Improved Anther Culture Media for Enhanced Callus Formation and Plant Regeneration in Rice (Oryza sativa L.)

Generic Workflow of a Highly Effective and Easy Anther Culture Method for Both Japonica and Indica Rice

As one of the most important staple crops in the world, rice plays a pivotal role in world food security. The creation of doubled haploids based on anther culture is an important technology for rice breeding. However, at present, rice anther culture technology still faces many problems, such as genotype dependency, especially genotypes of indica rice. In this study, fifteen rice genotypes, including twelve japonica rice genotypes and three indica rice genotypes, were randomly selected and used to study anther culture by using a modified M8 medium. The results showed that the total callus induction rates of these different rice genotypes ranged from 0.81 to 13.95%, with an average of 6.64%, while the callus induction rates calculated for the top ten highest callus inductions for each rice genotype ranged from 2.75 to 17.00%, with an average of 10.56%. There were varying gaps between the total callus induction rates and the callus induction rates in these different rice genotypes. The fact that the gaps for some rice genotypes were relatively large indicated that standard tiller or anther collection was not applicable to all rice genotypes and that there was still a lot of room for improvement in the callus induction rate of some rice genotypes through optimization of the sampling method. The plantlet regeneration rates ranged from 12.55 to 456.54%, with an average of 200.10%. Although there were many albinos from anther culture for some rice genotypes, these would still meet the requirement if the rice genotypes had higher callus induction rates or regeneration rates. The percentages of seed setting of regenerated green seedlings ranged from 14% to 84%, with an average of 48.73%. Genetic diversity analysis showed that the genetic background of these different rice genotypes was representative, and the phylogenetic tree and Principal Component Analysis (PCA) divided them into indica and japonica types. Therefore, in this study, an anther culture method suitable for both indica and japonica rice genotypes was established, which could improve doubled haploid breeding in rice.

Rapid breeding of an early maturing, high-quality, and high-y.ielding rice cultivar using marker‑assisted selection coupled with optimized anther culture

With the global shift towards healthier eating habits, the focus of the rice industry has evolved from quantity to quality. In China, the Yangtze River Basin is the main area consuming long-grain and high-quality indica rice. Hubei Province, a significant rice-producing area, currently cultivates a limited range of rice varieties, risking degradation and diminishing economic returns. Therefore, it is imperative to cultivate elite rice varieties tailored to the local production conditions and can significantly enhance the added value. This study bred the novel rice cultivar "Runxiangyu", characterized by early maturity, high quality, and high yield. It is a hybrid of Ezhong 5, known for its moderate height and excellent quality, albeit with a long growth period and lack of fragrance, and Yuzhenxiang, renowned for its high quality, short growth period, and fragrance but limited by its tall stature and poor tillering ability. The breeding process utilized optimized anther culture coupled with molecular marker-assisted selection (MAS) and phenotype analysis. In the field, the developed cultivar was 120.9 cm tall and had an entire growth period of 117.5 days, demonstrating moderate disease resistance and excellent heat tolerance. Its grains are fragrant, meeting the national standard of grade two high-quality rice set by the Food Quality Supervision and Inspection Center of the Ministry of Agriculture and Rural Areas). Exhibiting superior agronomic traits, such as plant type, height, growth period, and stress resistance, along with and quality attributes, including grain shape, chalkiness, fragrance, and taste, "Runxiangyu" was certified by the Agricultural Crop Variety Certification Commission of Hubei in 2022. These findings suggested that molecular MAS coupled with optimized anther culture and multi-site phenotype analysis is an efficient and rapid method for crop breeding.

Supplementary Information

The online version contains supplementary material available at 10.1007/s11032-024-01495-4.

Unveiling the Genetic Basis Underlying Rice Anther Culturability via Segregation Distortion Analysis in Doubled Haploid Population

Anther culture (AC) is a valuable technique in rice breeding. However, the genetic mechanisms underlying anther culturability remain elusive, which has hindered its widespread adoption in rice breeding programs. During AC, microspores carrying favorable alleles for AC are selectively regenerated, leading to segregation distortion (SD) of chromosomal regions linked to these alleles in the doubled haploid (DH) population. Using the AC method, a DH population was generated from the japonica hybrid rice Shenyou 26. A genetic map consisting of 470 SNPs was constructed using this DH population, and SD analysis was performed at both the single- and two-locus levels to dissect the genetic basis underlying anther culturability. Five segregation distortion loci (SDLs) potentially linked to anther culturability were identified. Among these, SDL5 exhibited an overrepresentation of alleles from the female parent, while SDL1.1, SDL1.2, SDL2, and SDL7 displayed an overrepresentation of alleles from the male parent. Furthermore, six pairs of epistatic interactions (EPIs) that influenced two-locus SDs in the DH population were discovered. A cluster of genetic loci, associated with EPI-1, EPI-3, EPI-4, and EPI-5, overlapped with SDL1.1, indicating that the SDL1.1 locus may play a role in regulating anther culturability via both additive and epistatic mechanisms. These findings provide valuable insights into the genetic control of anther culturability in rice and lay the foundation for future research focused on identifying the causal genes associated with anther culturability.

