Search

CL; International audience; The success of any in vitro mutagenesis programme depends on the establishment of reproducible in vitro plant regeneration procedures, optimization of mutagenic treatments, and efficient screening of the mutagenized populations for desired variations (Ahloowalia, 1998; van Harten et al., 1998; Jain, 2000, 2006, 2007). Cell and tissue culture techniques can improve effectiveness of mutation induction in several aspects. One of the major advantages of tissue culture is that a large number of regenerated plants can be handled at ease for mutagenic treatments (Jain, 2000; Predrieri, 2001). They offer a wide choice of plant material for mutagen treatment (in vitro axillary buds, organs, tissues, protoplasts, and cells) (Broertjes and van Harten, 1988; Constabel and Shyluk, 1994; Jain and Newton, 1989; Duncan, 1997) that is more suited to mutation induction techniques as compared to in vivo buds (Predrieri, 2001). The tissue structures from which plants originate are either multicellular or of single cell origin (Williams and Maheshwaran, 1986; Ochatt et al., 2005; Kulkarni et al., 2007). Normally chimeras are major problems in regenerated plants by mutagen treatment of multicellular structures such as shoot tips or axillary buds. By in vitro culture chimeras can easily be dissociated by several time subcultures of shoot cultures, say 4 generations M1V4 (Jain, 2000; Predrieri, 2001) This, as compared to in vivo buds, means less risk of obtaining chimera plants and a higher probability for mutated cells to express the mutation in the phenotype (Broertjes and van Harten, 1988). It also facilitates rapid completion of the propagation cycles of subculture aimed to separate mutated from nonmutated sectors (Ahloowalia, 1998). In vitro selection and cloning of selected mutants can also be effective when supported by adequate early evaluation tools (Ball, 1990; Duncan, 1997; Ochatt, 2006; Ochatt et al., 2005) as well large scale multiplication of mutant plants. Micropropagation is an ideal system for true-totype multiplication of mutant plants by using shoot tips or axillary buds (Ahloowalia, 1998). Furthermore, tissue culture provides high phytosanitary conditions, which is an ideal system for obtaining healthy starting material and is maintained throughout the plant regeneration process (Ochatt et al., 1990; Cassels, 1998; Predrieri, 2001; Kulkarni et al., 2007). Therefore, in vitro mutagenesis should always be carried out on healthy explants. Otherwise endogenous contamination may compromise obtaining mutations, as well as selection and/or the survival of mutants.

In vitro shoots of six pear (Pyrus communis L.) cultivars, Conference, Doyenné d'Hiver, Passe Crassane, Bartlett, Abbé Fetel and Butirra Precoce Morettini were irradiated with gamma rays (3.5 Gy). After three subcultures, microcuttings from the irradiated shoots and from additional non-irradiated microcuttings were rooted to establish plants for survey orchards. All trees were individually observed for variation in fruit traits and for productivity. Trees were selected for improved characters related to production such as early bearing and consistent productivity. Variations observed in fruit appearance concerned degree of russeting, fruit shape and size. The frequencies of the observed variations in fruit traits depended on the cultivar, ranging from 0.81% in Doyenné d'Hiver to 3.64% in Passe Crassane. Of the 97 variants selected, only two showed chimeral behavior.

This review describes in vitro mutation induction methods in fruits and the in vitro selection procedures available for early screening. Results obtained through in vitro mutation techniques, including somaclonal variation, are reviewed and compared with the current achievements and future prospects of transgenic breeding. Plant improvement based on mutations, which change one or a few specific traits of a cultivar, can contribute to fruit improvement without altering the requirements of fruit industry. Induced mutations have well defined limitations in fruit breeding applications, but their possibilities may be expanded by the use of in vitro techniques. Tissue culture increases the efficiency of mutagenic treatments for variation induction, handling of large populations, use of ready selection methods, and rapid cloning of selected variants. Molecular techniques can provide a better understanding of the potential and limitations of induced mutation e.g. molecular marker-assisted selection, which can lead to the early identification of useful variants. The relatively high number of research reports compared with the low number of cultivars released suggests that mutagenesis in combination with tissue culture is either ineffective or has yet to be exploited in fruits. Positive achievement recorded in other species seem to support the hypothesis that in vitro mutation induction has high potential also for fruit improvement. The possible contribution of a well-pondered and coordinated use of the numerous mutation induction, mutant selection, and field validation procedures available to advances in fruit breeding is discussed.