**1. Introduction**

As one of the earliest agricultural activities, plant breeding has begun in very ancient times (early on Neolithic revolution, 10,000 BC) parallel to human culture [1, 2]. For the plant breeding, genetic variations are the pre-request. Mutations (natural process that creates new variants [alleles] of genes) are the main source of all genetic variation in plants as well as in any other organisms [3, 4]. According to the historical records (an ancient book "Lulan"), the first spontaneous mutant plants (cereal crops) are found in China 2317 years ago [5, 6]. After that, a few

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researchers reported spontaneous variation in plants between 1590 and 1968 [6]. However, the first publications of induced mutations (through X-rays) for the plant breeding was published 89 years ago by Muller and Stadler [7, 8]. As a result of these studies, mutation breeding as a new approach was added to other plant breeding methods. Thereafter, a large amount of genetic variability has been induced by various mutagens (physical and chemical) and contributed to modern plant breeding. *Nicotiana tabacum* was the first commercial mutant cultivar (called: chlorine type), which produced by inducing mutations [6]. Induced mutations were used to improve tolerant plant varieties over the past 50 years in all over the world [8]. Currently, 3246 registered mutagenic plant varieties are there in FAO/IAEA mutant data base [9].

Increasing crop yields is a major demand for assuring food security. Mutagenesis is an important tool to improve crops and has not got any regulatory restrictions as genetically modified organisms (GMOs) [10]. Plant breeding is based on the genes. Initially, breeders select new phenotypes with valuable characters without knowing the genetic constitution. The emergence of molecular genetics is parallel to understand the details of inheritance of desirable/undesirable traits and genetically controlled, modern biotechnological breeding has paved a wide road. By using DNA recombinant technologies, the gene encoding a trait precisely manipulated to create novel phenotypes. The cloned gene in respect of the source or recipient of the genes can be transferred by breeding technology known as transgenic technology. In transgenic technology, the key step is the integration of desired foreign genes into the host plant genome. For plant transformation, there are primary tree methods such as the Agrobacterium-mediated, particle bombardment, and protoplast transformation. The Agrobacterium-mediated gene transfer method is one of the most practical and suitable method [2]. The first transgenic plant (tobacco [*N. tabacum*], which contain antibiotic resistance gene) was obtained in 1980 by Marc De Block through Agrobacteriummediated method [11]. Breeders can transfer encoding genes of new characters into plants genome through transgenic approaches. Its precision and the betterment of a trait without changing the genetic makeup of genome in elite genotypes are the main advantages [2]. Although transgenic technology has significant achievement in improving crops and has substantial commercial value, this technology has some technical obstacles. For example, in terms of highly recalcitrant to genetic transformation and regeneration, there are many economically important plant species or elite varieties of species. In addition to the technical obstacles, there are some debates about unpredictable risks of transgenic technology on environment and food safety, even though many of these debates are baseless. However, more advanced technologies have been developed to solve these ideas [2]. On the other hand, plant breeders use mutagenesis in plant breeding programs without restrictions such as the legislative constraints, licensing costs, and societal opposition of transgenic technology [10]. Although still limited to the content of the endogenous genome, mutagenesis and high-resolution screening will supply a very good complement to recombinant DNA technologies and genetically modified organisms (GMOs) in further improved new plant forms that are better adapted to change conditions of environment and the increasing global population [1].
