High frequency plant regeneration from immature embryos of an elite barley cultivar (Hordeum vulgare L. cv. Morex)

The improvement of the in vitro plant regeneration in barley with the epigenetic modifier of histone acetylation, trichostatin A

Genotype-limited plant regeneration is one of the main obstacles to the broader use of genetic transformation in barley breeding. Thus, developing new approaches that might improve responses of in vitro recalcitrant genotypes remains at the center of barley biotechnology. Here, we analyzed different barley genotypes, including "Golden Promise," a genotype commonly used in the genetic transformation, and four malting barley cultivars of poor regenerative potential. The expression of hormone-related transcription factor (TF) genes with documented roles in plant regeneration was analyzed in genotypes with various plant-regenerating capacities. The results indicated differential expression of auxin-related TF genes between the barley genotypes in both the explants and the derived cultures. In support of the role of auxin in barley regeneration, distinct differences in the accumulation of free and oxidized auxin were observed in explants and explant-derived callus cultures of barley genotypes. Following the assumption that modifying gene expression might improve plant regeneration in barley, we treated the barley explants with trichostatin A (TSA), which affects histone acetylation. The effects of TSA were genotype-dependent as TSA treatment improved plant regeneration in two barley cultivars. TSA-induced changes in plant regeneration were associated with the increased expression of auxin biosynthesis-involved TFs. The study demonstrated that explant treatment with chromatin modifiers such as TSA might provide a new and effective epigenetic approach to improving plant regeneration in recalcitrant barley genotypes.

Genetic Transformation of Apomictic Grasses: Progress and Constraints

The available methods for plant transformation and expansion beyond its limits remain especially critical for crop improvement. For grass species, this is even more critical, mainly due to drawbacks in in vitro regeneration. Despite the existence of many protocols in grasses to achieve genetic transformation through Agrobacterium or biolistic gene delivery, their efficiencies are genotype-dependent and still very low due to the recalcitrance of these species to in vitro regeneration. Many plant transformation facilities for cereals and other important crops may be found around the world in universities and enterprises, but this is not the case for apomictic species, many of which are C4 grasses. Moreover, apomixis (asexual reproduction by seeds) represents an additional constraint for breeding. However, the transformation of an apomictic clone is an attractive strategy, as the transgene is immediately fixed in a highly adapted genetic background, capable of large-scale clonal propagation. With the exception of some species like Brachiaria brizantha which is planted in approximately 100 M ha in Brazil, apomixis is almost non-present in economically important crops. However, as it is sometimes present in their wild relatives, the main goal is to transfer this trait to crops to fix heterosis. Until now this has been a difficult task, mainly because many aspects of apomixis are unknown. Over the last few years, many candidate genes have been identified and attempts have been made to characterize them functionally in Arabidopsis and rice. However, functional analysis in true apomictic species lags far behind, mainly due to the complexity of its genomes, of the trait itself, and the lack of efficient genetic transformation protocols. In this study, we review the current status of the in vitro culture and genetic transformation methods focusing on apomictic grasses, and the prospects for the application of new tools assayed in other related species, with two aims: to pave the way for discovering the molecular pathways involved in apomixis and to develop new capacities for breeding purposes because many of these grasses are important forage or biofuel resources.

TRA1: A Locus Responsible for Controlling Agrobacterium-Mediated Transformability in Barley

In barley (Hordeum vulgare L.), Agrobacterium-mediated transformation efficiency is highly dependent on genotype with very few cultivars being amenable to transformation. Golden Promise is the cultivar most widely used for barley transformation and developing embryos are the most common donor tissue. We tested whether barley mutants with abnormally large embryos were more or less amenable to transformation and discovered that mutant M1460 had a transformation efficiency similar to that of Golden Promise. The large-embryo phenotype of M1460 is due to mutation at the LYS3 locus. There are three other barley lines with independent mutations at the same LYS3 locus, and one of these, Risø1508 has an identical missense mutation to that in M1460. However, none of the lys3 mutants except M1460 were transformable showing that the locus responsible for transformation efficiency, TRA1, was not LYS3 but another locus unique to M1460. To identify TRA1, we generated a segregating population by crossing M1460 to the cultivar Optic, which is recalcitrant to transformation. After four rounds of backcrossing to Optic, plants were genotyped and their progeny were tested for transformability. Some of the progeny lines were transformable at high efficiencies similar to those seen for the parent M1460 and some were not transformable, like Optic. A region on chromosome 2H inherited from M1460 is present in transformable lines only. We propose that one of the 225 genes in this region is TRA1.

Effects of salt and heat pre-treatment factors on efficient regeneration in barley (Hordeum vulgare L.)

