In other difficult cases, the barriers to sexual crossing can sometimes be overcome by preparing protoplasts from vegetative (somatic) tissues from two sources. This involves treatment with cell-wall degrading enzymes, after which the protoplasts are encouraged to fuse by incubation in an optimal concentration of polyethylene glycol. A successful fusion of protoplasts from the two donors produces a new protoplast that is a somatic hybrid. Using tissue cultures, such cells can, in some cases, be induced to develop into new plants.
Somatic fusion is of particular interest for characters related to the chloroplast or mitochondrion. These plastids contain some genetic information in their specific, non-nuclear DNA, which is responsible for the synthesis of a number of essential proteins. In about two-thirds of the higher plants, plastids with their DNA are inherited in a "maternal" fashion—the cytoplasm of the male gamete is discarded after fusion of the egg and sperm cells. In contrast, in the minority of plants with biparental inheritance of plastid DNA, or when fusion of somatic protoplasts occurs, there is a mixing of the plastids from both parents. In this way, there is a potential for new plastid-nucleus combinations.
For chloroplasts, one application of plastid fusion is in the breeding of resistance to the effects of triazine herbicides. For mitochondria, an application relevant to plant breeding is in the imposition of male sterility. This is a convenient character when certain plants are to be employed as female parents for a hybrid cross. The transfer of male-sterile cytoplasm in a single step can avoid the need for several years of backcrosses to attain the same condition. Somatic hybridization has been used successfully to transfer male sterility in rice, carrot, rapeseed (canola), sugar beet, and citrus. However, this character can be a disadvantage in maize, where male sterility simultaneously confers sensitivity to the blight fungus, Helminthosporium maydis. This sensitivity can lead to serious losses of maize crops.
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