Effect of ovary induction on bread wheat anther culture: ovary genotype and developmental stage, and candidate gene association

Developments and prospects for doubled haploid wheat

Doubled haploid production is a valuable biotechnology that can accelerate the breeding of new wheat varieties by several years through the one-step creation of 100% homozygous plants. The technology also plays important role in studying the genetic control of traits in wheat, in marker-assisted selection, in genomics and in genetic engineering. In this paper, recent advances in androgenesis and gynogenesis techniques, emphasizing predominantly the in vitro culture phase, as well as the emerging innovative approaches in researching and producing wheat doubled haploids are reviewed. Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-based genome editing, that allows targeted mutagenesis and gene targeting, is being tested extensively as a powerful and precise tool to induce doubled haploids in wheat. The review provides the reader with recent examples of gene modifications in wheat to induce haploidy.

Microtubule organization changes severely after mannitol and n-butanol treatments inducing microspore embryogenesis in bread wheat

BACKGROUND

A mannitol stress treatment and a subsequent application of n-butanol, known as a microtubule-disrupting agent, enhance microspore embryogenesis (ME) induction and plant regeneration in bread wheat. To characterize changes in cortical (CMT) and endoplasmic (EMT) microtubules organization and dynamics, associated with ME induction treatments, immunocytochemistry studies complemented by confocal laser scanning microscopy (CLSM) were accomplished. This technique has allowed us to perform advanced 3- and 4D studies of MT architecture. The degree of MT fragmentation was examined by the relative fluorescence intensity quantification.

RESULTS

In uni-nucleated mannitol-treated microspores, severe CMT and EMT fragmentation occurs, although a complex network of short EMT bundles protected the nucleus. Additional treatment with n-butanol resulted in further depolymerization of both CMT and EMT, simultaneously with the formation of MT aggregates in the perinuclear region. Some aggregates resembled a preprophase band. In addition, a portion of the microspores progressed to the first mitotic division during the treatments. Bi-nucleate pollen-like structures showed a high MT depolymerization after mannitol treatment and numerous EMT bundles around the vegetative and generative nuclei after n-butanol. Interestingly, bi-nucleate symmetric structures showed prominent stabilization of EMT.

CONCLUSIONS

Fragmentation and stabilization of microtubules induced by mannitol- and n-butanol lead to new configurations essential for the induction of microspore embryogenesis in bread wheat. These results provide robust insight into MT dynamics during EM induction and open avenues to address newly targeted treatments to induce ME in recalcitrant species.

Unpollinated Ovaries Used to Produce Doubled Haploid Lines in Durum Wheat

The use of doubled haploid lines improves the efficiency of cultivar development and homozygous genotypes can be obtained in one generation, as opposed to conventional line production, which requires several cycles of self-pollination. However, in durum wheat (Triticum turgidum subsp. durum Desf.), the low efficiency of green plant regeneration and the very high frequency of albino plants hinder the application of this technique.We observed the success of using gynogenesis for durum wheat and the significant influence of growing conditions on ovary and callus development, and on plant regeneration. Our results suggested that the cold pretreatment for 2 weeks is efficient for durum wheat. Furthermore, the addition of 2,4-D, vitamins and glutamine, and the use of maltose as sugar source in media improved the ovary regeneration. We describe in this work an efficient method to regenerate green plants from in vitro durum wheat gynogenesis .

Bread Wheat Doubled Haploid Production by Anther Culture

The use of doubled haploid (DH) plants in plant breeding programmes is the fastest route to release new varieties (4-6 years), allowing for a rapid response to end-user needs. Microspore embryogenesis is one of the most efficient methods for DH plant production in bread wheat. In this process, microspores triggered by a stress treatment or by application of bioactive compounds are reprogrammed to follow an embryogenic pathway that leads to the production of haploid or DH plants. In this chapter, we describe a protocol for anther culture of bread wheat. This protocol is based on an osmotic and starvation treatment of the anthers followed by the application of a microtubule disrupting agent. Anthers are cultured in an ovary pre-conditioned medium with mature ovaries from cv. Caramba. This protocol has been applied to a wide range of genotypes and F1s from bread and spelt wheat.

Species with Haploid or Doubled Haploid Protocols

In this chapter, we present a list of species (and few interspecific hybrids) where haploids and/or doubled haploids have been published, including the method by which they were obtained and the corresponding references. This list is an update of the compilation work of Maluszynski et al. published in 2003, including new species for which protocols were not available at that time, and also novel methodologies developed during these years. The list includes 383 different backgrounds. In this book, we present full protocols to produce DHs in 43 of the species included in this list. In addition, this book includes a chapter for one species not included in the list. This makes a total of 384 species where haploids and/or DHs have been reported up to date.

