Introduction

Plant mutation breeding is a core area among modern approaches that is practiced as a part of plant breeding technology. The contribution of plant breeding combined with modern technologies for expansion of crop production toward improved food security and nutrition is recognized worldwide. With increasing population and concomitant decreasing land resources, improvement in crop yields based on fertilizer utilization, and the control insects, pests, and pathogens need to urgently be implemented and faces several challenges (Ahloowalia and Maluszynski 2001;Swaminathan 1998). In the early twentieth century, plant biologists determined that application of chemicals radiation technology could achieve an increased frequency of genetic modifications and efficiency in treated seeds (Oladosu et al. 2016). Thereafter, a variety of mutagens, such as physical or chemical, were utilized to induce an extensive range of genetic variability that has been prompted and contributed to current plant breeding (Solanki et al. 2011).

Radiation-induced mutation breeding is a remarkable method that presents superior mutant cultivars in contrast to conventional breeding like selection and crossing which is time-consuming and laborious with limited induced genetic alteration (Beyaz and Yildiz 2017; Hanafiah et al. 2010). In plant mutation, breeding hinges not only on its effectiveness but also on its efficiency owing to the convenience of physical or chemical mutagens. Mutagenic effectiveness, which is the rate of mutations produced by a mutagen based on the response of a cultivar to increasing doses of the mutagen determines the mutation rate about the destructive effects (Konzak 1965;Rajarajan et al. 2014). Therefore, for experimental purposes, irradiation doses range from low to high, approximately at 100 Grays, although, up to a kGy could be used in agricultural business or for varietal preferences (Maity et al. 2005). In the last few decades, several studies have focused on the utilization of radiation in gamma rays with a specific interest in superior cultivars of agricultural crops of economic interest (Jan et al. 2012). Examples of favorable traits induced after gamma exposure include dwarf or semi-dwarf growth pattern, earlier flowering and maturity, high yielding varieties, and resistance to insect and pathogen infestations (Li et al. 2007). Over the previous fifty years, induced mutation has accounted for enhanced resistance varieties (Kharkwal and Shu 2009), and the mutant database of the Food and Agricultural Organization lists almost 3246 certified mutagenic plant varieties (Beyaz and Yildiz 2017).

Ornamental plant mutation breeding has become more successful owing to additional changes in phenotypic characteristics, heterozygous nature, and high mutation frequency produced a large number of new varieties (Datta 2001; Maluszynski et al. 1995;Micke et al. 1990). Within the ornamental trade circuit, there has always been a demand for new cultivars because of a change in the sense of flavor and pattern (Yan et al. 2019). Moreover, ionizing radiations leads to induced somatic mutation breeding that helps to holds promise for development and has a higher possibility for genetic achievement leading to great access to variations in flower shape, color, uniformity, and other physiological characteristics (Datta 1997). Therefore, the objectives of plant mutation breeding are to pool resources of individual organizations to greater advantage and promote efficient use of radiation intensity through international collaborations to ensure to ensure higher and quality varieties (Spencer-Lopes et al. 2018).

The present paper reviews ornamental plant mutation breeding research that has been conducted previously using gamma irradiation technology. Thus, the propose of this paper is to emphasize the significance of gamma irradiation of various duration and doses of economically important ornamental crop species, which can be introduced in varietal amelioration programs.

