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Breeding objectives. In many of the traditional mango producing countries of South and South-east Asia, there is significant consumer resistance to replacement of local mango cultivars by newer selections. Serious production and postharvest problems that have a genetic basis, e.g. alternate and irregular bearing, susceptibility to anthracnose and mango malformation, tree shape and size, etc., cannot be resolved easily using an applied physiology approach. In recent years, paclobutrazol has been applied to alternate bearing mango cultivars to promote flowering in the off-years; however, this results in severe decline in trees that have been treated for successive years. Conventional mango breeding in India, therefore, has focused upon the development of new cultivars that are largely indistinguishable from traditional selections with respect to fruit size, appearance, taste, flavour and overall quality.

Mutation breeding has not significantly impacted mango cultivar improvement by production of useful off-types of existing selections; however, there is some evidence that somatic mutations can occur naturally in mango within seed-propagated polyembryonic cultivars. There are reported to be phenotypic differences within ‘Kensington Pride’ (polyembryonic) trees in Australia, and many of the polyembryonic cultivars of south-east Asia, e.g. ‘Arumanis’, ‘Golek’, etc., show significant variation, so that different phenotypes of ‘Arumanis’, for example, have been characterized as ‘Arumanis-1’, ‘Arumanis-2’, etc. Litz et al. (1993) reported that there was isozyme variation within a nucellar seedling population derived from two polyembryonic cultivars, ‘Carabao’ and ‘Philippine’.

The most important production and postharvest problem of mango in the humid tropics and subtropics is anthracnose, caused by C. gloeosporioides. The current strategies for control of this disease involve the use of moderately resistant cultivars, i.e. ‘Tommy Atkins’ and ‘Keitt’, etc., and at least weekly application of fungicides, i.e. benomyl or maneb or mancozeb from the time of flowering until harvesting (Dodd et al., 1998). This can result in as many as 25 spray applications in a season.

PROTOCOL. The effect of irradiation for inducing variation in embryogenic cultures of ‘Hindi’, ‘Keitt’ and ‘Tommy Atkins’ was reported by Litz (2009). Embryogenic mango cultures growing on semisolid maintenance medium were exposed to 0–200 Gy irradiation provided by ⁶⁰Co. The LD₅₀ of ‘Keitt’ is c.125 Gy; the LD₅₀ of ‘Tommy Atkins’ is c.100 Gy; however, the LD₅₀ of ‘Hindi’ could not be established within this dosage range.

There is increasing evidence that culture filtrates produced by pathogenic fungi and bacteria can be utilized not only to select for resistance to the pathogen in vitro (Hammerschlag, 1992), but also to induce the host resistance response. For the former approach to be used effectively, the prerequisite is a highly embryogenic suspension culture that can be challenged with the culture filtrate. Litz et al. (1991) observed that the culture filtrate of C. gloeosporiodes could be used as a selective agent in mango suspension cultures. Somatic embryos were recovered from embryogenic cells and PEMs that had survived exposure to culture filtrate, and regenerants appeared to show resistance to inoculation with the pathogen. Jayasankar et al. (1999) characterized the in vitro effects of C. gloeosporiodes phytotoxin that had been purified according to established protocols (Gohbara et al., 1977, 1978), presumably colletotrichin, and crude culture filtrate on the mortality and growth of ‘Hindi’ and ‘Carabao’ embryogenic cultures. This study established the LD₅₀ values for the effect of culture filtrate and phytotoxin on embryogenic cultures and the growth curves of challenged cultures. In a later study with the same mango cultivars (Jayasankar and Litz, 1998), embryogenic cultures were either exposed continuously for four cycles of challenge/selection/regrowth or were challenged for one, two, three and four complete cycles with the purified and partially purified culture filtrate of C. gloeosporioides. At the end of each cycle, surviving PEMs were cloned and either rechallenged or subcultured onto somatic embryo maturation medium.

ACCOMPLISHMENTS. At least three successive challenges with either crude filtrate or purified phytotoxin were necessary to stimulate the expression of antifungal genes in vitro. This was measured by coculturing the challenged material with a virulent strain of the pathogen. Coculture of the pathogen with resistant cultures resulted in the suppression of fungal growth, and the antifungal properties of the PEMs increased with each cycle of challenge and selection. There was enhanced production of extracellular chitinase and β-1,3-glucanase from selected, antifungal cultures. There was stable expression of the antifungal nature of the resistant lines in suspension cultures and in somatic embryos for more than 2 years after selection. Plants were regenerated. Several RAPD markers were associated with selected antifungal cultures (Jayasankar et al., 1998). There was no variation in RAPD markers of the unchallenged controls with respect to the parent trees, indicating that exposure to either the phytotoxin or culture filtrate is essential for antifungal expression. These results also demonstrated that embryogenic cultures are stable genetically, and the induced variation does not appear to be a result of somaclonal variation. Furthermore, it seems probable that the phytotoxins themselves are highly mutagenic.

In vitro-induced mutation followed by selection can be a highly efficient method for addressing a specific breeding problem of perennial trees for which there are both an effective selection agent and a highly embryogenic regeneration protocol. Unfortunately, there are relatively few such selection agents that can be utilized in this manner.