The origin of plant cell and tissue culture can be found in a treatise published during the mid-18th century, entitled La Physique des Arbes, that describes the formation of callus tissue following the for mation of a ring of cortex from elm trees. Over the next two centuries, the discovery of plant growth hormones, in particular auxins and cytokinins, and detailed analyses on the nutritional requirements of plants, led to the formulation of media that could maintain actively dividing cultures derived from gymnosperms, and both dicotyledon ous and monocotyledonous angiosperms. However, much of the prog ress and technological development in the in vitro propagation of plant cells, tissues, and organs has occurred during the last 25 years. Recently, plant tissue culture techniques have been used as basic tools in the rapidly expanding field of plant biotechnology for the development and clonal propagation of new and/or improved plant varieties. Plant tissue culture is used for the micropropagation of commercially valuable cultivars that include ornamentals, oil palm, Glycyrrhiza, Pyrethrum, pine, Eucalyptus, sugar cane, and potatoes. Cultured plant tissue is also used for the selection of cells and, ul timately, the regeneration of plants that are tolerant to physical stresses such as pathogens, drought, and temperature extremes, and to chemical stress agents such as salinity, herbicides, proteins, and pyrethrins. In addition, new plants have been produced by the fusion of protoplasts prepared from cultured cells of different species in cluding sunflower and french bean, tomato and potato, and various cultivars of Datura. Finally, bacterial vectors and various mechanical methods have been used to introduce foreign genes into cultured plant tissues. Genetic transformation can result in profound changes in the phenotype and/or biochemical profile of the regenerated trans genic plants that are not characteristic of the wild type. An impressive variety of technologies in tissue culture, genetic manipulation, and molecular biology have been developed for nu merous plant species. Many of these techniques, sometimes referred to as plant biotechnology, have been extensively summarized and compiled in a well-balanced collection of easy-to-follow protocols presented in three separate, but complimentary, volumes. Plant Cell, Tissue and Organ Culture consists of 22 chapters (with 86 figures) and 5 appendices. The chapters cover critical aspects of (a) the es sential requirements for the operation of a plant tissue culture lab oratory; (b) the basic procedures of micropropagation, regeneration, and somatic embryogenesis; (c) some specific applications of organ culture systems such as embryo rescue and culture, and anther and microspore culture for haploid and double haploid production; (d) elementary transformation technology; and (e) useful microtechnique and analytical protocols specifically adapted to cultured tissues and cells. The appendices provide a convenient summary of media for mulations and commercial suppliers for the materials described in the text.

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Plant Cell, Tissue and Organ Culture

This review describes in vitro mutation induction methods in fruits and the in vitro selection procedures available for early screening. Results obtained through in vitro mutation techniques, including somaclonal variation, are reviewed and compared with the current achievements and future prospects of transgenic breeding. Plant improvement based on mutations, which change one or a few specific traits of a cultivar, can contribute to fruit improvement without altering the requirements of fruit industry. Induced mutations have well defined limitations in fruit breeding applications, but their possibilities may be expanded by the use of in vitro techniques. Tissue culture increases the efficiency of mutagenic treatments for variation induction, handling of large populations, use of ready selection methods, and rapid cloning of selected variants. Molecular techniques can provide a better understanding of the potential and limitations of mutation breeding e.g. molecular marker-assisted selection, which can lead to the early identification of useful variants. The relatively high number of research reports compared with the low number of cultivars released suggests that mutagenesis in combination with tissue culture is either ineffective or has yet to be exploited in fruits. Positive achievement recorded in other species seem to support the hypothesis that in vitro mutation induction has high potential also for fruit improvement. The possible contribution of a well-pondered and coordinated use of the numerous mutation induction, mutant selection, and field validation procedures available to advances in fruit breeding is discussed.