Plant Mutation Breeding: History

Since the 1950s, radiation has emerged as a prevalent means of plant breeding to extend variation across an extensive range of grain and flower plant (Oldach 2011). The first spontaneous cereal mutant plants are originated in China 2317 years ago (Solanki et al. 2011;Van 1998). Furthermore, scientific records of spontaneous diversity in crop species have been made between 1590 to 1968 (Beyaz and Yildiz 2017). In 1889, Hugo de Vries introduced a variant in the evening primrose and snapdragon that no longer followed Mendel’s laws of inheritance at the time of experimenting or “rediscovery”. Subsequently, in 1901, he was the first to recognize that “mutation is a mechanism of creating variability” and regarded mutations as heritable adjustments through mechanisms distinct from recombination and segregation. In the twentieth century, his understanding of mutation as the source of genetic variation became the basis of mutation strategies in plant breeding and genetics. The first X-rays induced tobacco mutant variety, Chlorina, was introduced in Indonesia in 1936 (Broertjes 1988), and chromosome doubling consequences of colchicine treatment on plant chromosomes were determined in 1937 (Shu et al. 2012). In 1949, the first experiments on plant mutation using a ⁶⁰Co gamma-ray were performed and the first mutant propagated crop variety was released as a tulip variety Faraday with better flower shade and arrangement in 1954 (Broertjes 1988). Whereas natural occurring of ornamental plant mutation induced breeding has been practiced for centuries, in vitro and in vivo mutagenesis and somaclonal variation techniques ensued since 1930s, 1970s and 1980s, respectively (Schum 2003). From 2000 till date, mutagenesis has attained widespread consideration for its use in a promising new approach, “targeted induced local lesions in genomes” or TILLING.

According to Food and Agricultural Organization figures 77 induced mutant cultivars had been released in 1969, whichever rise adequately to 1,330 crop varieties (Bradshaw 2013). In addition, the evaluation of the durum wheat in Italy indicated the presence of approximately 70% mutant varieties, and 400 rice mutant varieties have been obtained from mutation breeding programs since 1994. Using the FAO/IAEA database, one can identify gamma-irradiation ornamental mutant varieties along with their induced attribute that are desirable for commercialization (Table 1, 2.1, and 2.2).

Application of Gamma Irradiation and Associated Mechanism

With the aid of atomic particles, ionizing radiation is produced utilizing the process wherein one or more electrons are discharged during collisions of the particles with atoms or molecules (Thomas 2012). Regarding gamma irradiation, radioisotopes are produced electromagnetic radiation and nuclear reactors, from sources such as Cobalt-60, and Caesium-137 which have dangerous effects and permeate the cell. ⁶⁰Co and ¹³⁷Cs are the most relevant gamma radiation sources for radiation process considering the relative high energy of their gamma rays and long half-life; 5.27 years for ⁶⁰Co and a relatively higher 30.1 years for ¹³⁷Cs. The Gray (Gy) is the derived System International (SI) unit for irradiated energy and is used to compute the absorbed dose of gamma irradiation. It is defined as 1 Gy = 1 Jkg^-1; the corresponding non-SI unit for irradiation is 100 rads = 1 Gy or 1 krad = 10 Gy. The absorbed irradiation dose rate (Gys^-1/Gymin^-1) indicates how much energy absorbed at a given time in a unit mass of the irradiated material (Kodym and Afza 2003) and doses are typically graded in three categories as high, medium, and low (more than 10 kGy, 1 to 10 kGy, and less than 1 kGy respectively). The high doses are suitable for the sterilization of food products, whereas low doses are suitable to induce mutations in seed material. Doses varying from 60 to 700 Gy have been used for many seeds propagated crops, e.g., rice, wheat, maize, beans, and rapeseed (Ahloowalia and Maluszynski 2001).

Presently, all industrial radiation processing facilities employ ⁶⁰Co as the gamma irradiation source. The officially released mutant varieties available from the Food and Agricultural Organization database suggest that among all radiation technology, gamma irradiation has been the approach most frequently practiced for inducing mutations in crops. The simplicity of the treatment rather than a higher efficiency of mutation induction is the main reason (Maluszynski et al. 2009). Exposure of plant substances, especially seeds, to gamma irradiation results in mutagenic modifications in living cells through a variety of processes. Two mechanisms occur after irradiation: a) the primary or physical response, which manifests on the molecular disturbance, and b) the secondary or chemical response from the ionized molecules which helps produce free radicals (Lagoda 2012). Considering gamma radiation has excessive energy and increased penetrability in exposed plant cells and tissues, DNA of the subject material may additionally undergo extreme modifications owing to direct physical strikes of radiation on the DNA or through the manufacture of reactive O^-2 species such as atomic oxygen, hydrogen peroxides, and hydroxy ions (Majeed et al. 2017;Majeed et al. 2018). Mutagenesis research strongly indicates that o rganisms c an i ntentionally f ix D NA when DNA is impaired because of irradiation or chemicals; however, viability decreases proportionally, and surviving colonies are mutated in the absence of such repair in unicellular organisms (Shu et al. 2012). As induced mutations, the deteriorated DNA in cells will sustain only when this damage is adjusted either precisely or improperly and even the result of inaccurate repairs will be locked in the genome (Shu and Lagoda 2007).

