1.  Introduction

Endosperm is a unique tissue in its origin, development and ploidy level. It is a product of double fertilization but unlike the embryo it is triploid and develops into a formless tissue (Bhojwani and Bhatnagar 1999). It is, therefore, an interesting tissue for morphogenesis. Any abnormality in the development of endosperm may cause the abortion of embryo resulting in sterile seeds (Johnston et al. 1980). The endosperm may be totally consumed by developing embryo leading to the formation of exalbuminous (non-endospermous) seeds or when it persists, the seed is called albuminous (endospermous). In albuminous seeds, it is used as a food source which may contain proteins, starch or fats and the embryo can utilize this food during seed germination.

Cellular totipotency of endosperm cells was first demonstrated by Johri and Bhojwani in 1965. To date, differentiation of shoots/embryos/plantlets from endosperm tissue has been reported for more than 64 species belonging to 24 families. In many of these reports the regenerants were shown to be triploid. A key factor in the induction of cell divisions in mature endosperm cultures is the association of embryo. The embryo factor is required only to trigger cell divisions; further growth occurs independent of the embryo. Triploid plants are usually seed-sterile. However, there are many examples where seedlessness caused by triploidy is of no serious concern or, at times, even advantageous. Some of the crops where triploids are already in commercial use include several varieties of apple, banana, mulberry, sugar beet and watermelon. Natural triploids of tomato produced larger and tastier fruits than their diploid counterparts (Kagan-Zur et al. 1990).

Traditionally, triploids are produced by crossing induced superior tetraploids and diploids. This approach is not only tedious and lengthy (especially for tree species) but in many cases it may not be possible due to high sterility of autotetraploids. The first step in the process is to produce tetraploids by colchicine treatment of germinating seeds, seedlings or vegetative buds. In most of these cases the rate of induction of tetraploids had been low (7-22%). Moreover, the treatment is lengthy and laborious. Once tetraploids have been produced, their cross with the diploid parent may not be successful in majority of the cases. In successful crosses the seed-set, seed germination and survival rate of the seedlings is low. Moreover, all sexually produced triploids do not behave uniformly, which may be due to segregation both at tetraploid level and subsequent population of crosses with putative diploid. In contrast, in vitro regeneration of plants from endosperm, a naturally occurring triploid tissue, offers a direct, single step approach to triploid production. The selected triploids, expected to be sexually sterile, can be bulked up by micropropagation.