Improvement of Rice Blast Resistance in TGMS Line HD9802S through Optimized Anther Culture and Molecular Marker-Assisted Selection

Rice blast caused by Magnaporthe oryzae is one of the most serious rice diseases worldwide. The early indica rice thermosensitive genic male sterile (TGMS) line HD9802S has the characteristics of stable fertility, reproducibility, a high outcrossing rate, excellent rice quality, and strong combining ability. However, this line exhibits poor blast resistance and is highly susceptible to leaf and neck blasts. In this study, backcross introduction, molecular marker-assisted selection, gene chipping, anther culture, and resistance identification in the field were used to introduce the broad-spectrum blast-resistance gene R6 into HD9802S to improve its rice blast resistance. Six induction media were prepared by varying the content of each component in the culture medium. Murashige and Skoog's medium with 3 mg/L 2,4-dichlorophenoxyacetic acid, 2 mg/L 1-naphthaleneacetic acid, and 1 mg/L kinetin and N6 medium with 800 mg/L casein hydrolysate, 600 mg/L proline, and 500 mg/L glutamine could improve the callus induction rate and have a higher green seedling rate and a lower white seedling rate. Compared to HD9802S, two doubled haploid lines containing R6 with stable fertility showed significantly enhanced resistance to rice blast and no significant difference in spikelet number per panicle, 1000-grain weight, or grain shape. Our findings highlight a rapid and effective method for improving rice blast resistance in TGMS lines.

Haploid Induction in Indica Rice: Exploring New Opportunities

Haploid plants are of significant interest to crop breeders due to their ability to expedite the development of inbred lines. Chromosome-doubling of haploids, produced by either in vitro or in vivo methods, results in fully homozygous doubled haploids. For nearly five decades, in vitro methods of anther and microspore culture have been attempted in many crops. In rice, in vitro methods are used with some success in japonica cultivars, although indica types have remained recalcitrant to a large extent. This review aims to explore the reasons for the lack of success of in vitro methods in indica rice and discuss new advancements in in vivo haploid induction protocols in other cereals and their relevance to rice. In particular, the current level of understanding of in vivo haploid inducer systems that utilize MTL and CENH3 mutants is analyzed in detail. One notable advantage of in vivo haploid induction systems is that they do not require tissue culture competence. This makes these methods more accessible and potentially transformative for research, offering a pragmatic approach to improving indica rice cultivars. By embracing these in vivo methods and harnessing the power of gene editing technologies like CRISPR/Cas9 systems, breeders can reshape their approach to indica rice improvement.

Embryo Rescue in Plant Breeding

Embryo rescue (ER) techniques are among the oldest and most successful in vitro tissue culture protocols used with plant species. ER refers to a series of methods that promote the development of an immature or lethal embryo into a viable plant. Intraspecific, interspecific, or intergeneric crosses allow the introgression of important alleles of agricultural interest from wild species, such as resistance or tolerance to abiotic and biotic stresses or morphological traits in crops. However, pre-zygotic and post-zygotic reproductive barriers often present challenges in achieving successful hybridization. Pre-zygotic barriers manifest as incompatibility reactions that hinder pollen germination, pollen tube growth, or penetration into the ovule occurring in various tissues, such as the stigma, style, or ovary. To overcome these barriers, several strategies are employed, including cut-style or graft-on-style techniques, the utilization of mixed pollen from distinct species, placenta pollination, and in vitro ovule pollination. On the other hand, post-zygotic barriers act at different tissues and stages ranging from early embryo development to the subsequent growth and reproduction of the offspring. Many crosses among different genera result in embryo abortion due to the failure of endosperm development. In such cases, ER techniques are needed to rescue these hybrids. ER holds great promise for not only facilitating successful crosses but also for obtaining haploids, doubled haploids, and manipulating the ploidy levels for chromosome engineering by monosomic and disomic addition as well substitution lines. Furthermore, ER can be used to shorten the reproductive cycle and for the propagation of rare plants. Additionally, it has been repeatedly used to study the stages of embryonic development, especially in embryo-lethal mutants. The most widely used ER procedure is the culture of immature embryos taken and placed directly on culture media. In certain cases, the in vitro culture of ovule, ovaries or placentas enables the successful development of young embryos from the zygote stage to maturity.