Mutation Induction of Ornamental Mutant Plants Using Gamma Irradiation

The intensity of the gamma irradiation can be expressed as morphological, biochemical, structural, and practical alterations in plant growth and development. For the principal objective of breeding, mutagenic treatment with low effects on physiology, and a distinct genetic response are preferred. The mutagenic treatment goal is to prompt mutations attributable to the hereditary enhancement of distinct features. Mutagenesis is used to Promote various constructive attributes such as those affecting self-compatibility, plant height, flowering time, fruit color, fruit ripening, and resistance to disease, and insects. Currently, the range in plant variety obtained through mutation initiation is multiplying constantly (Maluszynski et al. 1995;Piri et al. 2011). Plant development by irradiation is radiosensitive; therefore, the response depends upon a certain absorbed dose of irradiation. Some physiological features may be generated even though a declining tendency of these traits is observed at higher gamma irradiation doses (Maity et al. 2005).

For inducing mutations in ornamental plants, physical mutagens are very applicable to in vitro and in vivo propagation. The diverse plant parts suitable for spontaneous treatment comprise the shoot tip, adventitious buds and cuttings from stems, root cutting, leaves, petioles, somatic tissues, pedicels (Spencer-Lopes et al. 2018). The interesting aspect of in vitro selection is acquiring the most desired features of the plant through manipulation of the MS medium containing the altering agent (Lestari 2006). For in vitro cultured plant material, a large amount of tissues and micrograms of cell suspensions are required for the irradiation dose. High power radiation can be accomplished in sealed containers, whereas radiations with minor insertion ability need to be applied in unsealed vessels to attain a homogeneous effect. Gamma irradiation in non-hormone containing MS medium is being encouraged owing to the effects of the rays on media composition (Brunner 1995;Dix 1990).

Some reviews recommend that callus cultures are much more hypersensitive to radiation treatment and rely upon much lower doses ranges between 2 and 5 Gy than stem cuttings or seeds; comparatively greater doses in the range of 15 to 20 Gy can result in necrotic tissue or lose of developmental bility (Ahloowalia and Maluszynski 2001;IAEA 1997). For example, in Gerbera jamesonii, the number of shoots regenerated from gamma-irradiated cultured petiole reduced when explants were exposed to a radiation dose of 20 Gy (Hasbullah et al. 2012). After irradiation, subcultures of Exacam affine were grown on new MS media, and a 50% decrease in growth rate was observed at 32 Gy (Limtiyayotin et al. 2018). Sakr et al. (2017) demonstrated that Brunfelsia pauciflora shoot tips exposed to gamma irradiation at 5.0 Gy, and 10 Gy resulted in 100% survival during in vitro multiplication. The highest values average shoot number, total number of leaves, and the shoot length were recorded the multiplication stage with exposure to 10 Gy.

