Hatano Y., Katsumura Y., Mozumder A. (Eds.) Charged Particle and Photon Interactions with Matter - Recent Advances, Applications, and Interfaces

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940 Charged Particle and Photon Interactions with Matter

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943

Applications to Biotechnology:

Ion-Beam Breeding of Plants

Atsushi Tanaka

Japan Atomic Energy Agency

Takasaki,

Japan

Yoshihiro Hase

Japan Atomic Energy Agency

Takasaki,

Japan

33.1 introduCtion

Since the observation of mutation induction with x-rays in Drosophila by Muller (1927) and with

x-rays and gamma rays in maize and barley by Stadler (1928), a number of plant mutations have been

induced by ionizing radiation and used for plant science and breeding. The biological effects of ion

beams have also been investigated and it has been found that ion beams show a high relative bio-

logical effectiveness (RBE) in lethality, mutation, and so on compared to low linear energy transfer

(LET) radiation such as gamma rays, x-rays, and electrons (Blakely, 1992). As ion beams deposit

high energy on a local target, it is suggested that ion beams predominantly induce single- or double-

strand DNA breaks with damaged end groups whose reparability would be low (Goodhead,1995)

Contents

33.1 Introduction ..........................................................................................................................943

33.2 Ion-Beam

Irradiation ............................................................................................................944

33.3

Biological

Effects of Ion Beams...........................................................................................946

33.4

Mutation

Induction ............................................................................................................... 947

33.4.1

Determination

of Irradiation Dose ........................................................................... 947

33.4.2

Mutation Frequency.................................................................................................. 947

33.4.3 Determination

of Population Size for Mutation Induction.......................................948

33.4.4

Mutation

Spectrum ...................................................................................................949

33.4.5

Molecular

Nature of Mutation Induced by Ion Beams............................................. 950

33.5

Novel

Mutants Induced by Ion Beams................................................................................... 950

33.5.1

Model Plants .............................................................................................................950

33.5.2 Crops......................................................................................................................... 952

33.5.3 Ornamental

Flowers ................................................................................................. 952

33.5.4

Trees........................................................................................................................... 952

33.6 Other

Biological Effects of Ion Beams................................................................................. 953

33.6.1

Cross Incompatibility ............................................................................................... 953

33.6.2 Sex

Modication....................................................................................................... 953

33.6.3

Activation

of Endogenous Transposable Element .................................................... 953

33.7

Conclusion

and Perspective .................................................................................................. 953

Acknowledgments.......................................................................................................................... 955

References...................................................................................................................................... 955

944 Charged Particle and Photon Interactions with Matter

(Figure 33.1). Therefore, it seems plausible that ion beams can frequently produce large DNA altera-

tions such as inversions, translocations, and deletions rather than point mutations, producing char-

acteristic mutants induced by ion beams. However, the characteristics of ion beams for mutation

induction have not been elucidated yet. On the other hand, more than 2200 mutant varieties of cere-

als, beans, owers, etc., have been ofcially registered to FAO/IAEA organization (FAO/IAEA,

2006), most of which are induced by ionizing radiations, chiey gamma rays. As saturations for

gamma- or x-ray mutanization have been realized in several kinds of mutant phenotypes because of

its long history of about 80 years, new chemical or physical mutagens were expected to come into

existence.

Based on suggestions of the consultative committee for the advanced radiation technology in

Japan, the Takasaki Ion Accelerator Advanced Radiation Application (TIARA) was established and

basic research on induction of plant mutations by ion beams was started in 1991 (Fukuda et al., 2003).

We rst investigated the characteristics of ion beams for mutation induction. For over 17 years, bio-

logical effects of ion beams have been greatly elucidated, and novel mutants and varieties of crops

have been consistently and efciently induced by ion beams. At present, more than 100 research

activities have started to utilize the ion-beam irradiation method in several irradiation facilities and

research

organizations.