Mutation induction and tissue culture in improving fruits

It has been suggested that genetic modification could lead to crops with enhanced invasiveness and persistence. These new genotypes could invade natural ecosystems and cause undesirable change, either through spread of the crops themselves or through hybridization with wild relatives. We review the progress made in the genetic modification of the major UK crops, and identify those crops and traits in which genetic modification is most advanced. Data on the potential for the spread of transgenes through pollen movement and the relative performance of modified and unmodified crops are examined. It is concluded that the spread of modified crops and their hybrids with wild relatives can be modelled in the same manner as for unmodified crops (...)

Genetically modified crops and hybridization with wild relatives: a UK perspective

Plum hypocotyl slices were transformed with the coat protein (CP) gene of plum pox virus (PPV-CP) following cocultivation with Agrobacterium tumefaciens containing the plasmid pGA482GG/PPVCP-33. This binary vector carries the PPV-CP gene construct, as well as the chimeric neomycin phosphotransferase and β-glucuronidase genes. Integration and expression of the transferred genes into regenerated plum plants was verified through kan resistance, GUS assays, and PCR amplification of the PPV-CP gene. Twenty-two transgenic clones were identified from approximately 1800 hypocotyl slices. DNA, mRNA, and protein analyses of five transgenic plants confirmed the integration of the engineered CP gene, the accumulation of CP mRNA and of PPV-CP-immunoreactive protein. CP mRNA levels ranged from high to undetectable levels, apparently correlated with gene structure, as indicated by DNA blot analysis. Western analysis showed that transgenic plants produced amounts of CP which generally correlated with amounts of detected mRNA.

Transgenic plums (Prunus domestica L.) express the plum pox virus coat protein gene.

SummaryTaxonomic relationships
Plum pox virus (PPV) is a member of the genus Potyvirus in the family Potyviridae. PPV diversity is structured into at least eight monophyletic strains.

Geographical distribution
First discovered in Bulgaria, PPV is nowadays present in most of continental Europe (with an endemic status in many central and southern European countries) and has progressively spread to many countries on other continents.

Genomic structure
Typical of potyviruses, the PPV genome is a positive-sense single-stranded RNA (ssRNA), with a protein linked to its 5′ end and a 3′-terminal poly A tail. It is encapsidated by a single type of capsid protein (CP) in flexuous rod particles and is translated into a large polyprotein which is proteolytically processed in at least 10 final products: P1, HCPro, P3, 6K1, CI, 6K2, VPg, NIapro, NIb and CP. In addition, P3N-PIPO is predicted to be produced by a translational frameshift.

Pathogenicity features
PPV causes sharka, the most damaging viral disease of stone fruit trees. It also infects wild and ornamental Prunus trees and has a large experimental host range in herbaceous species. PPV spreads over long distances by uncontrolled movement of plant material, and many species of aphid transmit the virus locally in a nonpersistent manner.

Sources of resistance
A few natural sources of resistance to PPV have been found so far in Prunus species, which are being used in classical breeding programmes. Different genetic engineering approaches are being used to generate resistance to PPV, and a transgenic plum, ‘HoneySweet’, transformed with the viral CP gene, has demonstrated high resistance to PPV in field tests in several countries and has obtained regulatory approval in the USA.

Plum pox virus and sharka: a model potyvirus and a major disease

A system was developed which allows the transfer of foreign genes into apricot cultivars. We report the transformation and regeneration of Prunus armeniaca plants with Agrobacterium tumefaciens strain LBA 4404 containing various binary plasmids, pBinGUSint, carrying the marker gene s-glucuronidase (GUS) and pBinPPVm, carrying the coat protein gene of Plum Pox Virus (PPV). The marker gene GUS was used for optical evaluation of the efficiency of the transformation system. The coat protein gene of PPV was used to introduce coat protein mediated resistance against one of the most important pathogens of stone fruit trees in Europe and the whole Mediterranean area. This is the first report of the successful integration of a viral coat protein gene into a fruit tree species, opening a new perspective on the control of the disease.

Birgit Kalthoff

Papers

Regeneration of transgenic plants of Prunus armeniaca containing the coat protein gene of Plum Pox Virus.