Somaclonal Variation: Theoretical and Practical Considerations

Development, Management and Utilization of a Kiwifruit (Actinidia spp.) In Vitro Collection: A New Zealand Perspective

The New Zealand Institute for Plant and Food Research Limited (PFR) supports a large kiwifruit breeding program that includes more than twenty Actinidia species. Almost all the kiwifruit accessions are held as field collections across a range of locations, though not all plants are at multiple locations. An in vitro collection of kiwifruit in New Zealand was established upon the arrival of Pseudomonas syringae pv. Actinadiae-biovar 3 in 2010. The value of an in vitro collection has been emphasized by restrictions on importation of new plants into New Zealand and increasing awareness of the array of biotic and abiotic threats to field collections. The PFR in vitro collection currently holds about 450 genotypes from various species, mostly A. chinensis var. chinensis and A. chinensis var. deliciosa. These collections and the in vitro facilities are used for germplasm conservation, identification of disease-free plants, reference collections and making plants available to users. Management of such a diverse collection requires appropriate protocols, excellent documentation, training, sample tracking and databasing and true-to-type testing, as well as specialized facilities and resources. This review also discusses the New Zealand biosecurity and compliance regime governing kiwifruit plant movement, and how protocols employed by the facility aid the movement of pathogen-free plants within and from New Zealand.

The expression and perpetuation of inherent somatic variation in regenerants from embryogenic cultures of Pennisetum glaucum (L.) R. Br. (pearl millet)

Genetic analysis was conducted on the qualitative and quantitative traits of sexual progeny derived from embryogenic cultures of two inbred lines of Pennisetum glaucum (L.) R. Br. (pearl millet). These lines included a genetically stable inbred of Tift 23 BE and a genetic marker line, derived from Tift 23BE, which bore qualitative genetic markers for a dominant purple plant trait (P) and two recessive traits, early flowering (e1) and yellow stripe (ys). Tissue culture regenerant populations (R0) and progeny populations (R1) produced from these plants by selfing showed no qualitative genetic variation when derived from the genetically stable inbred Tift 23BE. In contrast, stably inherited qualitative variation for a number of genetic markers was observed in R0, R1, and R2 progeny of the genetic marker line. In a population of 1,911 plants regenerated over a 12-month period, 0.02% of the population lost or showed reduced expression of the purple plant trait and 92% of plants were chlorophyll deficient. Plants showing reduction or loss of anthocyanin synthesis also flowered later. None of the purple plants showed any significant variation in flowering time. The incidence of chlorophyll deficiency increased with time in culture, 51 % of the progeny regenerated after 1 month were chlorophyll deficient, while 100% of the plants regnerated after 12 months were chlorophyll deficient. Qualitative variation was also observed in control populations of the genetic marker line where 1 plant in a total of 1,010 lacked purple pigmentation and a total of 6% showed chlorophyll variation in the first generation (S0). The presence of qualitative variation in controls suggests that the inherent variation present in the original explant was expressed and perpetuated in vitro. Quantitative variation was observed for a number of traits in the first sexual cycle (R1) of the marker line but did not occur in a subsequent generation, suggesting that this variation was epigenetic.

Somaclonal variation in plants regenerated from cultures of soybean

Plants were regenerated from embryogenic and organogenic cultures derived from immature embryos of nine soybean (Glycine max L. Merr.) genotypes and extensive qualitative variation was noted in different regenerated families. Three lethal sectoral albinos were seen in the regenerated plants (R0). Variants observed in later selfed generations included twin seeds, multiple shoots, dwarfs, abnormal leaf morphology, abnormal leaflet number, wrinkled leaves, chlorophyll deficiency, partial sterility and complete sterility. The frequency of possible mutations ranged from 0 to 4% in R plants as determined by studies of corresponding R1, R2, R3 and R4 families. No significant differences were seen in the frequencies of possible mutations for embryogenic as compared to organogenic culture derived plants. Chlorophyll deficiency, sterility and wrinkled leaf traits were followed in two or more generations and showed that these traits were inherited stably. The known traits of this nature are controlled by single recessive nuclear genes. Other traits occurred more randomly and not in all generations. The genetic basis of the random variation is not known at the present time. This study indicates that heritable somaclonal variation does occur in tissue culture derived plants of soybean.

