INSTABILITY OF TRANSGENES IN PLANTS AND ITS IMPLICATIONS FOR PLANT BREEDING

摘要

A major commitment is being made to the exploitation of transgenes for crop improvement. Numerous studies involving many transgenes and plant species indicate that transgenes are often silent, or display variable expression during development or between sexual generations. Transgene expression is affected by the number of transgenes, the genetic background and the environment. Instability in expression of chalcone synthase genes in petunia in the presence of a transgene(s) encoding chalcone synthase under the control of the cauliflower mosaic virus 35S promoter is described. Chalcone synthase is a key enzyme in floral anthocya-nin production. Such transgenic plants vary in floral phenotype. Some have purple flowers, others white flowers and many have flowers with non-random white and purple sectors. Flower type is characteristic of a particular transformant. Somatic variants are observed frequently. Loss of purple pigment production is associated with post-transcriptional loss of mRNA from trans and endogenous chalcone synthase genes (co-suppress ion). Studies on the structure of the chalcone synthase RNAs produced in transgenic plants, amplified using poly-merase chain reaction, have revealed several kinds of aberration. It is therefore postulated that aberrant RNAs provoke chalcone synthase RNA turnover. A speculative scheme to explain how RNA degradation occurs, involving antisense RNA, is presented. The implications of transgene behaviour and instability in plant breeding are discussed.