Light has been found to increase the proportion of tracheary elements differentiating in callus cultures derived from xylem-parenchyma of Pinus radiata D. Don grown on an induction medium containing activated charcoal but no phytohormones. The differentiation rate increased from 20% when callus was grown in darkness to 45% when callus was grown with a 16 h or 24 h photoperiod. When callus was grown with a 16 h photoperiod, tracheary elements were observed 2 days after transfer of callus to the induction medium, as compared to 5 days when callus was cultured in darkness. The differentiation rate was also influenced by the concentration of activated charcoal added to the induction medium, the optimum concentration being 5 g l )1 . Exclusion of activated charcoal from the induction medium decreased the differentiation rate to 2%. The activities of the lignin-related enzymes L-phenylalanine ammonia lyase and cinnamyl alcohol dehydrogenase were significantly higher in cell cultures grown with a 16 h photoperiod as compared to when grown in darkness. The results show that light had a stimulating effect on tracheary element differentiation and the activities of lignin-related enzymes in P. radiata callus cultures. The new growth conditions markedly improve this cell culture system and make it particularly useful for functional gene testing and cell-wall analysis of in vitro grown tracheary elements of coniferous gymnosperms.
In this review we examine the history and progression of conifer genetic engineering. The review includes the methods used, the conifer species transformed, the genes inserted and the regeneration of genetically engineered conifer trees. We cover both Biolistic ® and Agrobacterium-mediated transformation, and we detail transformation events with and without plant regeneration. We show that almost all conifer transformation work uses nptII as a selective marker, and very often uidA is included as a reporter gene. Further, we show that a range of genes that are of commercial interest for forest tree plantations have been introduced, such as herbicide resistance, insect resistance and those related to wood properties. We briefly discuss the future for biotechnology in the context of socially acceptable enhanced plantation forestry and under consideration of benefits and risks.
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