Richard Nakano

Bio: Richard Nakano is an academic researcher from University of California, Riverside. The author has contributed to research in topics: Callus & Ploidy. The author has an hindex of 3, co-authored 3 publications receiving 308 citations.

Chromosome complement as a determinant of the morphogenic potential of tobacco cells

Abstract: A B S T R A C T Polysomatism in Nicotiana tabacum L. 'Wisconsin 38' was confirmed. Pith samples from the region of the stem 3.5-10.5 cm below the apex contained nearly equal proportions of diploid and tetraploid cells and samples obtained further down, 15.5-22.5 cm, showed predominantly tetraploid (circa 70%) and smaller proportions of diploid (9%), octaploid (16%), and aneuploid (5%) cells. Cultures of the callus from pith explants showed no evidence of diploid cells after 1 year, but did show roughly half 4n and 8n euploid and half -aneuploid cells. The callus after 6 years in vitro consisted entirely of aneuploid cells. The attainment of this predominance of aneuploid cells could account for the decline of callus growth and organ formation of tobacco tissue cultures. Tobacco tissue cultures started from single cells disclosed that totipotentiality was not restricted to diploid cells but was possessed by and expressed with apparently equal ease by tetraploid cells. The morphogenetically depressed situation was associated with a highly variable aneuploidy. With increase in somatic age the frequency of aneuploid cells increased and the level of ploidy among the aneuploid cells shifted from sub-tetraploidy to above tetraploidy.

Morphogenetic behavior of tobacco tissue cultures and implication of plant senescence

Abstract: Decreases in the growth and organ-forming capacities characterized continuously cultured tobacco (Nicotiana tabacum 'Wisconsin 38') callus The root-initiation ability was completely lost in l 2-year-old cultures The rate of shoot formation decreased to a low plateau in cultures that reached 1'2-3 years of age since the explanting An inverse relationship between callus growth and in vitro clonal age was also observed Studies with callus clones started from individually isolated pith cells showed that the growth and organ-forming potentials of somatic cells varied, signifying that cell alterations had occurred in vivo Both totipotent and non-totipotent cell lines were obtained Subculturing the single-cell lines through several passages disclosed that the morphogenetically depressed state was irreversible and instability was characteristic of the totipotent lines In the latter, a change toward the morphogenetically repressed level was observed These findings are discussed in relation to the phenomenon of senescence It is suggested that an accumulation of somatic mutations, ie, genetic alterations resulting in reductions in the morphogenetic potential of cells, is a basis underlying senescence

Somaclonal variation — a novel source of variability from cell cultures for plant improvement

Abstract: It is concluded from a review of the literature that plant cell culture itself generates genetic variability (somaclonal variation). Extensive examples are discussed of such variation in culture subclones and in regenerated plants (somaclones). A number of possible mechanisms for the origin of this phenomenon are considered. It is argued that this variation already is proving to be of significance for plant improvement. In particular the phenomenon may be employed to enhance the exchange required in sexual hybrids for the introgression of desirable alien genes into a crop species. It may also be used to generate variants of a commercial cultivar in high frequency without hybridizing to other genotypes.

The polyploidy and its key role in plant breeding.

Abstract: This article provides an up-to-date review concerning from basic issues of polyploidy to aspects regarding the relevance and role of both natural and artificial polyploids in plant breeding programs. Polyploidy is a major force in the evolution of both wild and cultivated plants. Polyploid organisms often exhibit increased vigor and, in some cases, outperform their diploid relatives in several aspects. This remarkable superiority of polyploids has been the target of many plant breeders in the last century, who have induced polyploidy and/or used natural polyploids in many ways to obtain increasingly improved plant cultivars. Some of the most important consequences of polyploidy for plant breeding are the increment in plant organs ("gigas" effect), buffering of deleterious mutations, increased heterozygosity, and heterosis (hybrid vigor). Regarding such features as tools, cultivars have been generated with higher yield levels, improving the product quality and increasing the tolerance to both biotic and abiotic stresses. In some cases, when the crossing between two species is not possible because of differences in ploidy level, polyploids can be used as a bridge for gene transferring between them. In addition, polyploidy often results in reduced fertility due to meiotic errors, allowing the production of seedless varieties. On the other hand, the genome doubling in a newly formed sterile hybrid allows the restoration of its fertility. Based on these aspects, the present review initially concerns the origin, frequency and classification of the polyploids, progressing to show the revolution promoted by the discovery of natural polyploids and polyploidization induction in the breeding program status of distinct crops.

Somaclonal and gametoclonal variation

Abstract: 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.

Mitotic chromosome doubling of plant tissues in vitro

Abstract: In vitro chromosome doubling can be induced by several antimitotic agents. The most commonly used are colchicine, oryzalin and trifluralin. The process of induced chromosome doubling in vitro consists of a typical succession of sub-processes, including an induction phase and a confirmation protocol to measure the rate of success. The induction step depends on a large number of variables: media, antimitotic agents, explant types, exposure times and concentrations. Flow cytometry is the pre-eminent method for evaluation of the induced polyploidization. However, alternative confirmation methods, such as chromosome counts and morphological observations, are also used. Since polyploidization has many consequences for plant growth and development, chromosome doubling has been intensively studied over the years and has found its way to several applications in plant breeding. This review gives an overview of the common methods of chromosome doubling in vitro, the history of the technique, and progress made over the years. The applications of chromosome doubling in a broader context are also discussed.

Cytogenetics of plant cell and tissue cultures and their regenerates

Abstract: After a short introduction, the cytogenetics of plant cell and tissue cultures and their regenerates will be discussed. In the first section discussion will focus on cytogenetic conditions “in vivo”, i.e., in the original explant: (I) widespread ocurrence of polysomaty as a consequence of endoreduplication; (2) aneusomaty, an important, though rare, cause of chromosome number variation in vivo; (3) occurrence of chromosome structural changes in differentiated tissues, especially in association with aging; (4) mixoploidy and/or gene mutations, either nuclear or organellar, present as mosaics or periclinal chimeras, especially in vegetatively propagated plants. In section two the discussion will follow with nuclear processes at and during callus induction: (1) mitosis induction in diploid (haploid) and endoreduplicated cells and initiation of cell lines with different ploidy levels; (2) chromosome endoreduplication prior to mitosis induction as a mechanism of polyploidization; (3) nuclear fragmentation (ami...