RIKEN Accel. Prog. Rep. 48 (2015)

Ⅲ-4. Radiation Chemistry & Biology

Extra-early-flowering wheat mutants produced by heavy-ion-beammutagenesis†
A. Nishiura,＊1 Y. Kazama,＊2 T. Abe,＊2 N. Mizuno,＊3 S. Nasuda,＊3 and K. Murai＊1To avoid harvesting in the rainy season, early floweringor early heading is one of the most important characteristics
for bread wheat (Triticum aestivum) in East Asia, including
Japan. A detailed understanding of the flowering
mechanism in wheat plants is of value not only for wheatbreeding but also for basic scientific research. From thelarge scale mutant panel of diploid einkorn wheat (Triticummonococcum)strain
KU104-1
developedby
heavy-ion-beam irradiation1), we identified four extra
early-flowering mutants, named extra-early-flowering1(exe1), exe2, exe3, and exe4. The four exe mutants fell into
twogroups,
namelyTypeI(moderatelyextra-early-flowering type; exe1 and exe3) and Type II(extremely extra-early-flowering type; exe2 and exe4),
based on a field experiment.
In a growth chamber, leaf-emergence timing in wild-type(WT) and exe mutant plants was examined under long-day(LD) or short-day (SD) conditions. Under LD conditions,
WT plants averaged about 180 days from sowing toflag-leaf unfolding. In contrast, Type I exe mutants took
about 65 days and Type II about 50 days. WT plants
transited from the vegetative to reproductive growth phaseat the 9-leaf stage, while exe mutants showed earlier phasetransition: at the 5-leaf stage for exe1 and exe3 (Type I) andthe 4-leaf stage for exe2 and exe4 (Type II). In WT plants,
the flag leaf occurred at the 18-leaf stage, while in exe1,exe2, exe3, and exe4, the flag leaf appeared at the 9-leaf,
8-leaf, 9-leaf, and 7-leaf stages, respectively. Under SD
conditions, WT plants transited from the vegetative toreproductive growth phase at the 13-leaf stage. In contrast,exe
Fig. 1. Extra-early-flowering (exe) mutant plants grown inthe field.
†Condensed from the article in Breeding Science 64, 213 (2014)
Department of Bioscience, Fukui Prefectural University＊2
RIKEN Nishina Center＊3
Graduate School of Agriculture, Kyoto University＊1exe1 and exe3 mutants (Type I) transited at the 8-leaf stage,
while exe2 and exe4 mutants (Type II) transited at the 4-leafstage. Interestingly, the phase transition was early (at the4-leaf stage) in Type II exe mutants under both SD and LDconditions. Comparing Type I exe mutants between SD andLD conditions indicates that these mutants retained
photoperiodic sensitivity. However, Type II exe mutants had
lost almost all photoperiodic sensitivity.
In wheat, it was found that three genes mainly control theflowering, namely VERNALIZATION 1 (VRN1), VRN2 andVRN3. The analysis of VRN1, a flowering-promoter gene,
showed that it was more highly expressed in seedlings at theearly developmental stages in Type II mutants than in TypeI mutants. The up-regulation of VRN1 expression in exe
mutants was associated with earliness in flowering under
LD conditions. The extremely extra-early flowering in TypeII mutants was associated with a more rapid up-regulationof VRN1 expression than in Type I mutants. It is notablethat a similar level of VRN1 expression was observed at thephase transition in each exe mutant: at the 5-leaf stage forType I mutants and at the 4-leaf stage for Type II mutants.
This finding suggests that the level of VRN1 expression iscorrelated with the phase transition from vegetative toreproductive growth and may act as a threshold forflowering competency in wheat plants.
The original KU104-1 is an einkorn wheat strain thatcarries a null allele of the VRN2 gene, a repressor offlowering. Thus, our results indicate that the level of VRN1
expression controls earliness in flowering in exe mutants
independently of VRN2. In bread wheat, it has been known
that winter cultivar quantitatively requires prolonged cold
temperature to yield vernalization saturation. Some wintercultivars require 2-4 weeks of low temperature to reach themaximum vernalization effect on heading, and others
require more than 4 weeks. Positional cloning of the gene
was performed for the duration required for vernalization,and the results demonstrated that this trait is controlled by
the recessive VRN1 gene on the A genome at the protein
level2). A previous study also revealed that the binding
ability of the VRN1 protein to TaHOX1 was associated withthe requirement of low-temperature duration to reachvernalization saturation. These findings, together with thepresent exe study, support our model of gene network forflowering: VRN1 plays a central role in the floweringpathway3).
References

1) K. Murai et al.: Nucl. Instr. Meth. Phys. Res. B 314, 59 (2013).
2) G. Li et al.: Plant J. 76, 742 (2013).
3) S. Shimada et al.: Plant J. 58, 668 (2009).

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