33.2 ion-beam irradiation

Currently, there are four facilities available for plant ion-beam breeding in Japan. Across the world,

there are few facilities for ion-beam breeding, but some exist in China, although they remain

obscure. The four facilities are as follows: TIARA of the Japan Atomic Energy Agency, RIKEN RI

Beam Factory (RIBF), the Wakasa Wan Energy Research Center Multi-purpose Accelerator with

Synchrotron and Tandem (W-MAST), and the Heavy Ion Medical Accelerator in Chiba (HIMAC)

of the National Institute of Radiological Sciences (NIRS). Table 33.1 shows the physical properties of

the ion beams frequently used in these facilities. Here, we describe the protocol of the ion-beam

irradiation

in TIARA, which was originally described in the report by Tanaka et al. (1997a).

the case of carbon ions, these are generated by the electron cyclotron resonance (ECR) ion

source and accelerated by an azimuthally varying eld (AVF) cyclotron to obtain 18.3MeV/u

ions. At the target surface, the energy of the carbon ions slightly decreases to 17.4MeV/u, result-

ing in the estimated 122keV/μm mean LET in the target material (0.5mm thickness) as water equiva-

lent, and the range of ions is ca. 1.1mm. These physical properties were calculated using an ELOSS

code program, a type of modied OSCAR code program (Tanaka et al., 1997c). Particle uences of

the ions were determined using a diethyleneglycol-bis-allylcarbonate (CR-39) lm track detector.

In general, ion beams are scanned at around 70 × 70mm; these exit the vacuum chamber through

a beam window of 30μm titanium foil (Figure 33.2). The sample is placed under the beam window

Energy deposition Irradiation to cell

Gamma rays: low LET radiation

Spur (~3 nm)

Ion beams: high LET radiation

300 nm

Gamma rays: 2000 spurs/Gy

Carbon ions: 4 tracks/Gy

Nucleus

Core

(~3 nm)

Nucleus

• • • • • • • • • • • •

Figure 33.1 Energy transfer of gamma rays and ion beams on a cell.

Applications to Biotechnology: Ion-Beam Breeding of Plants 945

and irradiated in the atmosphere. In the case of Arabidopsis or tobacco seeds, 100–3000 seeds are

sandwiched between Kapton lms (8μm thickness) to make a seed monolayer for homogeneous

irradiation. In the case of rice or barley seeds, the embryo side faces the ion beams. Tissue cultures,

such as those of ornamental explants, calluses, and shoot primordia, are contained in an aseptic petri

dish that is irradiated directly, except that the lid of the dish is replaced by the Kapton lm in order

to decrease the loss of ion-beam energy. A sample is irradiated within 2min for any dose.

AVF cyclotron

18.3 MeV/u

ions

Scanning: ×70 mm

for homogeneous irradiation

Titanium foil 30 mm:

from vacuum to atmosphere

Kapton film 8 µm:

to reduce energy loss of ions

Figure 33.2 Schematic of ion-beam irradiation.

table 33.1

ion-beam

rradiation

Facilities and the p

hysical

arameters

ofthe

adiations

Facility

ion

beam

energy

(mev/u)

let

(kev/μm)

range

(mm)

TIARA,

Japan Atomic

Energy

Agency

(http://www.taka.jaea.go.jp/index_e.html)

He 12.5 19 1.6

He 25.0 9 6.2

C 18.3 122 1.1

C 26.7 86 2.2

Ne 17.5 441 0.6

RIBF,

RIKEN

Nishina Center

(http://www rarf riken.go.jp/Eng/index.html)

C 135 23 43

N 135 31 37

Ne 135 62 26

Ar 95 280 9

Fe 90 624 6

W-MAST,

The Wakasa Wan Energy Research Center

(http://www.werc.or.jp/english/index.htm)

H 200 0.5 256

C 41.7 52 5.3

HIMAC,

National

Institute ofRadiological Sciences

(http://www.nirs.go.jp/ENG/index.html)

C 290 13 163

Ne 400 30 165

Si 490 54 163

Ar 500 89 145

Fe 500 185 97

Note:

Representative

ion radiations are shown in each facility.