An efficient callus induction and plant regeneration system has been developed using salt and heat as pre-treatment factors for three barley genotypes viz. BB-3, BB-6 and BHL-18. Different concentrations of NaCl (1.5, 2.5, 3.5, 4.5, 5.5 and 6.5 g/L) were used and its effects were determined on the basis of the viability of callus (CV), plant regeneration (PR), relative growth rate (RGR) and tolerance index (TI). The BB-6 showed highest performance on tolerance based on CV (14.72%), PR (7.69%), RGR (0.91%) and TI (0.42%) at 6.5 g/L NaCl. Various NaCl concentrations displayed significantly differences at P < 0.01 level as compared with the control. Plant regeneration capability was recorded after heat pre-treatment using calli at 30, 35 and 40 °C. In this study, BHL-18 produced highest callus induction (59.71%) after desiccated at 40 °C for BB-6. Highest regeneration was recorded around 41.66% when 4 weeks old calli were pre-treated at 35 °C. Furthermore, heat pre-treatment factors were very effective for enhancing plant regeneration (25-41.66%) which was 1.8-2.14 fold higher compared to the control (13.88-19.44%). Hence, heat treated calli displayed higher tolerance level to survive in NaCl-induced treatment for determining abiotic stress and increased regeneration rate at 35 °C temperature in BB-6 barley genotype.

Protocol for efficient regulation of in vitro morphogenesis in einkorn (Triticum monococcum L.), a recalcitrant diploid wheat species

Einkorn (Triticum monococcum L.) is A-genome diploid wheat that has a potential to become a useful model for understanding the biology and genomics in Triticeae. Unfortunately, the application of modern technologies such as genetic engineering, RNAi-based gene silencing and genome editing is not available for einkorn as there is no efficient in vitro tissue culture and plant regeneration system. In the present study an efficient and simple protocol for plant regeneration via direct or indirect somatic embryogenesis and organogenesis has been developed. Various auxins used as sole inductors in einkorn displayed low effect for morphogenesis (0–8%) and plant regeneration (1–2 shoots per explant). The addition of Daminozide, the inhibitor of biosynthesis of gibberellins, together with auxin significantly improved the formation of morphogenic structures, especially when Dicamba (51.4%) and Picloram (56.6%) were used for combination; furthermore, the simultaneous addition of cytokinin into induction medium significantly promoted in vitro performance. Among the tested cytokinins, the urea-type substances, such as TDZ and CPPU were more effective than the adenine type ones, BA and Zeatin, for the regulation of morphogenesis; especially, TDZ was more effective than CPPU for shoot formation (11.73 vs. 7.04 per regenerating callus). The highest morphogenic response of 90.2% with the production of more than 10 shoots per initial explant was observed when 3.0 mg/L Dicamba, 50.0 mg/L Daminozide and 0.25 mg/L TDZ were combined together. Along with the identification of appropriate induction medium, the optimal developmental stage for einkorn was found as partially transparent immature embryo in size of around 1.0 mm. Although in the present study the critical balance between plant growth regulators was established for einkorn only, we assume that further the proposed strategy could be successfully applied to other recalcitrant cereal species and genotypes.

Hv-CBF2A overexpression in barley accelerates COR gene transcript accumulation and acquisition of freezing tolerance during cold acclimation

C-Repeat Binding Factors (CBFs) are DNA-binding transcriptional activators of gene pathways imparting freezing tolerance. Poaceae contain three CBF subfamilies, two of which, HvCBF3/CBFIII and HvCBF4/CBFIV, are unique to this taxon. To gain mechanistic insight into HvCBF4/CBFIV CBFs we overexpressed Hv-CBF2A in spring barley (Hordeum vulgare) cultivar 'Golden Promise'. The Hv-CBF2A overexpressing lines exhibited stunted growth, poor yield, and greater freezing tolerance compared to non-transformed 'Golden Promise'. Differences in freezing tolerance were apparent only upon cold acclimation. During cold acclimation freezing tolerance of the Hv-CBF2A overexpressing lines increased more rapidly than that of 'Golden Promise' and paralleled the freezing tolerance of the winter hardy barley 'Dicktoo'. Transcript levels of candidate CBF target genes, COR14B and DHN5 were increased in the overexpressor lines at warm temperatures, and at cold temperatures they accumulated to much higher levels in the Hv-CBF2A overexpressors than in 'Golden Promise'. Hv-CBF2A overexpression also increased transcript levels of other CBF genes at FROST RESISTANCE-H2-H2 (FR-H2) possessing CRT/DRE sites in their upstream regions, the most notable of which was CBF12. CBF12 transcript levels exhibited a relatively constant incremental increase above levels in 'Golden Promise' both at warm and cold. These data indicate that Hv-CBF2A activates target genes at warm temperatures and that transcript accumulation for some of these targets is greatly enhanced by cold temperatures.

