Large scale selection of aluminum-resistant mutants from plant cell culture: expression and inheritance in seedlings

Isolation of aluminum-tolerant cell lines of tobacco in a simple calcium medium and their responses to aluminum

Aluminum (Al)-tolerant cell lines (ALT301 and ALT401) of tobacco were isolated in a simple calcium (Ca) solution from ethyl methane sulfonate (EMS)-treated suspension cultured tobacco cells (Nicotiana tabacum L. cv. Samsun, a cell line SL) at the logarithmic phase of growth. A highly tolerant cell line ALT301 exhibited the accumulation of Al and the deposition of callose to the same extent as the parental SL cells during the exposure to Al. However, the Al-treated ALT301 cells grew much better than the Al-treated SL cells after transfer to Al-free growth medium. Compared to SL cells, ALT301 cells were more tolerant to toxicity of copper and iron, but not to that of lanthanum. These results suggest that ALT301 cells have an internal tolerance mechanism, which makes cells grow normally in spite of Al accumulation and Al-induced lesion represented by the deposition of callose. This tolerance mechanism seems also to be effective against copper and iron toxicity. A slightly tolerant cell line ALT401 also accumulated Al to the same degree as SL cells, but deposited significantly less callose than did SL cells (43% of SL). The growth of ALT401 cells after Al treatment was only slightly better than that of SL cells. Thus, it seems that ALT401 cells have a mechanism to protect cells only from the Al-induced deposition of callose, which is not enough to overcome the Al-induced inhibition of growth. The different phenotypes exhibited by these Al-tolerant cell lines suggest that the deposition of callose is not directly related to the inhibition of growth in Al-treated cells.

The genetics of metal tolerance in vascular plants

In many parts of the world soils are detrimental to plant growth owing to elevated levels of metal ions, caused either by natural processes or by the result of man's activities. Many plants have evolved ecotypes or varieties that are able to grow more-or-less normally on these soils. This paper reviews our knowledge of the genetics of this phenomenon. The nature of tolerance and the problems of its measurement are discussed. Tolerance is frequently measured by an index produced by comparing growth in a contaminated environment with growth in a control environment. It is argued that this measurement is inappropriate for many genetical studies, and that it is frequently more useful to use growth at a single critical level of metal as a measure of tolerance. Polygenic inheritance provides a null hypothesis that has to be tested in a genetical analysis. Examples of major genes for tolerance to aluminium, arsenic, boron, cadmium, copper and manganese are discussed. Even where major genes have been demonstrated, it is probable that other minor genes, 'modifiers', are present as well. Because of the nature of tolerance as a character, dominance and epistasis are likely to vary with the level of metal at which an analysis is performed. Tolerance is generally found to be dominant at some levels of the metal. Studies which have mapped tolerance genes, particularly to aluminium and salt, are discussed. The specificity of tolerance is a matter of some confusion. Some studies indicate that tolerances evolve independently to different metals, but others have suggested that tolerance to one metal may often confer a degree of tolerance to some other metals. Very little is known about the molecular genetics of tolerance, and the mechanisms of tolerance to most metals. The possible role of phytochelatins and metallothionein-like proteins in metal tolerance is discussed. The distribution of tolerance in natural populations suggests that tolerance is a disadvantage in uncontaminated environments, but how this 'cost' arises is not known. There is some evidence that the disadvantage to tolerance may be associated more with the modifiers of tolerance than with the primary tolerance gene. The study of the genetics of tolerance is of importance in planning breeding programmes to produce tolerant crops for use in areas where metal contamination is a limiting factor in productivity. It can also assist in understanding the mechanisms of tolerance, as exemplified by the study of the mechanism of arsenic tolerance in Holcus lanatus. Important areas for further research are discussed. Contents Summary 541 I. Introduction 542 II. Introduction 542 III. Transmission genetics of tolerance 544 IV. Specificity of tolerance 550 V. Molecular genetics of tolerance 552 VI. Ecological genetics of tolerance 553 VII. Conclusions 555 Acknowledgements 556 References 556.

Comparison of the response to aluminum toxicity in gametophyte and sporophyte of four tomato (Lycopersicon esculentum Mill.) cultivars

We tested pollen from four tomato cultivars differing in sensitivity to aluminum in the sporophyte to determine if Al sensitivity was also expressed in pollen. Pollen sensitivity to Al was measured by the ability to germinate and grow in a control solution after a short period in a high concentration of Al. The response was ranked and compared to the Al sensitivity ranking of the four cultivars based on top growth in Al toxic soil. In addition, seedlings from the most and least sensitive cultivars, based on pollen germination, were compared for Al sensitivity in nutrient solutions. Treatment with Al significantly reduced pollen germination in the two more sensitive cultivars, but not in the two more resistant cultivars. However, the ranking was not the same as that based on the shoot growth of the sporophyte. Root growth as a criterion of sporophytic Al sensitivity produced results similar to pollen germination. The study suggests that although the correspondence is better for some phenotypic responses of the sporophyte than others, Al tolerance appears to be another character expressed in both pollen and sporophyte.

Partitioning of variation derived from tissue culture of winter wheat

The presence of somaclonal variation is well documented in wheat, but information is needed which identifies point(s) during the tissue culture process at which variation is most likely induced. Field experiments were designed to partition the total somaclonal variation among three potential sources: single embryo-derived calli, regenerant (R0) plants of a common embryo-derived callus, and spike-derived lines of a common R0 plant. Three populations of winter wheat ('TAM 105', 'Vona', and 'KS75210') totaling 72 lines were evaluated in replicated drilled plots in the R2 and R3 generations. The principal source of variation was influenced by parent genotype. Considering all traits, somaclonal variation in the 'TAM 105' and 'Vona' populations was predominantly attributed to tillers from the same regenerant plant. This source, as well as the original R0 plant source, contributed to variation in the 'KS75210' population, depending on the trait measured. Embryos did not consistently provide a significant source of variation. The presence of somaclonal variation was not always associated with a downward shift in population mean compared to parental controls. Significant population increases were noted for spike density and biomass, and some lines showed significantly higher grain protein content without a yield reduction, but these responses were again genotype-specific.