Development of In Vitro Anther Culture for Doubled Haploid Plant Production in Indica Rice (Oryza sativa L.) Genotypes

Anther culture is an efficient biotechnological tool in modern plant breeding programs to produce new varieties and parental lines in hybrid seed productions. However, some bottlenecks-low induction rate, genotype dependency, albinism-restrict the widespread utilization of in vitro anther culture in rice breeding, especially in Oryza sativa ssp. indica (indica) genotypes, while an improved efficient protocol can shorten the process of breeding. Three different induction media (N₆NDK_, N₆NDZ, Ali-1) and four plant regeneration media (mMSNBK1, MSNBK3, MSNBKZ1, MSNBKZ2) were tested with five indica rice genotypes to increase the efficiency of in vitro androgenesis (number of calli and regenerated green plantlets). The production of calli was more efficient on the N₆NDK medium with an average 88.26 calli/100 anthers and N₆NDZ medium with an average of 103.88 calli/100 anthers as compared to Ali-1 with an average of 6.96 calli/100 anthers. The production of green plantlets was greater when calli was produced on N₆NDK medium (2.15 green plantlets/100 anthers) compared when produced on to N₆NDZ medium (1.18 green plantlets/100 anthers). Highest green plantlets production (4.7 green plantlets/100 anthers) was achieved when mMSNBK1 plant regeneration medium was used on calli produced utilizing N₆NDK induction medium. In the best overall treatment (N₆NDK induction medium and mMSNBK1 plant regeneration medium), four tested genotypes produced green plantlets. However, the genotype influenced the efficiency, and the green plantlets production ranged from 0.4 green plantlets/100 anthers to 8.4 green plantlets/100 anthers. The ploidy level of 106 acclimatized indica rice plantlets were characterized with flow cytometric analyses to calculate the percentage of spontaneous chromosome doubling. Altogether, 48 haploid-, 55 diploid-, 2 tetraploid- and 1 mixoploid plantlets were identified among the regenerant plantlets, and the spontaneous chromosome doubling percentage was 51.89%. Utilization of DH plants have been integrated as a routine method in the Hungarian rice breeding program. The tetraploid lines can be explored for their potential to offer new scopes for rice research and breeding directions in the future.

Improvement of Anther Culture to integrate Doubled Haploid Technology in Temperate Rice (Oryza sativa L.) Breeding

Doubled haploid (DH) plant production, such as anther culture (AC), is an effective tool used in modern rice breeding programs. The improved efficient protocols applied can shorten the process of breeding. The effect of combinations of plant growth regulators (2.5 mg/L NAA, 1 mg/L 2,4-D and 0.5 mg/L kinetin; 2 mg/L 2,4-D and 0.5 mg/L BAP) in the induction medium were compared in AC for five rice breeding materials and combinations. Induction of calli ranged from 264.6 ± 67.07 to 468.8 ± 123.2 calli/100 anthers in AC of rice genotypes. Two basal media (MS and N₆) and two combinations of growth regulators (1 mg/L NAA, 1 mg/L BAP and 1 mg/L kinetin; 1.5 mg/L BAP, 0.5 mg/L NAA and 0.5 mg/L kinetin) were used as regeneration media. The in vitro green plant production was the highest with the application of the N₆NDK induction medium (NAA, 2,4-D and kinetin) and the MS-based regeneration medium (1 mg/L NAA, 1 mg/BAP and 1 mg/L kinetin) in anther culture of the '1009' genotype (95.2 green plantlets/100 anthers). The mean of five genotypes was 24.48 green plantlets/100 anthers for the best treatment. Flow cytometric analyses conducted identified the microspore origin of the haploid calli produced in AC, while the uniformity of spontaneous DH plants was checked in the DH₁ and DH₂ generations. Spontaneous chromosome doubling ranged from 38.1% to 57.9% (mean 42.1%), depending on the breeding source. The generated and selected DH lines were tested in micro- and small-plot field experiments to identify promising lines for a pedigree breeding program. The improved AC method was integrated in a Hungarian temperate rice pedigree breeding program.

Callus induction and regeneration in high-altitude Himalayan rice genotype SR4 via seed explant

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In the present study, an efficient protocol for callusing and regeneration via direct and indirect means was established using mature seeds as source of explants as they are accessible round the year and receptive to transformation via callus.

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Highest frequency (96.6%) of callus induction was obtained in MS media containing 3.0 mg/L 2, 4-D.

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Highest regeneration frequency (100%) from callus was obtained from MS media supplemented with 5.0 mg/L BAP and 0.5 mg/L NAA with highest number of shoots having an average shoot length 9.14 ± 0.204 (cm) after four weeks of culture.