Regeneration capacity of mature embryo-derived callus in barley ( Hordeum vulgare L.)

In this study, induction of regenerable callus from mature embryos in eight Turkish barley varieties was analysed by using different plant growth regulators (PGRs). Varying concentrations (0.5-4 mg l -1 ) of 2,4-dichlorophenoxyacetic acid (2,4-D) and dicamba (3,6-dichloro-o-anisic acid) were tested for callus induction from mature embryos. Highest percent of callus induction was observed in Bornova 92 variety (98.3%) on MS medium supplemented with 4 mg l -1 dicamba. Calli were transferred to regeneration media with 0.5 mg l -1 dicamba, 0.5 mg l -1 zeatin riboside (ZR) and 2 mg l -1 thidiazuron (TDZ). Low concentrations of dicamba induced multiple shoots during callus regeneration. When the effect of precultivation with 2,4-D or dicamba on the shoot induction were evaluated, lower concentrations (< 4 mg l -1 ) of auxins have been found optimal. On the regeneration medium with 0.5 mg l -1 dicamba, shoots were able to elongate up to 20 cm and shoot numbers were between 1-23 per callus. The use of ZR led to formation of short shoot buds and somatic embryos in 2 weeks period. The effect of TDZ was different from other PGRs by inducing green solid sectors on calli surfaces (Total 51 sectors/20 callus/Akhisar variety). Five plantlets have been grown from these solid cell clumps and transferred to specific media for root formation. As a result, five varieties (Süleyman Bey, Bornova 92, Vamyk Hoca, Kaya and Akhisar) tested in our study showed the potential to produce regenerable callus by using low amounts of dicamba or TDZ. The optimization process starts from culturing embryos to plantlet formation took nearly 4 weeks.

Efficient generation of transgenic barley: the way forward to modulate plant-microbe interactions

Stable genetic transformation represents the gold standard approach to the detailed elucidation of plant gene functions. This is particularly relevant in barley, an important experimental model widely employed in applied molecular, genetic and cell biological research, and biotechnology. Presented are details of the establishment of a protocol for Agrobacterium-mediated gene transfer to immature embryos, which enables the highly efficient generation of transgenic barley. Advancements were achieved through comparative experiments on the influence of various explant treatments and co-cultivation conditions. The analysis of representative numbers of transgenic lines revealed that the obtained T-DNA copy numbers are typically low, the generative transmission of the recombinant DNA is in accordance with the Mendelian rules and the vast majority of the primary transgenics produce progeny that expresses the respective transgene product. Moreover, the newly established protocol turned out to be useful to transform not only the highly amenable cultivar (cv.) 'Golden Promise' but also other spring and winter barley genotypes, albeit with substantially lower efficiency. As a major result of this study, a very useful tool is now available for future functional gene analyses as well as genetic engineering approaches. With the aim to modify the expression of barley genes putatively involved in plant-fungus interactions, numerous transgenic plants have been generated using diverse expression cassettes. These plants represent an example of how transformation technology may contribute to further our understanding of important biological processes.

Ethylene influences green plant regeneration from barley callus

The plant hormone ethylene is involved in numerous plant processes including in vitro growth and regeneration. Manipulating ethylene in vitro may be useful for increasing plant regeneration from cultured cells. As part of ongoing efforts to improve plant regeneration from barley (Hordeum vulgare L.), we investigated ethylene emanation using our improved system and investigated methods of manipulating ethylene to increase regeneration. In vitro assays of regeneration from six cultivars, involving 10 weeks of callus initiation and proliferation followed by 8 weeks of plant regeneration, showed a correlation between regeneration and ethylene production: ethylene production was highest from 'Golden Promise', the best regenerator, and lowest from 'Morex' and 'DH-20', the poorest regenerators. Increasing ethylene production by addition of 1-aminocyclopropane 1-carboxylic acid (ACC) during weeks 8-10 increased regeneration from Morex. In contrast, adding ACC to Golden Promise cultures during any of the tissue culture steps reduced regeneration, suggesting that Golden Promise may produce more ethylene than needed for maximum regeneration rates. Blocking ethylene action with silver nitrate during weeks 5-10 almost doubled the regeneration from Morex and increased the Golden Promise regeneration 1.5-fold. Silver nitrate treatment of Golden Promise cultures during weeks 8-14 more than doubled the green plant regeneration. These results indicate that differential ethylene production is related to regeneration in the improved barley tissue culture system. Specific manipulations of ethylene were identified that can be used to increase the green plant regeneration from barley cultivars. The timing of ethylene action appears to be critical for maximum regeneration.