In commercial cultivars, vegetative propagation is desirable because these plants can produce new plants through plant parts. The application of irradiation dramatically enhances the frequency of somatic cell mutations for favorable characteristics. Although mutation is regarded as a one-cell phase, multicellular apices are also typically recognized as an alternative self-sufficient group of cell layers alike corpus, epidermis, sub epidermis, and a wide variety of meristematic cells (Broertjes 1988;Brunner 1995). Patil et al. (2017) reported that in a chrysanthemum cultivar exposed to irradiation of 0.5 to 3.0 Krad, the maximum plant survival rate was observed in control plant, and the minimum values were observed in the treatments doses of 2.5 and 3.0 Krad. At the time of bud initiation, plant height, and flowering were least in 2.5 and 3.0 Krad irradiation treatments. Shoot cultures of orchid exposed irradiation at 15 Gy to 45 Gy revealed a reduced average shoot length, total fresh weight, and leaf area (Billore et al. 2019). Flower color exerts a great role on consumer demand, and in the marketing sector, it reflects aesthetic gratification; therefore, gamma irradiation might also be used to create a new flower color that could be acceptable to all consumers. In chrysanthemum, multiple shoots have been irradiated with gamma rays at 10, 30, 50, 70, 90, and 110 Gy. Subsequently, subculturing of the explant was performed thrice from the M1V1 to M1V4 shoots irradiated at 50 Gy, and much more variation was observed in the treated plant where previously only yellow flower color was obtained (Lamseejan et al. 2000).

Many researchers have tried to improve novel marigold cultivars with the aid of breeding, although very little work has been conducted on mutation breeding (Heslot 1968). Marigold seed was irradiated with 100, 200, 300, and 400 Gy, and the stimulating effect was observed at 100 Grays wherein nearly all the characteristics studied showed a positive correlation, including growth and yield attributes (Singh et al. 2009). Shafiei et al. (2019) revealed that 25 Gy gamma-ray irradiation is considered an appropriate dose to generate a mutation frequency in the chrysanthemum cut flower. The purple rose cultivar showed the greatest change in petal color with a mutation rate of 54.56 %, and a variation of 32.11% was detected in the pink rose cultivar flowers 25 Gy treatment. In nature, some plant species prefer a lower dose of irradiation, although some species produce a great response in flower color with a higher dose of gamma radiation. Using gamma irradiation and EMS, Heslot (1968) observed a wide range of mutant flowers in diploid and tetraploid rose cultivars where 4 and 8 Krad with 8 ppm EMS showed high-frequency mutation. Rose cultivars with different scented were irradiated with various doses of gamma irradiation. After two years of irradiation, the mutant flower with a different color and shape was observed with the optimum dose of mutant induction being 4-5 Krad of radiation (Datta 1997;Gupta and Shukla 1970). In petunia plants, survival rate and plant height were decreased after exposure to high mutagen doses where 40 and 50 Gy and mutagenesis was in plant, especially in leaves. Owing to their heterozygous and polyploidy nature, flower color mutants from Dahlia being formed and commercialized using gamma irradiation (Berenschot et al. 2008).

The response of seedling survival rate (%), maturity, size, shape, flower color, yield, lethality, and other physiological factors to variable doses of gamma irradiation levels observed in several studies is presented in Table 3. It can be noted that selection of expected outcomes is possible through application of distinct doses of gamma rays and they can be used for further commercial cultivation of ornamental flowers.

Impact of Gamma Irradiation on Functional Changes in Ornamental Plants

Evaluation of biological changes resulting from physicochemical changes is imperative. To induce probable anatomical changes of potential plants, the biological effectiveness of gamma irradiation rays is established based on the intercommunication within cell molecules, especially, water helps to initiate free radicals which may displace cellular parts (Desai and Rao 2014). After exposure to ionizing radiation, the promoted highly reactive oxygen species comprising hydroxyl radicals, hydrogen peroxide, and superoxide radicals (OH, H₂O₂, and O^2-, respectively) can separate carbohydrates, protein, nucleic acids, and lipids from cell membrane resulting in successive damage to cellular components and creating oxidative stress (Suzuki et al. 2012). In addition, phenolic compounds, anthocyanins, flavonoids, and carotenoids are known as phytochemicals that can contribute to protecting against the damaging effects of oxidant- antioxidant imbalance (Hong et al. 2018;Iloki-Assanga et al. 2015). Gamma irradiation of increasing doses intensifies the activity of free radicals and enzymes such as antioxidant catalase, peroxidase, and superoxide dismutase. In general, an irradiated seed has more fractions of amino acid, protein, lipids, and polysaccharides than non-irradiated plants. According to the several studies, the total carbohydrate and protein content decreased with increasingly higher doses of gamma rays owing to hydrolyzing enzymes and higher metabolic activities in germinating seeds (Maity et al. 2004).