946 Charged Particle and Photon Interactions with Matter

33.3 biologiCal eFFeCts oF ion beams

Several biological endpoints such as survival, growth, and chromosomal aberration have been

investigated in order to elucidate the effects of ion beams on plants. Most efforts were focused on

the LET dependence of survival of plants from treated seeds (Hirono et al., 1970; Mei et al., 1994).

general, all experiments showed a peak RBE for the survival of treated seeds above ca. 230

keV/μm.

Using carbon ions with different LET, Hase et al. (2002) found that the RBE for plant survival

and chromosome aberrations in tobacco increased with increasing LET and showed the highest

value at 230keV/μm. Shikazono et al. (2002) found that the LET of the peak RBE for lethality of

Arabidopsis was over 221keV/μm for carbon ions and over 350keV/μm for neon and argon ions.

These data suggest that the LET for maximum RBE for plant lethality is higher in seeds than in

mammalian

cells (Figure 33.3).

The

effects of LET on chromosomal aberrations were also investigated. The frequencies of

mitotic cells with chromosome aberrations, such as chromosome bridges, acentric fragments,

and lagging chromosomes, were much higher for ion beams than for gamma rays (Hase et al.,

1999). The highest RBE was 52.5 at 230 keV/μm of LET (Hase et al., 2002). The frequency

of cells with chromosome aberrations did not decrease after fractionated irradiation with

carbon ions, although a clear decrease was observed after exposure to electrons (Shimono

etal., 2001).

For mutation induction, Kazama et al. (2008) showed that a LET of 30keV/μm (N ion) was

most effective for inducing albino plants of Arabidopsis, indicating that the relationships with LET

may differ between mutation induction, plant lethality, chromosome aberrations, and double-strand

breaks (DSBs).

Recently, DNA DSBs of tobacco protoplasts were quantied by pulsed-eld gel electrophoresis

(Yokota et al., 2007). Initial DSB yields were dependent on LET and the highest RBE was obtained

at 124 and 241keV/μm carbon ions. It is interesting that ion beams yielded short DNA fragments

more frequently than gamma rays. The DSB rejoining efciency, however, is comparable to

mammalian cells (CHO-K1) (Yokota et al., 2009). These observations are well-grounded for the

effective

induction of biological endpoints caused by ion beams.

100

1000100

RBE

LET (keV/µm)

Tobacco

(seeds)

Arabidopsis

(seeds)

Carbon

Argon

Neon

Carbon

Figure 33.3 The RBE of lethality as a function of LET.

Applications to Biotechnology: Ion-Beam Breeding of Plants 947

33.4 mutation induCtion

33.4.1 deterMination of irradiation doSe

Determination of the irradiation dose for irradiation processes is the most important step and

involves hard work. Although the best irradiation dose could be determined as that which causes

the highest mutation rate of phenotype interest, determining it is very difcult because it takes much

time and labor. Alternatively, to determine appropriate doses for mutation induction, survival rate,

growth

rate, or chlorophyll mutation, etc. could be used as indicators.

Figure

33.4 shows the survival curves of Arabidopsis dry seeds for several ion beams and electrons

as a control of low LET radiations. The effect of ion beams is higher than that of electrons but it

varies by energy and kind of ions. Until now, 18.3MeV/u carbon ions are well used and show high

mutation rates and acquisition of novel mutants. However, it is not fully understood what kind of ion

and ion energy is the best for mutation induction. It likely depends on plant species and structures,

especially on genome size and ploidy, water content, and on what kind of mutation a researcher

wants to induce. According to several results obtained so far, it is suggested that effects of ion beams

on mutation induction are not due to the kind of ion but roughly due to its LET. Ion beams with LET

∼10 to ∼500

keV/μm

would be suitable.

for the dose, 50% of lethal dose, namely LD

, was previously thought to be the best choice

for mutation induction for x-ray or gamma-ray irradiation. Recently, Yamaguchi et al. (2009a)

clearly showed that the dose at the shoulder end of survival curves is sufcient to efciently

induce chlorophyll mutation not only by ion beams but also by low LET radiations in rice. In

Arabidopsis seeds, 18.3 MeV/u carbon ions, 150Gy, a dose that is three quarters of the shoulder

end of the survival, was frequently used for mutation induction (Hase et al., 2000; Tanaka et al.,

2002; Sakamoto et al., 2003). Irradiation doses that show 100%–80% growth rate (around the

shoulder end of the growth curve) were well utilized for mutation induction from plantlet samples.