Albino Plant Formation in Androgenic Cultures: An Old Problem and New Facts

High frequency of albino plant formation in isolated microspore or anther cultures is a great problem limiting the possibility of their exploitation on a wider scale. It is highly inconvenient as androgenesis-based doubled haploid (DH) technology provides the simplest and shortest way to total homozygosity, highly valued by plant geneticists, biotechnologists and especially, plant breeders, and this phenomenon constitutes a serious limitation of these otherwise powerful tools. The genotype-dependent tendency toward albino plant formation is typical for many monocotyledonous plants, including cereals like wheat, barley, rice, triticale, oat and rye - the most important from the economical point of view. Despite many efforts, the precise mechanism underlying chlorophyll deficiency has not yet been elucidated. In this chapter, we review the data concerning molecular and physiological control over proper/disturbed chloroplast biogenesis, old hypotheses explaining the mechanism of chlorophyll deficiency, and recent studies which shed new light on this phenomenon.

Trichostatin A Affects Developmental Reprogramming of Bread Wheat Microspores towards an Embryogenic Route

Microspores can be developmentally reprogrammed by the application of different stress treatments to initiate an embryogenic pathway leading to the production of doubled haploid (DH) plants. Epigenetic modifications are involved in cell reprogramming and totipotency in response to stress. To increase microspore embryogenesis (ME) efficiency in bread wheat, the effect of the histone deacetylase inhibitor trichostatin A (TSA) has been examined in two cultivars of wheat with different microspore embryogenesis response. Diverse strategies were assayed using 0-0.4 µM TSA as a single induction treatment and after or simultaneously with cold or mannitol stresses. The highest efficiency was achieved when 0.4 µM TSA was applied to anthers for 5 days simultaneously with a 0.7 M mannitol treatment, producing a four times greater number of green DH plants than mannitol. Ultrastructural studies by transmission electron microscopy indicated that mannitol with TSA and mannitol treatments induced similar morphological changes in early stages of microspore reprogramming, although TSA increased the number of microspores with 'star-like' morphology and symmetric divisions. The effect of TSA on the transcript level of four ME marker genes indicated that the early signaling pathways in ME, involving the TaTDP1 and TAA1b genes, may be mediated by changes in acetylation patterns of histones and/or other proteins.

Genetic variation of salt-stressed durum wheat (Triticum turgidum subsp. durum Desf.) genotypes under field conditions and gynogenetic capacity

Agriculture has new challenges against the climate change: the preservation of genetic resources and the rapid creation of new varieties better adapted to abiotic stress, specially salinity. In this context, the agronomic performance of 25 durum wheat (Triticum turgidum subsp. durum Desf.) genotypes (nineteen landraces and six improved varieties), cultivated in two semi-arid regions in the center area of Tunisia, were assessed. These sites (Echbika, 2.2 g l^-1; Barrouta, 4.2 g l^-1) differ by their degree of salinity of the water irrigation. The results showed that most of the agronomic traits (e.g. spike per meter square, thousand kernels weight and grain yield) were reduced by salinity. Durum wheat landraces, Mahmoudi and Hmira, and improved varieties, Maali and Om Rabia showed the widest adaptability to different quality of irrigation water. Genotypes including Jneh Kotifa and Arbi were estimated as stable genotypes under adverse conditions. Thereafter, salt-tolerant (Hmira and Jneh Khotifa) and the most cultivated high-yielding (Karim, Razzak and Khiar) genotypes were tested for their gynogenetic ability to obtain haploids and doubled haploid lines. Genotypes with good induction capacity had not necessarily a good capacity of regeneration of haploid plantlets. In our conditions, Hmira and Khiar exhibited the best gynogenetic ability (3.1% and 2.9% of haploid plantlets, respectively).

Bcl-2△21 and Ac-DEVD-CHO Inhibit Death of Wheat Microspores

Microspore cell death and low green plant production efficiency are an integral obstacle in the development of doubled haploid production in wheat. The aim of the current study was to determine the effect of anti-apoptotic recombinant human B-cell lymphoma-2 (Bcl-2△21) and caspase-3-inhibitor (Ac-DEVD-CHO) in microspore cell death in bread wheat cultivars AC Fielder and AC Andrew. Induction medium containing Bcl-2△21 and Ac-DEVD-CHO yielded a significantly higher number of viable microspores, embryo-like structures and total green plants in wheat cultivars AC Fielder and AC Andrew. Total peroxidase activity was lower in Bcl-2△21 treated microspore cultures at 96 h of treatment compared to control and Ac-DEVD-CHO. Electron paramagnetic resonance study of total microspore protein showed a different scavenging activity for Bcl-2△21 and Ac-DEVD-CHO. Bcl-2△21 scavenged approximately 50% hydroxyl radical (HO^•) formed, whereas Ac-DEVD-CHO scavenged approximately 20% of HO^•. Conversely, reduced caspase-3-like activities were detected in the presence of Bcl-2△21 and Ac-DEVD-CHO, supporting the involvement of Bcl-2△21 and Ac-DEVD-CHO in increasing microspore viability by reducing oxidative stress and caspase-3-like activity. Our results indicate that Bcl-2△21 and Ac-DEVD-CHO protects cells from cell death following different pathways. Bcl-2△21 prevents cell damage by detoxifying HO^• and suppressing caspase-3-like activity, while Ac-DEVD-CHO inhibits the cell death pathways by modulating caspase-like activity.