Ⅲ-4. Radiation Chemistry & Biology

RIKEN Accel. Prog. Rep. 52 (2019)

An early-flowering einkorn wheat mutant with deletions ofPHYTOCLOCK 1/LUX ARRHYTHMO and VERNALIZATION 2exhibits a high level of VERNALIZATION 1 expression induced
by vernalization†
A. Nishiura,∗1 S. Kitagawa,∗1 M. Matsumura,∗1 Y. Kazama,∗2 T. Abe,∗2 N. Mizuno,∗3 S. Nasuda,∗3and K. Murai∗1The early-ﬂowering or early-heading phenotype inbread wheat (Triticum aestivum) cultivars is importantas it can produce an early harvest. This characteristic isparticularly beneﬁcial in East Asia as it allows harvesting
to before the onset of the rainy season. To understandthe molecular mechanism of ﬂowering in wheat, we developed a large-scale mutant panel in diploid einkorn wheat(T. monococcum) using a heavy-ion beam.1) Einkorn
wheat seeds were exposed to a heavy-ion beam andthen sown in the ﬁeld. Selfed seeds from each spike of
M1 plants were used to generate M2 lines. Every year
over the past 15 years, we have obtained approximately
1,000 M2 lines and built up a mutant panel with 10,000
M2 lines. This mutant panel is being systematicallyscreened for mutations aﬀecting reproductive growth, especially for the ﬂowering-time mutants. From the large
scale mutant panel, we have identiﬁed four extra earlyﬂowering mutants, named extra early-flowering1 (exe1 ),exe2, exe3, and exe4.2) The four exe mutants fall into twogroups namely Type I (moderately extra early-ﬂowering
type: exe1 and exe3 ) and Type II (extremely extra earlyﬂowering type: exe2 and exe4 ). An analysis of VERNALIZATION 1 (VRN1 ), a ﬂowering promoter gene,shows that it is more highly expressed in seedlings at
early developmental stages in both Type I and II mutants than wild-type (WT). These ﬁndings indicate thatthe diﬀerence in earliness between Type I and II mutants
is associated to the level of VRN1 expression.
The diﬀerences between the diurnal gene expression
patterns in the ﬁeld were examined for four clock-related
genes and three clock downstream genes in WT andexe3 mutant plants grown in the ﬁeld, respectively. The
biggest diﬀerence was found for a clock-related gene,
PHYTOCLOCK 1/LUX ARRHYTHMO, which is abbreviated to Wheat PCL1 (WPCL1 ). The WPCL1 was
not expressed in the exe3 mutant plants, whereas it was
highly expressed during sunset in WT plants. PCR analysis of DNA markers indicated that the exe3 mutant
had a deletion of WPCL1 in the genome, which was cosegregated with the mutant phenotype in the segregationline.
We conﬁrmed that the original strain KU104-1 carried a mutation that produced a null allele of a ﬂowering repressor gene VERNALIZATION 2 (VRN2 ). As†∗1∗2∗3Condensed from the article in J. Plant Physiol. 222, 28 (2018)
Faculty of Bioscience and Biotechnology, Fukui PrefecturalUniversityRIKEN Nishina Center
Graduate School of Agriculture, Kyoto UniversityFig. 1. Model for the extra early-flowering phenotype in theexe3 mutants compared with wild-type. (A) Wild-type,(B) exe3 mutant

a result, the exe3 mutant has both WPCL1 and VRN2
loss-of-function mutations. The analysis of plant development in a growth chamber showed that vernalization
treatment accelerated ﬂowering time in the exe3 mutant
under short day (SD) as well as long day (LD) conditions and the early-ﬂowering phenotype was correlatedwith the earlier up-regulation of VRN1. The deletionof WPCL1 aﬀects the SD-speciﬁc expression patterns ofsome clock-related genes, clock downstream genes, andphotoperiod pathway genes, suggesting that the exe3
mutant causes a disordered SD response. The present
study indicates that VRN1 expression is associated withthe biological clock and the VRN1 up-regulation is notinﬂuenced by the presence or absence of VRN2.
A model for the extra early-ﬂowering phenotype ofexe3 mutant is shown in Fig. 1. The disruption of clockfunction also aﬀects the expression of the ﬂorigen gene
WFT through the VRN1 expression. A high level ofWFT expression was observed in the exe3 mutant under LD conditions, suggesting that the disrupted clock
somehow induces WFT expression. Under SD conditions, another ﬂorigen gene could be up-regulated by thedisrupted clock function and accelerate ﬂowering in theexe3 mutant. The up-regulation of VRN1 is controlled
by the vernalization pathway and clock function. It is not
related to VRN2 and determines the earliness in wheat.
References
1) K. Murai et al., Nucl. Instrum. Methods Phys. Res. B
314, 59 (2013).
2) A. Nishiura et al., Breed. Sci. 64, 213 (2014).

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