Irradiation doses that show more than ∼80% regeneration or growth rate of calli were used in the

case of tissue culture (data not shown).

33.4.2 Mutation freQuency

Mutation frequency was investigated on a gene locus basis using known visible Arabidopsis mutant

phenotypes, such as transparent testa (tt), in which the seed coat is transparent because of the lack of

pigments, and glabrous (gl), in which no trichomes are produced on leaves and stems (Shikazono

100

0 500 1000 1500

Survival (%)

Dose (Gy)

2 MV electron

12.5 MeV/u He

5.0 MeV/u He

18.3 MeV/u C

13.0 MeV/u Ne

Figure 33.4 Survival curves of Arabidopsis seeds irradiated with electrons and ion beams.

948 Charged Particle and Photon Interactions with Matter

et al., 2003). Mutation rate of tt and gl loci was 1.9 × 10

−6

per diploid cell per locus per dose

(Shikazono et al., 2005) (Table 33.2). As the dose was used at 150Gy, mutation rate is ca. 2.85 × 10

−4

The average mutation frequencies of tt and gl loci induced by C ions were 20-fold higher than those

induced by electrons. Mutation frequency is generally calculated as a unit per dose for radiation-

induced mutations. However, it is important to compare the mutation frequency as the number of

mutants per irradiated population when considering the use of mutants for practical purposes, for

example, in agriculture. Carbon ions can produce Arabidopsis mutants at a rate four times higher

than by electrons, because carbon ions need one-fth of the dose (i.e., RBE = 5) to induce the same

biological

effects as electrons (Tanaka et al., 1997a).

Mutation

rates were also investigated in other plants with regard to phenotype. In chrysanthemum,

the mutation rates of ower color induced by carbon ions, with oral petal and leaf as irradiation

samples, were ca. 16% and 7%, respectively, which are approximately half the rates induced by

gamma rays (Nagatomi et al., 1997); whereas mutation frequencies of ower color and shape

in carnation were 2.8%, 2.3%, and 1.3% for ion beams, gamma rays, and x-rays, respectively

(Okamura et al., 2003). For hairless mutation in sugarcane, the rate was 17.1% for 80Gy of helium

ions, which is almost double the maximum rate for 200Gy of gamma rays (Degi et al., 2004). In rice,

chlorophyll mutations were investigated in detail (Yamaguchi et al., 2009a). The mutation fre-

quency increased linearly with dose by the irradiation of 18.3 and 26.7MeV/u carbon ions, 25MeV/u

helium ions, and gamma rays. Mutation frequency per M

plant was equal to or higher in those three

ion

beams compared to gamma rays in several doses investigated.

33.4.3 deterMination of population Size for Mutation induction

Since mutation basically occurs at random under the laws of probability, it is clear that the maximum

number of samples possible should be used for irradiation. If the mutation frequency for a particular

gene is known, the minimum size to induce at least one mutant for the desired gene can be calcu-

lated. Dr. Redei and Dr. Koncz (Redei and Koncz, 1992) suggested that 23,025 seeds of Arabidopsis

should be enough only if all the treated seeds survive and produce large M

families, when the

mutation rate is 1 × 10

−4

and genetically effective cell number (GECN) is 2. As the mutation rate

is ca. 2.85 × 10

−4

for 18.3MeV/u carbon ions at 150Gy as above, about 8,000 seeds are necessary

to induce at least one desired mutant. Whereas the mutation rate is 2.85 × 10

−4

and the number of

Arabidopsis genes is thought to be about 27,000, it may be calculated that about 3,500 seeds are

necessary on an average to obtain one mutant for certain loci and that about eight genes will be

mutated

at the same time by this irradiation.

