL1 is the most likely candidate gene for controlling the early flowering phenotype in the einkorn wheat mutant.

A Distorted Circadian Clock Causes Early Flowering and Temperature-Dependent Variation in Spike Development in the Eps-3Am Mutant of Einkorn Wheat

“…In terms of sequence conservation, we observed that the Eps-3A m locus in einkorn wheat had higher resemblance to the syntenic barley sequence than to that of bread wheat ( Figure 2A and Table S2). During our genetic characterization of Eps-3A m , we noted that two adjacent marker loci, TmLUX (PAV_261_262) and TmPUMILIO (PAV_295_296), were scored as null alleles in KT3-5 (Figure 2A), suggesting a putative deletion of both genes in agreement with previous findings (Mizuno et al 2012). DNA gel blot analysis confirmed this result ( Figure S2).…”

“…Moreover, recent analysis of eam8 (HvELF3) and eam10 (HvLUX1) mutants showed misexpression of circadian clock gene-related transcripts (Faure et al 2012; Campoli et al 2013), which was in line with previous findings in Arabidopsis (Hicks et al 2001). So far, cereal PCL1/LUX was proposed as a candidate conferring early flowering in eam10 and KT3-5 mutants (Mizuno et al 2012; Campoli et al 2013). Authors have concluded that the mechanism of the PCL1/LUX protein action is similar in both einkorn wheat and barley; it represses expression of Vrn3/FT through the negative regulation of Ppd-1/Ppd-H1, thus delaying flowering.…”

“…Authors have concluded that the mechanism of the PCL1/LUX protein action is similar in both einkorn wheat and barley; it represses expression of Vrn3/FT through the negative regulation of Ppd-1/Ppd-H1, thus delaying flowering. When PCL1/LUX is knocked-out, it causes the early heading phenotype, especially when the mutant harbors the wild-type (wt) allele of Ppd-H1; i.e., photoperiod sensitivity (Mizuno et al 2012; Campoli et al 2013).Recently, delayed fluorescence (DF) has been proposed as a method for quick but high-resolution analysis of the circadian clock in virtually any species (Gould et al 2009). The DF phenomenon, discovered by Strehler and Arnold (1951) is a luminescence produced by the photosynthetic apparatus after excitation with ambient light as a result of charge recombination in the photosystem II (PSII) and subsequent emission of a photon (Rutherford et al 1984).…”

“…However, low TmLHY protein levels cannot fulfill the required activating effect on the TmPRR95 and high transcript levels of TmPRR95 cannot be maintained, resulting in becoming low and arrhythmic ( Figure 4B). It has been suggested that the early heading of KT3-5 was driven by elevated Ppd1 expression and subsequent accumulation of WFT transcript (Mizuno et al 2012). Evidence found by Campoli et al (2013) also supports that HvLUX1 (barley ortholog of PCL1/LUX) acts on HvFT through Ppd-H1, as it shows a clear interaction with this gene, e.g., eam10 mutant lines harboring the wild-type Ppd-H1 allele flowered earlier than those with the recessive ppd-H1 mutant allele (Campoli et al 2013).…”

EARLY FLOWERING3 Redundancy Fine-Tunes Photoperiod Sensitivity

“…Interestingly, in both crop groups, mutations in ELF3 genes have been shown to contribute to the expansion in range of species through alteration of photoperiod responsiveness and the associated impact on yield (Faure et al, 2012;Matsubara et al, 2012;Weller et al, 2012;Zakhrabekova et al, 2012;Alvarez et al, 2016;Lu et al, 2017). In crop species, a role for ELF4 function has been clearly established only in the legume species pea (Pisum sativum; Liew et al, 2009), but mutations in LUX orthologs have also been shown to cause early flowering and impaired photoperiod response in pea and in the cereals einkorn wheat (Triticum aestivum) and barley (Hordeum vulgare; Mizuno et al, 2012;Campoli et al, 2013;Liew et al, 2014), suggesting that the EC genes are likely to be intimately linked to the photoperiod response mechanism in both crop groups.…”