Overcoming incompatibility and promoting genetic recombination in flowering plants

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Abstract
A critical review of observations regarding physiology of pollen-style incompatibility in plants led to the conclusion that the widely occurring evolutionary relationships between genetic systems of self-incompatibility, floral morphology, site of inhibition, and pollen cytology are best understood in the light of two-phase S gene physiology — S specificity determination (transcription), and S protein synthesis or precursor conversion (translation). Precocity may occur either in S gene action or in S precursor conversion and may thereby alter the relationships between various features of incompatibility. It is concluded that in sporophytic systems interspecific incompatibility substances, governed by the Primary Specificity, may be generally derived from the tapetum alone, whereas intraspecific self-incompatibility substances, governed by the Secondary Specificity, may be produced by both the meiocytes and the tapetum. It is suggested that the ability of mentor pollen to overcome incompatibility may be a result of transfer from compatible to incompatible pollen of a “regulator” for the production of a pollen growth promoting substance (PGS). The regulator “switches-on” the recipient's own S genetic element(s) controlling the production of PGS, thus making the normally incompatible pollen behave as compatible. There is evidence suggesting that pollen growth capacity is species specific and is largely inherent in the S gene complex. Genetic transformation by means of irradiated pollen and through grafting are briefly discussed. It is argued that wide application of somatic hybridisation by protoplast fusion will be seriously limited by genomic and cytoplasmic incompatibilities, and that one of the major contributions of this technique will lie in cases of extreme chromosome elimination from the hybrid giving an end product similar to that of transformation techniques. It is concluded that with a better understanding of the phenomenon of transformation in higher organisms, be it through irradiated pollen, grafting, or protoplast fusion, the scope of induced genetic recombination may be greatly enhanced.