In ornamental plants, somatic mutation of flower color is directed owing to either an increase or decrease in the concentration of pigments; this could be because of the obstruction of synthesis of pigments during flower development, or the mutation may be arise because of the influence of a new pigment (Datta 1997). During vegetative propagation, the mutant cells of plants remain in the dormant stage in the M₁V₂ generation and reveal their mutant characteristics depending on nuclear factors such as chromosome number, size, heterochromatin, polyploidy level, centromere number and nuclear DNA content (Datta 2001;Zhu et al. 2003). In different chrysanthemum cultivars after gamma irradiation, chlorophyll variegated leaves have been established by mutagenic treatment that provides additional elegance to these ornamental plants, especially at the time of flowering (Datta 1997;Datta and Teixeira 2006). Schum and Preil (1998) found that irradiation induced alteration in leaf pigmentation in Begonia, Bougainvillea, Hibiscus, Lantana, Petunia. The purple or pink colored flower mutant cultivar of chrysanthemum had a higher content of phenolic acids, anthocyanin, and flavonoids than the original chrysanthemum cultivars (Ryu et al. 2019).

Hasbullah et al. (2012) observed that after different doses of irradiation, the chlorophyll content in Gerbera jamesonii flowers was relatively low compared to that in the control plants. In addition, soluble protein content in flower reduced gradually after exposure to irradiation. In Gerbera, the activity of antioxidant enzymes such as ascorbate peroxidase catalase, superoxide dismutase, and glutathione reductase increased significantly with the increase in dose rate as compared to that in the control (Ghani and Sharma 2019). Gamma doses at 10 kGy resulted in a significant increase in the concentration of chlorophyll a and b; low doses activated photosynthetic pigment system and sugar content in plants was significantly increased at the developmental stage (Jan et al. 2013). In addition, high doses of gamma irradiation can impair photosynthetic pigment activity thereby disrupting photosynthetic capacity (Strid et al. 1990). In chrysanthemum, the shoot was irradiated with gamma-rays of 20, 30, 50, and 70 Gy and sucrose levels were observed to increase significantly in leaves and fructose level first increased and then decreased again. In contrast, glucose contents declined rapidly and increased later where soluble sugar was found in the maximum in the 20 Gy doses (Noh and Kim 2003). In rose Folklore cultivar, anthocyanin, and cyanidin content has maximum in control plants where induced mutant showed a lower amount of substance (Kaicker 1990). Lata (1987) observed the effect of the mutation on rose where anthocyanin contents decreased gradually with the increase of irradiation.

Prospects of Ornamental Plant Molecular Mutation Breeding

Molecular mutation breeding has become an effective tool for crop improvement and development and is becoming progressively robust and costs effective. Plant mutation breeding is a significantly rapid throughput technique that is applicable in thousands of elements and can be analyzed within a few weeks. The several techniques used have illustrated that the trend in genome sequencing is in line with the basic stage of initiating a redesigned proposal that has been advanced after the application of mutation breeding (FAO/IAEA 2018). For the identification and evaluation of mutated plants, DNA fingerprinting, and molecular mappings like random amplified polymorphic DNA (RAPD), single nucleotide polymorphism (SNPs), amplified fragment length polymorphisms (AFLP), simple sequence repeat (SSR) and sequence-tagged microsatellite sites (STMS) have undoubtedly supported plant molecular mutation breeding technology (Beetham et al. 1999;Raina et al. 2016). Mutation induction and molecular breeding techniques have been combined to increase advanced crossing and to introduce foreign particles or genes into receptive plant cultivars (Linqing 1991).

