Plant tissue culture has become one of the fundamental tools of plant science research. It is extensively employed in the production, conservation and improvement of plant resources. The presence of somaclonal variation in populations derived from tissue culture is affecting the use of tissue culture negatively and has remained a major problem. Conversely, it is a source of new desirable clones/variants with better agronomic traits. In this review, we summarize the possible causes, detection methods and desirability of variants. Somaclonal variation is one of the most researched and reviewed topics. Hence, we restricted ourselves to outlining various examples which may be used as important references for researchers who intend to identify and/or characterize somaclonal variants while using tissue culture for research and production. Emphasis is placed on the negative effects of somaclonal variation. However, this review also includes examples of some useful variants generated as a result of somaclonal variation.

Somaclonal variation in plants: causes and detection methods

For several years it has been recognized that introduction of plant cells into culture results in genetic changes. These genetic alterations have been recovered in the plants regenerated from cell cultures. More recently it has been recognized that this method of introducing genetic changes into crop plants could be used to develop new breeding lines. The technology of introducing genetic variation by using cell culture has been termed somaclonal and gametoclonal variation. Somaclonal variation has been detected and documented in tomato. In particular, several single gene mutations induced by somaclonal variation have been mapped to specific loci. As this variation represents a new tool for the plant breeder, breeding strategies for the use of this variation are presented and discussed. Somaclonal and gametoclonal variation are new tools for the geneticist and plant breeder that permit reduction in the time period for new variety development and that permit access to new classes of genetic variation.

Somaclonal and gametoclonal variation

The generation and analysis of plant chimeras and other genetic mosaics have been used to deduce patterns of cell division and cell fate during plant development and to demonstrate the existence of clonally distinct cell lineages in the shoot meristems of higher plants. Cells derived from these lineages do not have fixed developmental fates but rely on positional information to determine their patterns of division and differentiation. Chimeras with cells that differ genetically for specific developmental processes have been experimentally generated by a variety of methods. This review focuses on studies of intercellular interactions during plant development as well as of the coordination of cells during meristem function and organogenesis. Recent experiments combining mosaic analysis with molecular analysis of developmental mutants have begun to shed light on the nature of the signals involved in these processes and the mechanisms by which they are transmitted and received among cells.

What chimeras can tell us about plant development.

There has been a remarkable escalation of interest in tissue-culture derived plant variation (somaclonal variation) in the last few years. Earlier authors were aware to some extent that abnormalities could result from a tissue culture cycle (42,65,67). However it is only more recently that the thought has been seriously entertained that some of this variation may be useful for varietal improvement (95,100). It is our contention that the lateness of this realization was a consequence of the fact that so few tissue culturists were engaged in careful analysis of the regenerated plants and also failed to see cell culture manipulation in a genetic context. We were committed to the idea of variation but envisaged it would only happen after specific manipulations (somatic hybridization or DNA-mediated transformation). Underlying the development of these important means of modifying the plant genome was the presupposition that tissue culture was cloning.

Somaclonal Variation and Crop Improvement

A wide range of phenotypic variation occurred among protoplast — derived plants of tetraploid potato cultivar ‘Bintje’. The variant plants had alterations in growth and vigour, and in leaf and stem characteristics. The results suggest that the altered morphologies are caused predominantly by changes in ploidy levels. Some alterations could be attributed typically to octoploidy and aneuploidy. The occurrence of mixoploidy indicates that at least part of the observed variation arose during culture stage. The exogeneous cytokinin or auxin level and their combination during in vitro phase influenced the frequency of the variants observed. The origin of variation is discussed.

Phenotypic variation and ploidy level of plants regenerated from protoplasts of tetraploid potato (Solanum tuberosum L. cv. 'Bintje').

It was investigated what potentialities for mutation breeding of potato are offered by using adventitious sprouts that arise in vitro from leaf explants (rachis, petiole, leaflet-disc) after X-irradiation. Mutation frequency and chimerism were studied in subterranean and aerial parts in three vegetative generations (vM1, vM2, vM3). Plants obtained from irradiated series produced a very high mutation frequency, a wide mutation spectrum and a very low rate of chimerism. Mutations were observed also in control series, especially in plants derived from the rachis and petiole explant group.

In vitro adventitious bud techniques for vegetative propagation and mutation breeding of potato (Solanum tuberosum L.). II. Significance for mutation breeding

A fast neutron irradiation experiment using two different dose rates has been performed on the potato variety Desiree in order to obtain chimera-free mutants. Both tubers and tuber parts (cross sections) were de-eyed at different moments before and after irradiation. Adventitious sprouts successively developing on the callus in the holes of excised eyes were separated from the treated tubers and planted individually. So called ‘grouped’ sprouts were separated. Results indicated that (practically) chimera-free mutants can be obtained by de-eying tuber parts shortly before irradiation, by applying relatively high dose-rates and by taking adventitious sprouts which develop not earlier than three months after irradiation.

Preventing chimerism in potato (Solanum tuberosum L.)

Dihaploids of the potato variety Desiree were used to obtain preliminary information about the doses and dose rates most suitable for mutation breeding purposes. The sprouts that developed after irradiation were removed in succession and then planted separately in pots. A broad distinction of 3 successive groups of sprout production can be made. The low dose rate object (50 rad/min) demonstrated an increase in overall as well as in uniform mutation frequency in the successive groups, while the high dose rate group (1000 rad/min) showed a converse behaviour with regard to overall mutation frequency.

Dihaploid potatoes in mutation breeding: Some preliminary results

Ivy leaf of potato (Solanum tuberosum), a radiation-induced dominant mutation for leaf shape

While investigating the formation of adventitious sprouts from L1 tissue in potato as part of our work on potato mutation, we tested the ‘Jorgensen’ method.

H. Bouter

Papers

In vitro adventitious bud techniques for vegetative propagation and mutation breeding of potato (Solanum tuberosum L.). II. Significance for mutation breeding

Preventing chimerism in potato (Solanum tuberosum L.)

Dihaploid potatoes in mutation breeding: Some preliminary results

Ivy leaf of potato (Solanum tuberosum), a radiation-induced dominant mutation for leaf shape

The ‘Jørgensen’ method, does it produce adventitious sprouts from potato?