Ⅲ-4. Radiation Chemistry & Biology
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Irradiation of seeds with C-ion and Ar-ion beams for Streptocarpus
mutagenesis to understand leaf meristem genetics
K. Nishii,∗1,∗2 M. Möller,∗1 N. Kelso,∗1 H. France,∗1 S. Barber,∗1 Y. Hayashi,∗3 K. Ishii,∗3 and T. Abe∗3
Streptocarpus Lindl. (Gesneriaceae) attract developmental biologists owing to their variable shoot/leafforms, which include caulescents with ordinary shootapical meristem (SAM) and acaulescent (unifoliatesand rosulates) lacking SAMs (e.g., Figs. 1a–c). Anisocotyly (Fig. 1b), the unequal cotyledon development
producing a macrocotyledon also marks this genus/family.1) Streptocarpus rexii Lindl. is rosulate and has
been studied as a model to understand their unique
cotyledon and leaf development. Previous results haveshown relocated expression of SAM genes in their leaf
meristems.1) Thus, the genetic and functional shift ofmeristems from the shoot apex laterally to the leaf
might result in their unique acaulescent forms. Thiswork is important to understand plant meristems andtheir eﬀects on plant form. To date, a genetic map,transcriptome, and genome have been generated forS. rexii,2) which are important resources to isolate theunderlying genetic loci. Here, we use forward genetics
utilizing heavy-ion mutagenesis to facilitate this aim.3)

transported to RIKEN (Wako, Japan). C-ion (12 C6+ )and Ar-ion (40 Ar17+ ) irradiations were conducted atRIBF. The high linear energy transfer (LET) value ofC-ion was set to 30 keV µm−1 , and that of Ar-ion to184 keV µm−1 . For C-ion, doses of 50, 100, 150, 200,300 and 400 Gy, and for Ar-ion, 10, 20 and 50 Gy wereused, labelled C50, C100, C150, C200, C300, C400,
Ar10, Ar20, and Ar50, respectively. The lethal eﬀects
of the treatments were assessed as the germination rateand seedling growth. The seeds were sown in Petridishes on ﬁlter paper soaked with tap water and examined 2 and 3 weeks later. The growth of seedlings(the cotyledon size) was measured 7 weeks after sowing. The ﬂower phenotype was also observed for mutations in the M1 generation from 20–26 plants per treatment. To obtain larger M2 populations for screening,
M1 seeds of selected treatments were sown and cultivated. The vegetative phenotype was observed while
focusing on mutations in leaf initiation. Germination
was observed in all treatments; however, it was delayed in C400 and Ar50. The ﬁnal germination rate
was low only for Ar50 (Table 1). A strong suppressionof seedling growth was observed in C300, C400, andAr50, and all seedlings eventually died. Over 20% ofM1 plants of C150, C200, Ar10 and Ar20 showed ﬂower
mutations, such as split corollas and shift in symmetry(Figs. 1c–e).
Table 1. Eﬀects of heavy-ion irradiation on Streptocarpus
rexii.

Fig. 1. Images of Streptocarpus rexii wild type and heavyion irradiated mutants. a-b) S. rexii wild type. b)
Anisocotyly. c) Wild type ﬂower. d-e) C200 ﬂower
mutations. f) C200 qualiﬁer “JW” M1 plant showing
delayed leaf initiation phenotype.

Seeds of S. rexii (RBGE 20170766*A) cultivated inthe glasshouses at Royal Botanic Garden Edinburgh(RBGE) were harvested in October 2018, certiﬁed at
Science & Advice for Scottish Agriculture (SASA; Phytosanitary Certiﬁcate No. SG/SASA/2018/033), and∗1∗2∗3

Royal Botanic Garden Edinburgh
The Research Institute for Integrated Science, Kanagawa
University
RIKEN Nishina Center- 191 -Considering all results together, C150, C200, andAr20 were just below the lethal dosage, and showed
clear ﬂower mutations. Thus, they were selected forlarge scale mutant screening. More than 1000 M1
plants of C150, C200, and Ar20 were cultivated to obtain M2 seeds. The estimated fertility rates (in MayⅢ-4. Radiation Chemistry & Biology

RIKEN Accel. Prog. Rep. 56 (2023)2021) were 62, 47, and 40% for C150, C200, and Ar20,
respectively. Five of C200 M1 plants, and 25 of Ar20
showed a delayed leaf initiation phenotype (Table 1;Fig. 1f). M2 seeds of these were harvested from four
C200 and 12 Ar20 plants. Now, these mutant phenotypes were observed for mutant stability in following
generations.
References
1) K. Nishii et al., Dev. Genes Evol. 220, 25 (2010).
2) K. Nishii et al., Plant Direct 6, e388 (2022).
3) T. Abe et al., Nucl. Phys. News 25, 30 (2015).

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