Marker proteins for embryogenic differentiation patterns in pea callus

Polypeptide pattern alterations during somatic embryogenesis were investigated using callus cultures of two Pisum sativum genotypes. Both genotypes show the formation of two different callus lines from the same explant after six to eight weeks in culture: a nodular yellowish callus line, which forms somatic embryoids in suspension cultures (e(+)) and a white compact callus line with no regenerative capacity (e(-)). The cytosol proteins of the two different callus lines were separated in a semi-preparative two-dimensional system and the polypeptide patterns were compared. Two protein bands were found (P1: Mr=45000 D, pI=7.0-7.1; P2: Mr=7000 D, pI=<4.5), which were characteristic of the putatively embryogenic (e(+)) callus line in all tissues investigated (two genotypes × two explant sources). These proteins found in nodular (e(+)) pea cultures are very similar to two proteins found in Daucus carota suspension cultures preceding the formation of somatic embryos.

Changes in methylation of tissue cultured soybean cells detected by digestion with the restriction enzymes HpaII and MspI

Each of the tandemly arranged 5S RNA genes of soybean contain two CCGG sites which, if unmethylated, can be digested by both MspI and HpaII. Methylation of the internal cytosine (CmeCGG) prevents digestion by HpaII but allows digestions by MspI.Suspension cultures were prepared from soybean plants and the DNA from these cultures was examined for the susceptibility of 5S RNA genes to digestion by MspI and HpaII. 5S genes from DNA extracted from intact plants can be partially digested with MspI but not at all by HpaII. In contrast, shortly after cells were cultured the 5S RNA could be hydrolyzed by both HpaII and MspI. After prolonged cell culture, the 5S genes from some cell lines were found to have become partially or even completely resistant to HpaII digestion. The results suggest that lack of methylation can occur when cells are cultured and that such methylation may play a role in the heritable changes observed in cell culture.

Characterization of biomass production, cytology and phenotypes of plants regenerated from embryogenic callus cultures of Pennisetum americanum x P. purpureum (hybrid triploid napiergrass)

Five hundred and twenty-four plants of a triploid, sexually sterile hybrid napiergrass (Pennisetum americanum x P. purpureum; 3x=21) were regenerated from embryogenic callus cultures obtained from segments of young inflorescences. Replicated field trials were conducted for two consecutive years to compare the biomass yield, phenotype and cytology of tissue culture regenerants (TC) and vegetatively propagated (V) plants. In the first year total biomass yield of TC plants was significantly greater than V plants but there was no significant difference in the second year. TC plants had more tillers compared to V plants. V plants did not show any morphological variability. The TC population also exhibited a high degree of phenotypic stability (96%). There were 23 phenotypic variants in the TC population of 524, most of them being more dwarf and late-flowering. Detailed morphological analysis of the TC-variant plants suggests that they very likely arose from only a few variant cell lines. Cytological analysis indicated stability of the triploid status in randomly selected regenerants. Two of the morphological variants were hexaploids (6x=42). It is concluded that embryogenic callus cultures can provide useful alternative for the rapid propagation of hybrid napier-grass which is commonly propagated by cuttings.

Somaclonal variation in wheat: genetic and cytogenetic characterisation of alcohol dehydrogenase 1 mutants

The progeny of 551 regenerants of the hexaploid wheat cultivar 'Millewa' were analysed for somaclonal mutants at the threeAdh-1 loci in hexaploid wheat. Seventeen regenerants gave rise to progeny having altered ADH1 zymograms. Progeny with altered zymograms in 13 of these regenerants were aneuploid. The remaining 4 regenerants gave rise to euploid progeny with altered ADH1 zymograms. The genetics of three of these somaclonal mutants is described in detail. These regenerants were interpreted to possess a 4Aα isochromosome, a 3BS/4Aα translocation and a 7BS/4Aα translocation, respectively.

An unstable anthocyanin mutation recovered from tissue culture of alfalfa (Medicago sativa) : 1. High frequency of reversion upon reculture

A white-flowered mutant ("WFM") was regenerated from tissue culture of a purple-flowered plant of tetraploid alfalfa (Medicago sativa L.). When WFM was recultured, many regenerated plants (>20%) were purple-flowered. Genetic analysis established that a functional allele, C2, of a locus required for anthocyanin pigmentation was in the simplex condition (C2c2c2c2) in the donor genotype when it mutated to an unstable recessive ("mutable") allele, c2-m4, which is carried by WFM. Tissue culture experiments demonstrated that c2-m4 reverts to function at a high frequency in vitro. Results indicate that reversion occurs early in culture and may be the result of a genome shock associated with callus formation. Reversion also occurs in planta, but at a much lower frequency than in vitro. The c2-m4 allele is transmitted to progeny which revert in tissue culture. Revertant alleles, like the progenitor allele, are stable and are sexually transmitted. The action of a transposable element which is especially active in vitro is suggested.