Fertile pea plants regenerate from protoplasts when calluses have not undergone endoreduplication

Protoplast Regeneration and Its Use in New Plant Breeding Technologies

The development of gene-editing technology holds tremendous potential for accelerating crop trait improvement to help us address the need to feed a growing global population. However, the delivery and access of gene-editing tools to the host genome and subsequent recovery of successfully edited plants form significant bottlenecks in the application of new plant breeding technologies. Moreover, the methods most suited to achieve a desired outcome vary substantially, depending on species' genotype and the targeted genetic changes. Hence, it is of importance to develop and improve multiple strategies for delivery and regeneration in order to be able to approach each application from various angles. The use of transient transformation and regeneration of plant protoplasts is one such strategy that carries unique advantages and challenges. Here, we will discuss the use of protoplast regeneration in the application of new plant breeding technologies and review pertinent literature on successful protoplast regeneration.

Analysis of Ploidy in Haploids and Doubled Haploids

Determination of the ploidy level is an essential step when trying to produce doubled haploids (DHs) in any species. Each species and method used to produce DHs has its own frequency of DH production, which means that the rest of plants produced stay haploid. Since haploids are of little use for breeding purposes, it is necessary to distinguish them from true DHs. For this, several methodologies are available, including flow cytometry, chromosome counting, chloroplast counting in stomatal guard cells, measurement of stomatal size and length, counting of nucleoli, evaluation of pollen formation and viability, analysis of cell size, and analysis of morphological markers. However, not all of them are equally easy to use, affordable, reliable, reproducible, and resolutive and therefore useful for a particular case. In this chapter, we revise these methods available to assess the ploidy level of plants, discussing their respective advantages and limitations, and provide some troubleshooting tips and hints to help decide which to choose in each case.

Nuclear genome stability in long-term cultivated callus lines of Fagopyrum tataricum (L.) Gaertn

Long-term cultivated Fagopyrum tataricum (L.) Gaertn. (Tartary buckwheat) morphogenic and non-morphogenic callus lines are interesting systems for gaining a better understanding of the mechanisms that are responsible for the genetic stability and instability of a plant tissue culture. In this work, we used histological sections and transmission electron microscopy to identify and describe the morphology of the nuclei of all of the analysed callus lines. We demonstrated that the embryogenic callus cells had prominent round nuclei that did not contain heterochromatin clumps in contrast to the non-morphogenic callus lines, in which we found nuclei that had multiple lobes. Flow cytometry analysis revealed significant differences in the relative DNA content between the analysed calli. All of the analysed morphogenic callus lines had peaks from 2C to 8C as compared to the non-morphogenic callus lines, whose peaks did not reflect any regular DNA content and exceeded 8C and 16C for the line 6p1 and 16C and 32C for the callus line 10p2A. The results showed that non-morphogenic calli are of an aneuploid nature. The TUNEL test enabled us to visualise the nuclei that had DNA fragmentation in both the morphogenic and non-morphogenic lines. We revealed significantly higher frequencies of positively labelled nuclei in the non-morphogenic lines than in the morphogenic lines. In the case of the morphogenic lines, the highest observed frequency of TUNEL-positive nuclei was 7.7% for lines 2-3. In the non-morphogenic calli, the highest level of DNA damage (68.5%) was revealed in line 6p1. These results clearly indicate greater genome stability in the morphogenic lines.

From Stress to Embryos: Some of the Problems for Induction and Maturation of Somatic Embryos

Although somatic embryogenesis has been successfully achieved in numerous plant species, little is known about the mechanism(s) underlying this process. Changes in the balance of growth regulators of the culture medium, osmolarity, or amino acids as well as the genotype and developmental stage of the tissue used as initial explant may have a pivotal influence on the induction of somatic embryogenic cultures. Moreover, different stress agents (ethylene, activated charcoal, cold or heat or electrical shocks), as well as abscisic acid, can also foster the induction or further development of somatic embryos. In the process, cells first return to a stem cell-like status and then either enter their new program or dye when the stress level exceeds cell tolerance. Recalcitrance to differentiation of somatic cells into embryos is frequently observed, and problems such as secondary or recurrent embryogenesis, embryo growth arrest (at the globular stage or during the transition from torpedo to cotyledonary stage), and development of only the aerial part of somatic embryos can appear, interfering with normal germination and conversion of embryos to plants. Some solutions to solve these problems associated to embryogenesis are proposed and two very efficient somatic embryogenesis protocols for two model plant species are detailed.

