Ⅲ-4. Radiation Chemistry & Biology

RIKEN Accel. Prog. Rep. 53

Genetic analysis of an early flowering rice mutant induced
by argon ions
R. Morita,∗1 H. Ichida,∗1 K. Ichinose,∗1 Y. Shirakawa,∗1 Y. Hayashi,∗1 T. Sato,∗1,∗2 and T. Abe∗1
Heading date, also known as ﬂowering time, is one of
the important traits for a crop such as rice to achievea high yield by making eﬀective use of sunlight andtemperature.1) Although numerous genes inﬂuencing
the heading date have been identiﬁed and characterized in rice, it is necessary to identify novel genes that
determine the heading date to understand the mechanism to control this trait. To identify a novel geneinﬂuencing the heading date, we isolated a rice mutant(Ar-G150) that exhibited an early ﬂowering phenotype
from a rice (Oryza sativa L. cv. Nipponbare) M2 population derived from imbibed seeds irradiated with argon ions (2.5 Gy, 95 MeV/n, LET: 286 keVµm−1 ). To
determine the mode of inheritance of the early ﬂowering trait, we produced F2 progenies derived froma cross between Ar-G150 and their original variety,Nipponbare. We grew Nipponbare, Ar-G150, and theF2 progenies in a greenhouse under natural photoperiod conditions from June 17, 2016, and recorded theheading date of each plant. The heading date of Nipponbare were from September (Sept.) 2nd to Sept.
6th. The heading date of Ar-G150 were from August (Aug.) 23rd to Aug. 26th, approximately 10 days
earlier than Nipponbare. Segregation for the earlyﬂowering phenotype was observed in F2 progenies, i.e.,seven F2 plants showed normal ﬂowering (the heading dates were from Sept. 2nd to Sept. 5th), and 23
F2 plants showed early ﬂowering (the heading dates
were from Aug. 23rd to Aug. 27th). The segregationof normal- and early-ﬂowering phenotypes in the F2progenies (7:23) showed a good ﬁt to a 1:3 ratio (thetwo-tailed p-value from Fisher’s exact test was 1.00),
indicating that Ar-G150 possessed a single dominant
mutated gene with an early-ﬂowering trait. To characterize the mutated phenotype in detail or determine
the causative gene of early ﬂowering, the homozygousline for the mutated gene is needed. Thus, we cultivated 16 F3 lines (approximately 20 plants per line)
derived from the self-pollination of early-ﬂowering F2plants to select the homozygous line using a paddy
ﬁeld in 2019. In 5 out of the 16 F3 lines, all F3 plants
exhibited early ﬂowering, indicating that the mutatedgene in their parental plant was homozygous. In the remaining 11 lines, segregation for the early- and normalﬂowering phenotype was observed in F3 plants, indicating that the mutated gene in the parental plant washeterozygous. The number of homozygous F2 and heterozygous F2 (5:11) showed a good ﬁt to a 1:2 ratio (thetwo-tailed p-value from Fisher’s exact test was 1.00).
∗1∗2RIKEN Nishina Center
Graduate School of Agricultural Science, Tohoku University- 210 -Fig. 1. Days to heading of WT and Ar-G150 grown undera short-day condition. Values are means ± standard
error (n = 5). Student’s t-test was used to compare thetwo groups.

These results indicated that the early-ﬂowering character was determined by a single gene.
Using the homozygous F3 plants, we started tocharacterize the early-ﬂowering phenotype of Ar-G150.
First, we determined the days to heading under a controlled photoperiod condition using a growth chamber
to reveal whether the mutated gene aﬀects the photoperiod response. We grew Nipponbare and Ar-G150
plants under a short-day condition (10 h light/14 hdark), and measured the days to heading. Since rice
is a short-day plant and the critical photoperiod ofNipponbare is estimated to be 15.6 h,2) this condition
prompts rice plants to develop ﬂowers. Under this condition, the days to heading of Ar-G150 (57.8 ± 1.0 d,average ± standard error) was slightly less than thedays to heading of Nipponbare (61.2 ± 1.2 d), althoughno signiﬁcant diﬀerences were observed between thedays to heading of both lines (p > 0.05), suggesting that the mutated gene have little or no eﬀect ofpromoting ﬂowering under the short-day conditions(Fig. 1). We also started to perform the whole genome
re-sequencing analysis of homozygous F3 mutants toreveal the causative gene. The information of thecausative gene will help us understand the mechanism
to control the heading date of rice in more detail.
References
1) B. Wang, J. Li, Plant Cell 31, 1416 (2019).
2) T. Horie, H. Nakagawa, Jpn. J. Crop Sci. 59, 687 (1990).