Fertile transgenic Brachiaria ruziziensis (ruzigrass) plants by particle bombardment of tetraploidized callus

Tissue Culture and Somatic Embryogenesis in Warm-Season Grasses—Current Status and Its Applications: A Review

Warm-season grasses are C4 plants and have a high capacity for biomass productivity. These grasses are utilized in many agricultural production systems with their greatest value as feeds for livestock, bioethanol, and turf. However, many important warm-season perennial grasses multiply either by vegetative propagation or form their seeds by an asexual mode of reproduction called apomixis. Therefore, the improvement of these grasses by conventional breeding is difficult and is dependent on the availability of natural genetic variation and its manipulation through breeding and selection. Recent studies have indicated that plant tissue culture system through somatic embryogenesis complements and could further develop conventional breeding programs by micropropagation, somaclonal variation, somatic hybridization, genetic transformation, and genome editing. This review summarizes the tissue culture and somatic embryogenesis in warm-season grasses and focus on current status and above applications including the author’s progress.

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.

Protocol development for somatic embryogenesis, SSR markers and genetic modification of Stipagrostis pennata (Trin.) De Winter

BACKGROUND

Stipagrostis pennata (Trin.) De Winter is an important species for fixing sand in shifting and semi-fixed sandy lands, for grazing, and potentially as a source of lignocellulose fibres for pulp and paper industry. The seeds have low viability, which limits uses for revegetation. Somatic embryogenesis offers an alternative method for obtaining large numbers of plants from limited seed sources.

RESULTS

A protocol for plant regeneration from somatic embryos of S. pennata was developed. Somatic embryogenesis was induced on Murashige & Skoog (MS) medium supplemented with 3 mg·L-1 2,4-D subsequently shoots were induced on MS medium and supplemented with 5 mg·L-1 zeatin riboside. The highest shoots induction was obtained when embryogenic callus derived from mature embryos (96%) in combination with MS filter-sterilized medium was used from Khuzestan location. The genetic stability of regenerated plants was analysed using ten simple sequence repeats (SSR) markers from S. pennata which showed no somaclonal variation in regenerated plants from somatic embryos of S. pennata. The regenerated plants of S. pennata showed genetic stability without any somaclonal variation for the four pairs of primers that gave the expected amplicon sizes. This data seems very reliable as three of the PCR products belonged to the coding region of the genome. Furthermore, stable expression of GUS was obtained after Agrobacterium-mediated transformation using a super binary vector carried by a bacterial strain LBA4404.

CONCLUSION

To our knowledge, the current work is the first attempt to develop an in vitro protocol for somatic embryogenesis including the SSR marker analyses of regenerated plants, and Agrobacterium-mediated transformation of S. pennata that can be used for its large-scale production for commercial purposes.

Plant regeneration from embryogenic callus derived from shoot apices and production of transgenic plants by particle inflow gun in dwarf napier grass (Pennisetum purpureum Schumach.)

Napier grass (Pennisetum purpureum Schumach.) is a highly productive C4 tropical forage grass that has been targeted as a potential bioenergy crop. To further increase the efficiency of bioethanol production by molecular breeding, a reliable protocol for genetically transforming napier grass is essential. In this study, we report the creation of transgenic napier grass plants derived from embryogenic callus cultures of shoot apices. Embryogenic callus was initiated in three accessions of napier grass and a napier grass×pearl millet hybrid using Murashige and Skoog (MS) medium supplemented with 2.0 mg L^-1 2,4-dichlorophenoxyacetic acid (2,4-D), 0.5 mg L^-1 6-benzylaminopurine (BAP) and 50 µM copper sulfate (CuSO₄). Of the accessions tested, a dwarf type with late-heading (DL line) had the best response for embryogenic callus formation. Highly regenerative calli that formed dense polyembryogenic clusters were selected as target tissues for transformation. A plasmid vector, pAHC25, containing an herbicide-resistance gene (bar) and the β-glucuronidase (GUS) reporter gene was used in particle bombardment experiments. Target tissues treated with 0.6 M osmoticum were bombarded, and transgenic plants were selected under 5.0 mg L^-1 bialaphos selection. Although a total of 1400 target tissues yielded nine GUS-positive bialaphos-resistant calli, only one transgenic line that was derived from target tissue with the shortest culture term produced four transgenic plants. Thus, the length of time that the target tissue is in callus culture was one of the most important factors for acquiring transgenic plants in napier grass. This is the first report of successfully producing transgenic napier grass plants.