Effects of heavy-ion beams on chromosomes of common wheat, Triticum aestivum

Abstract

To investigate the nature of plant chromosomes irradiated by heavy-ion beams, the effects of nitrogen (N) and neon (Ne) ion beams on hexaploid wheat chromosomes were compared with those of X-ray. Chromosome aberrations, such as short, ring and dicentric chromosomes appeared in high frequency. The average numbers of chromosome breaks at LD-50 by irradiation with X-ray, N and Ne ion beams were 32, 20 and 20, respectively. These values may be underestimated because chromosome rearrangement without change in chromosome morphology was not counted. Thus, we subsequently used a wheat line with a pair of extra chromosomes from an alien species (Leymus racemosus) and observed the fate of the irradiated marker chromosomes by genomic in situ hybridization. This analysis revealed that 50 Gy of neon beam induced about eight times more breaks than those induced by X-ray. This result suggests that heavy-ion beams induce chromosome rearrangement in high frequency rather than loss of gene function. This suggests further that most of the novel mutations produced by ion beam irradiation, which have been used in plant breeding, may not be caused by ordinary gene disruption but by chromosome rearrangements.

Introduction

High linear energy transfer (LET) radiations such as neutron and heavy ions are present in space and known to have stronger biological effects than low-LET radiations, such as X- or γ-rays. In space exploration, the human body is exposed to high-LET radiations [1]. Thus, high-LET radiations are threats not only for humans but also for other organisms taken to space for provision of food. Because of this, it is necessary to estimate the distinct biological damage on organisms caused by high-LET radiations.

Recently, heavy-ion beams have been used as novel and efficient mutagens in plant breeding. Many plant variations were induced by irradiation, and many novel experimental materials and practical cultivars were generated [2], [3], [4], [5], [6], [7]. It is well known that high-LET ion beams have higher biological effects than low-LET radiation such as X- and γ-rays. Ion beams cause mutations ten times more effectively than other sources of radiation such as X-rays [3]. The spectrum of mutants produced by ion beams is different from that produced by X-ray probably because ion beams cause more localized and dense ionization in cells [2], [8]. Similarly, neutron irradiation shows a different spectrum from low-LET mutagens [9], [10]. However, the nature of the heavy-ion beam may be different from that of the neutron beam because the results in ion beam therapy are more effective than those in neutron beam therapy [11]. The study of Shikazono et al. [12] revealed that mutations induced by ion beams in Arabidopsis thaliana contain inversions, translocations, and short deletions at the DNA level at a similar frequency as point-like mutations at the nucleotide level. In this study, we investigated the effects of heavy-ion beams on chromosome breakage using a wheat line with marker chromosomes.

Common wheat (Triticum aestivum L., 2n = 6x = 42) is suitable for studies on chromosome rearrangement because of its size (about 30 times larger than Arabidopsis) and ploidy level. Because it is an allohexaploid, individuals with severe chromosome aberrations such as deletion of homozygotes and nullisomics can still survive [13], [14]. Homoeologous genes on the other genomes, having similar function, compensate for the genes on the lost chromosomes. Common wheat is, therefore, highly tolerant to chromosome aberrations compared with diploid organisms. This phenomenon allows us to adequately observe the effects of mutagens on chromosome aberration without the bias of survival rate of cells with aberrant chromosomes. In the present study, we used common wheat and an experimental line with a pair of alien chromosomes. Genomic in situ hybridization (GISH) clearly demonstrated the nature of aberrations at the chromosomal level.