Induced mutation through irradiation by gamma rays can assist in the production of a higher frequency of single sub substitutions and base deletions than deletion mutation and large mutations (Li et al. 2017;Shirasawa et al. 2016). Usually quantitative trait locus mapping is useful to analyze the individual genomic location and gene distribution in plants (Lander and Botstein 1989). There are several distinct molecular markers (AFLP, ISSR, RAPD, and SSR) among which SSR markers are a powerful mechanism that can predict high genetic variation in different plant varieties to determine induced mutation (Das et al. 2011;Tautz and Renz 1984). Molecular screening or filtering is established for induced mutations as specific function related genes that permit the hereditary preference of selected mutant plants in the mutation breeding program. After exposure of Petunia x hybrida 50 and 100 Gy gamma irradiation, PhMT2 and PhA PX g enes w ere identified u sing Q RT-PCR. Their expression level indicated that different doses led to different levels of DNA after irradiation and suggested that a low dose rate is unable to activate protection mechanisms (Dona et al. 2013).

Using AFLP markers in chrysanthemum, 866 bands combinations of 12 primers were produced that reflected 83% of the total polymorphism. Genetic diversity with a higher percentage of polymorphism was detected with 30 Gy irradiation using the primer combination of M-CAT/E-ACC (Kang et al. 2013). This ISSR analysis determined that gamma irradiation on the plant is an active mutation breeding method because it can discriminate mutant loci between them (Wang et al. 2017). Rapid markers have been used to pursue the effect of radioactivity at the genetic level and for genetic diversity among radio mutant plants (Kaul et al. 2011a;Kumar et al. 2012). The mutant plants introduced in Helichrysum bracteatum through exposure to irradiation were, identified using RAPD and SSR markers. It was observed that significant mutation was induced with 20 Gy dose rates through changes in genetic material at the DNA level in the H. bracteatum plant genome. These results indicated that RAPD and ISSR marker techniques are a very reliable and convenient approach in the detection of mutant DNA polymorphism among treated gamma irradiation individuals (El-Khateeb et al. 2017). Using RT-PCR mediated cloning of CmCCD4a, it was demonstrated that the accumulation of carotenoid content in the biosynthesis pathway is the key factor which is regulated by CmCCD4a gene (Yoshioka et al. 2012).

The prospective improvement of in vitro culture selection procedures for both disease (virus, bacteria, and fungus) and insect resistance would be remarkable in the molecular mutation breeding sector. To achieve genotypes with expected traits, combination of the molecular breeding and irradiation techniques for microspore culture; anther culture; cell suspension; regeneration of haploid; diploid; tetraploid and doubled haploid plants; and chromosome doubling could be employed (Raina et al. 2016;Schwarzacher 1994). In plant genomes, next-generation sequencing (NGS) has promoted a tremendous output of induced mutations (Gupta et al. 2017). To produce desired mutant plants at a higher level of frequency, it is assumed that changing the pattern of gene expression is a reliable approach and fixing these changes by clarifying gene expression mechanisms in different plant characteristics which is imperative for crop improvements (Yamaguchi 2018). Marker-assisted back crossing may be a probable consequences of new concepts and techniques for introgression of mutant alleles which assists in the expansion of the activity to identify effective traits through genes mutations helps the selection of genotype, analytical choice, doses of the mutagenic agent, and, improved experimental layout of mutation breeding programs (Spencer-Lopes et al. 2018).

Conclusion

Application of gamma radiation is used extensively to promote a novel cultivar in modern ornamental plant breeding. Although it can elicit adverse responses from plants through physiological and biochemical changes, it can compete successfully to produce predicted results for creating renewed varieties. Gamma irradiation has the potential to develop new varieties that require less time for establishments in different locations. Sometimes conventional breeding can be very challenging given its mode of propagation; thus, gamma irradiation could be a promising technique to overcome this breeding related obstacle. From many examples presented, it is clear that a lower amount of gamma irradiation doses produces positive and effective influences on morphological as well as physicochemical characteristics in various kinds of ornamental plants; however higher doses of irradiation have a deleterious effect with some exceptions. It is not possible to recommend the optimum doses of gamma irradiation; the primary fact is that it varies from plant to plant and responses are different from their growth level to other physical traits. Therefore, gamma irradiation is expected to play a preeminent role in the future to further strengthen and ensure successful, and well-established plant breeding programs.