Based

on our experience, the minimum population size to isolate one phenotypic mutation (not

one gene) is likely to be around 2000–5000M

seeds for Arabidopsis, rice, and other crops (unpub-

lished results). However, it is not yet fully understood how many seeds will be needed for plants with

table 33.2

mutation

Frequency i

nduced

by Carbon i

ons

and electrons

mutagen

(

dose)

no. of

1 plants

no. of

2 plants

utant group (loci)

mutation

Frequency (×10

−6

)

locus/diploid

Cell/

dose (gy))

Carbon

ions

(150Gy)

26,200 104,088 tt

(tt3–tt7, tt18, tt19) 1.9 (20 times)

gl (gl1–gl3, ttg1, ttg2)

Electrons

(750Gy)

ca.

17,600 80,827 tt (tt3–tt7, tt18, tt19) 0.097

(gl1–gl3, ttg1, ttg2)

Applications to Biotechnology: Ion-Beam Breeding of Plants 949

different genome size, gene number, and ploidy. On the other hand, it seems that a smaller population

size

would be enough for mutation induction in explants or tissue cultures.

33.4.4 Mutation SpectruM

The mutation spectrum has been investigated rst on the pink ower color of chrysanthemum cv.

Taihei by Nagatomi et al. (1997) (Table 33.3). The mutation induction of the regenerated plants from

irradiated explants of oral petals was investigated. Most ower-color mutants induced by gamma

rays were light pink, and a few were dark pink in color. In contrast, the color spectrum of the ion-

beam-induced mutants shifted from pink to white, yellow, and orange (Table 33.3). Furthermore,

ower mutants induced by C ions showed complex patterns and striped color types that have never

been obtained by gamma-ray irradiation in this cultivar. Mutation spectra of ower color and shape

of carnation were also investigated by Okamura et al. (2003) (Table 33.4). When carnation variety

Vital, whose character is spray type and cherry pink owers with frilly petals, was tested, ower-color

mutants such as pink, white, and red were obtained by x-ray irradiation, while the color spectra of the

mutants obtained by carbon ion irradiation were widely variant such as pink, light pink, salmon,

red, yellow, complex, and striped types. It was suggested that the mutation spectrum of ower color

induced

by ion beams is broad and that novel mutation phenotypes can be inducible.

theother hand, nodistinctdifferenceofspectrum forvisible mutants has beenfound in Arabidopsis

(Shikazono et al., 2003, 2005), rose (Yamaguchi et al., 2003), and rice (Yamaguchi et al., 2009a). It is

not apparent whether this difference in spectrum by ion beams is due to the kind of plants investigated

or to the phenotypes or genes investigated. Further examples and their analyses are needed to obtain the

general feature of ion beams in mutation spectrum.

table 33.4

mutation

pectrum

of Flower Color in Carnation

mutagen

mutation Frequency (×10

−1

ight

pink

ink

ark

pink

almon

yellow Cream s

tripe

inute

striped Complex

EMS 0 5.2 0 1.0 0 0 0 3.1 0 0

Soft

x-rays 1.7 8.4 0 3.4 0 0 0 0 0 0

Gamma rays 1.7 2.6 0 1.7 0 0 0 0 11.3 0

Carbon ions 2.4 4.7 2.4 3.5 2.4 2.4 1.2 3.5 0 2.4

Note:

Original

variety “Vital” with cherry color and serrated petals was used.

table 33.3

mutation

pectrum

of Flower Color in Chrysanthemum

mutagen

mutation Frequency (%)

white

ight

pink

ark

pink

range

yellow

Complex/

tripe

Unirradiated 0 0.3 0 0 0 0

Gamma rays 0 27.7 2.1 0 0 0

Carbon ions 0.3 4.6 0.3 0.3 0.2 10.2

Note:

Original

variety “Taihei” with pink-color petals was used.