Autotetraploid plant regeneration by indirect somatic embryogenesis from leaf mesophyll protoplasts of diploid Gentiana decumbens L.f

Somaclonal variation, often manifested as the increased ploidy of plants observed following in vitro culture, can be advantageous in ornamental species or those used for secondary metabolite production. Polyploidy occurs especially when plantlets are produced by protoplast and callus cultures. Plants were regenerated from green leaf mesophyll protoplasts of diploid Gentiana decumbens L.f. through somatic embryogenesis. A yield of more than 9 × 10⁵ protoplasts per gram of fresh weight was achieved by incubating fully expanded young leaves in an enzyme mixture containing 1.0% (w/v) cellulase and 0.5% (w/v) macerozyme. Protoplasts, cultured in agarose beads using a modified Murashige and Skoog medium, divided and formed microcalli, with the highest plating efficiency obtained on medium containing 2.0 mg l^-1 1-naphthaleneacetic acid and 0.1 mg l^-1 thidiazuron. Callus proliferation was also promoted by including thidiazuron in agar-solidified medium, while somatic embryogenesis was induced from microcalli on medium supplemented with 1.0 mg l^-1 kinetin, 0.5 mg l^-1 gibberellic acid, and 80 mg l^-1 adenine sulfate. Flow cytometric analysis and chromosome counting revealed that all regenerants were tetraploid.

In vitro auxin treatment promotes cell division and delays endoreduplication in developing seeds of the model legume species Medicago truncatula

The role of auxins in the morphogenesis of immature seeds of Medicago truncatula was studied, focusing on the transition from the embryo cell division phase to seed maturation. We analyzed seed development in vitro, by flow cytometry, and through the determination of the kinetics of seed fresh weight and size. Thus, seeds were harvested at 8, 10 and 12 days after pollination and cultured in vitro on a medium either without auxin or supplemented with indole-3-butyric acid (IBA) or naphthalene acetic acid (NAA) at 1 mg l(-1). All parameters studied were determined every 2 days from the start of in vitro culture. The results showed that both auxins increased the weight and size of seeds with NAA having a stronger effect than IBA. We further demonstrated that the auxin treatments modulate the transition between mitotic cycles and endocycles in M. truncatula developing seed by favoring sustained cell divisions while simultaneously prolonging endoreduplication, which is known to be the cytogenetical imprint of the transition from the cell division phase to the storage protein accumulation phase during seed development.

Flow cytometry enables identification of sporophytic eliciting stress treatments in gametic cells

Flow cytometry was used to quantify the effect of individual and combined stress treatments on elicitation of androgenesis by analyzing the relative nuclear DNA content of in vitro cultured microspores of Pisum sativum L. Differences in relative nuclear DNA content of microspores within anthers after stress treatments were clearly evident from the flow cytometry profiles, and permitted us to predict whether a combination of stresses were elicitors or enhancers of androgenesis. This is the first report to assess the effect of various stress treatments in a plant species based on relative nuclear DNA content and to use this information to categorize them as 'elicitors' or 'enhancers'. Flow cytometry represents a simple, quick and reliable way to analyze and discriminate the effect of various stress treatments on elicitation of androgenesis. These results form a solid basis for further efforts designed to enhance responses and to extend double haploid technology to other legumes.

Genetic improvement of grass pea for low neurotoxin (β-ODAP) content

Grass pea is a promising crop for adaptation under climate change because of its tolerance to drought, water-logging and salinity, and being almost free from insect-pests and diseases. In spite of such virtues, global area under its cultivation has decreased because of ban on its cultivation in many countries. The ban is imposed due to its association with neurolathyrism, a non-reversible neurological disorder in humans and animals due to presence of neurotoxin, β-N-oxalyl-L-α,β-diaminopropionic acid (β-ODAP) in its seedlings and seeds. The traditional varieties of grass pea contain 0.5-2.5% β-ODAP. Exploitable genetic variability for β-ODAP has been observed for development of low ODAP varieties, which along with improved agronomic and detoxification practices can help reduce the risk of lathyrism. Collaborative efforts between ICARDA and NARS have resulted in development of improved varieties such as Wasie in Ethiopia, Ratan, Prateek and Mahateora in India, and BARI Khesari-1 and BARI Khesari-2 in Bangladesh with <0.10% β-ODAP. Soil application of 15-20 kg ha(-1) zinc sulphate, early planting, and soaking seeds in water have shown significant effects on β-ODAP. Because of the often cross-pollination nature, the current breeding procedures being followed in grass pea requires paradigm shift in its approach for a possible genetic breakthrough.

Flow cytometry in plant breeding

Since the first report on the flow cytometric study of plant material 35 years ago, analyzing the nuclear DNA content of field bean, an ever increasing number of applications of FCM has been developed and applied in plant science and industry, but a similar length of time elapsed before the appearance of the first complete volume devoted to FCM of plant cells. Most published information on the uses of FCM addresses various aspects of animal (including human) cell biology, thus failing to provide a pertinent substitute. FCM represents an ideal means for the analysis of both cells and subcellular particles, with a potentially large number of parameters analyzed both rapidly, simultaneously, and quantitatively, thereby furnishing statistically exploitable data and allowing for an accurate and facilitated detection of subpopulations. It is, indeed, the summation of these facts that has established FCM as an important, and sometimes essential, tool for the understanding of fundamental mechanisms and processes underlying plant growth, development, and function. In this review, special attention is paid to FCM as applied to plant cells in the context of plant breeding, and some new and less well-known uses of it for plants will be discussed.