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Direct multiple shoot regeneration from seed explants was obtained using various concentrations of TDZ and BAP with highest regeneration frequency was observed in MS media supplemented with 6 mg/L of TDZ with maximum number of shoots.

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The present study is attempt to develop highly efficient regeneration protocol via both direct and indirect ways utilizing mature seeds as source explant and thus can be utilized for molecular studies for genetic advancement of Himalayan rice genotype SR4 through transformation.

SR4 genotype of rice is high altitude Himalayan rice prone to various abiotic stresses such as cold stress and therefore gives a poor yield. An efficient protocol for callusing and regeneration via direct and indirect means was established using mature seeds as an explant which can be utilized for molecular studies for genetic advancement of Himalayan rice genotype SR4 through transformation. Highest frequency (96.6%) of callus induction was obtained on MS media 3.0 mg/L 2, 4-D. While maximum regeneration frequency (100%), number of shoots with maximum length 9.14 ± 0.204 (cm) from callus was recovered from MS media amended with 5.0 mg/L BAP in combination with 0.5 mg/L NAA with highest number of shoots having an average shoot length 9.14 ± 0.204 (cm) after four weeks of culture. Direct multiple shoot regeneration from seed explants was obtained using various concentrations of TDZ and BAP with highest regeneration frequency was observed on MS media fortified with 6 mg/L of TDZ with maximum number of shoots. The shoots developed roots on MS media supplemented with 0.6 mg/L IBA.

P, B. N. D, Samantaray S. Androgenesis in indica rice: A comparative competency in development of doubled haploids

Rice is critical to global food security which demands immediate attention to meet the ever-growing population. Development of improved variety is the major focus area of research, in which doubled haploid (DH) technology plays a vital role. Since, androgenesis shows its potential in DH production, this method was not capitalized specially in indica rice due to due to its recalcitrant nature to tissue culture. Success of androgenesis is governed by many important factors such as stage of anther, pre-treatment conditions, accurate concentrations of media, and plant growth regulators. Though reports of androgenesis are abundant in rice, most of them either used japonica or a specific cultivar of indica rice ecotypes. In this study, a media combination was established which is successful in producing doubled haploids from F₁s of Savitri x Pokkali, IR20 x Mahulata along with the popular indica hybrids of Arize 8433DT, Arize 6453, Arize Bold, and Swift Gold. Out of 12 different media combinations tested, and 5 different durations of cold-treatments studied, N6 media with 2,4-D (2.0 mg/l) and BAP (0.5 mg/l) with 7^th day cold pre-treatment was found to be most effective in all of the F₁s for callus induction. Among all the F₁s, rice hybrid, Arize 8433DT showed highest of 52% callus induction. In case of green shoot regeneration, MS media with NAA (0.5 mg/l), BAP (2.0 mg/l) and Kn (1.0 mg/l) (MS+C4) was found to be the most efficient of six treatments studied with highest of 58.25% regeneration in Arize 8433DT. Further, MS+C4 in combination with proline (5.0 mg/l) increased the regeneration rate to 85.99%. Besides, MS media with NAA (1.0 mg/l), Kn (0.1 mg/l) and 50 g/l sucrose was found to be most efficient for supporting root induction in all F1s. This study claims the establishment of genotype independent androgenic protocol for indica rice which could be capitalized in indica rice improvement.

A Temporary Immersion System Improves Regeneration of In Vitro Irradiated Recalcitrant Indica Rice (Oryza sativa L.) Embryogenic Calli

The development of gamma ray-mutated rice lines is a solution for introducing genetic variability in indica rice varieties already being used by farmers. In vitro gamma ray (⁶⁰Co) mutagenesis reduces chimeras and allows for a faster selection of desirable traits but requires the optimization of the laboratory procedure. The objectives of the present work were sequencing of matK and rbcL, the in vitro establishment of recalcitrant rice embryogenic calli, the determination of their sensitivity to gamma radiation, and optimization of the generation procedure. All sequenced genes matched perfectly with previously reported matK and rbcL O. sativa genes. Embryogenic calli induction improved using MS medium containing 2 mg L^-1 2,4-D, and regeneration was achieved with MS medium with 3 mg L^-1 BA and 0.5 mg L^-1 NAA. The optimized radiation condition was 60 Gy, (LD20 = 64 Gy) with 83% regeneration. An immersion system (RITA^®, Saint-Mathieu-de-Tréviers, France) of either 60 or 120 s every 8 h allowed systematic and homogeneous total regeneration of the recalcitrant line. Other well-known recalcitrant cultivars, CR1821 and CR1113, also had improved regeneration in the immersion system. To our knowledge, this is the first study reporting the use of an immersion system to allow for the regeneration of gamma-ray mutants from recalcitrant indica rice materials.