Efficient in vitro regeneration of fertile plants from corm explants of Hypoxis hemerocallidea landrace Gaza -- the "African Potato"

We present efficient protocols for the regeneration of fertile plants from corm explants of Hypoxis hemerocallidea Fisch. and C. A. Mey. landrace Gaza, either by direct multiple shoot formation or via shoot organogenesis from corm-derived calluses. The regeneration efficiency depended on plant growth regulator concentrations and combinations. Multiple direct shoot formation with high frequency (100% with 5-8 shoots/explant) was obtained on a basal medium (BM) supplemented with 3 mg/l kinetin (BM1). However, efficient indirect regeneration occurred when corm explants were first plated on callus induction medium (BM2) with high kinetin (3 mg/l) and naphthalene acetic acid (NAA 1 mg/l), and then transferred to shoot inducing medium (BM3) containing BA (1.5 mg/l) and NAA (0.5 mg/l). Shoot regeneration frequency was 100% and 30-35 shoots per explant were obtained. The regenerated shoots were rooted on a root inducing medium (BM4) containing NAA (0.1 mg/l). Rooted plantlets were transferred to the greenhouse. The regenerants were morphologically normal and fertile. Flow cytometric analyses and chloroplast counts of guard cells suggested that the regenerants were diploid. Efficient cloning protocols described here, have the potential not only to substantially reduce the pressure on natural populations but also for wider biotechnological applications of Hypoxis hemerocallidea-an endangered medicinal plant.

Lathyrus sativus (grass pea) and its neurotoxin ODAP

Lathyrus sativus (grass pea) is a high-yielding, drought-resistant legume consumed as a food in Northern India and neighboring countries as well as in Ethiopia. Its development into an important food legume, however, has been hindered by the presence of the neurotoxin - beta-N-oxalyl-L-alpha,beta-diaminopropionic acid (beta-ODAP) in seeds which, if consumed in large quantities for prolonged periods, can cause irreversible paralysis. Recently, some low-toxin lines have been developed that may prove safe for both animal and human foods. Cultivation of L. sativus should thus be considered in suitable regions because the demand for legume animal feed protein products is expected to increase. This paper addresses advances in understanding L. sativus from the perspective of its taxonomy, genetics, ecology, chemistry, nutrition, medicine, biology and for animal nutrition.

Visual selection and maintenance of the cell lines with high plant regeneration ability and low ploidy level in Dianthus acicularis by monitoring with flow cytometry analysis

Efficient plant regeneration system from cell suspension cultures was established in D. acicularis (2n=90) by monitoring ploidy level and visual selection of the cultures. The ploidy level of the cell cultures closely related to the shoot regeneration ability. The cell lines comprising original ploidy levels (2C+4C cells corresponding to DNA contents of G1 and G2 cells of diploid plant, respectively) showed high regeneration ability, whereas those containing the cells with 8C or higher DNA C-values showed low or no regeneration ability. The highly regenerable cell lines thus selected consisted of compact cell clumps with yellowish color and relatively moderate growth, suggesting that it is possible to select visually the highly regenerable cell lines with the original ploidy level. All the regenerated plantlets from the highly regenerable cell cultures exhibited normal phenotypes and no variations in ploidy level were observed by flow cytometry (FCM) analysis.

Sites and regulation of polyamine catabolism in the tobacco plant. Correlations with cell division/expansion, cell cycle progression, and vascular development

We previously gave a picture of the homeostatic characteristics of polyamine (PA) biosynthesis and conjugation in tobacco (Nicotiana tabacum) plant organs during development. In this work, we present the sites and regulation of PA catabolism related to cell division/expansion, cell cycle progression, and vascular development in the tobacco plant. Diamine oxidase (DAO), PA oxidase (PAO), peroxidases (POXs), and putrescine N-methyltransferase expressions follow temporally and spatially discrete patterns in shoot apical cells, leaves (apical, peripheral, and central regions), acropetal and basipetal petiole regions, internodes, and young and old roots in developing plants. DAO and PAO produce hydrogen peroxide, a plant signal molecule and substrate for POXs. Gene expression and immunohistochemistry analyses reveal that amine oxidases in developing tobacco tissues precede and overlap with nascent nuclear DNA and also with POXs and lignification. In mature and old tissues, flow cytometry indicates that amine oxidase and POX activities, as well as pao gene and PAO protein levels, coincide with G2 nuclear phase and endoreduplication. In young versus the older roots, amine oxidases and POX expression decrease with parallel inhibition of G2 advance and endoreduplication, whereas putrescine N-methyltransferase dramatically increases. In both hypergeous and hypogeous tissues, DAO and PAO expression occurs in cells destined to undergo lignification, suggesting a different in situ localization. DNA synthesis early in development and the advance in cell cycle/endocycle are temporally and spatially related to PA catabolism and vascular development.