Tissue culture and associated biotechnological interventions for the improvement of coconut (Cocos nucifera L.): a review

Towards Pathogen-Free Coconut Germplasm Exchange

Coconut (Cocos nucifera L.) is an important palm species that serves as the mainstay of several industries and contributes to the livelihoods of millions of smallholder farmers. International exchange of coconut germplasm has been undertaken for several decades to facilitate the conservation of selected varieties within global genebanks and for the distribution to farmers and scientists. In vitro systems are a convenient and an efficient method for the exchange of coconut germplasm. However, it is possible that these tissue culture systems can transfer lethal pathogens causing a threat to the importing countries. In this review, the following topics are discussed: the major disease-causing agents of concern, the various tissues that could be used for coconut germplasm exchange, and the techniques available for the detection and elimination of disease-causing agents from various transmission systems. Additionally, the lack of clear, science-backed guidelines to facilitate the exchange of in vitro coconut materials is raised, along with recommendations for future studies to ensure the safe movement of coconut germplasm without biosecurity risks.

Coconut Callus Initiation for Cell Suspension Culture

The development of a cell suspension culture system for the scaling up of coconut embryogenic callus (EC) production would drastically improve efforts to achieve the large-scale production of high-quality clonal plantlets. To date, the hard nature of coconut EC appeared to be the main constraint for developing cell suspension cultures. Hence, this study attempted to acquire friable EC through the following approaches: The manipulation of (1) medium type and subculture frequency, (2) a reduced 2,4-dichlorophenoxy acetic acid concentration during subculture, (3) the nitrate level and the ammonium-to-nitrate ratio, and the addition of amino acid mixture, (4) the addition of L-proline, and (5) the reduction of medium nutrients. Unfortunately, none of these culture conditions produced friable coconut EC. Even though friable EC was not achieved via these approaches, some of the conditions were found to influence the formation of compact EC, therefore these results are important for further studies focused on somatic embryogenesis in coconut and other species.

Primary Root Excision Induces ERF071, Which Mediates the Development of Lateral Roots in Makapuno Coconut (Cocos nucifera)

Coconut (Cocos nucifera L.) is widely recognized as one of nature's most beneficial plants. Makapuno, a special type of coconut with a soft, jelly-like endosperm, is a high-value commercial coconut and an expensive delicacy with a high cost of planting material. The embryo rescue technique is a very useful tool to support mass propagation of makapuno coconut. Nevertheless, transplanting the seedlings is a challenge due to poor root development, which results in the inability of the plant to acclimatize. In this study, primary root excision was used in makapuno to observe the effects of primary root excision on lateral root development. The overall results showed that seedlings with roots excised had a significantly higher number of lateral roots, and shoot length also increased significantly. Using de novo transcriptome assembly and differential gene expression analysis, we identified 512 differentially expressed genes in the excised and intact root samples. ERF071, encoding an ethylene-responsive transcription factor, was identified as a highly expressed gene in excised roots compared to intact roots, and was considered a candidate gene associated with lateral root formation induced by root excision in makapuno coconut. This study provides insight into the mechanism and candidate genes involved in the development of lateral roots in coconut, which may be useful for the future breeding and mass propagation of makapuno coconut through tissue culture.

Is CRISPR/Cas9 a way forward to fast-track genetic improvement in commercial palms? Prospects and limits

Commercially important palms (oil palm, coconut, and date palm) are widely grown perennial trees with tremendous commercial significance due to food, edible oil, and industrial applications. The mounting pressure on the human population further reinforces palms' importance, as they are essential crops to meet vegetable oil needs around the globe. Various conventional breeding methods are used for the genetic improvement of palms. However, adopting new technologies is crucial to accelerate breeding and satisfy the expanding population's demands. CRISPR/Cas9 is an efficient genome editing tool that can incorporate desired traits into the existing DNA of the plant without losing common traits. Recent progress in genome editing in oil palm, coconut and date palm are preliminarily introduced to potential readers. Furthermore, detailed information on available CRISPR-based genome editing and genetic transformation methods are summarized for researchers. We shed light on the possibilities of genome editing in palm crops, especially on the modification of fatty acid biosynthesis in oil palm. Moreover, the limitations in genome editing, including inadequate target gene screening due to genome complexities and low efficiency of genetic transformation, are also highlighted. The prospects of CRISPR/Cas9-based gene editing in commercial palms to improve sustainable production are also addressed in this review paper.

Improving Coconut Using Modern Breeding Technologies: Challenges and Opportunities

Coconut (Cocos nucifera L.) is a perennial palm with a wide range of distribution across tropical islands and coastlines. Multitude use of coconut by nature is important in the socio-economic fabric framework among rural smallholders in producing countries. It is a major source of income for 30 million farmers, while 60 million households rely on the coconut industry directly as farm workers and indirectly through the distribution, marketing, and processing of coconut and coconut-based products. Stagnant production, inadequate planting materials, the effects of climate change, as well as pests and diseases are among the key issues that need to be urgently addressed in the global coconut industry. Biotechnology has revolutionized conventional breeding approaches in creating genetic variation for trait improvement in a shorter period of time. In this review, we highlighted the challenges of current breeding strategies and the potential of biotechnological approaches, such as genomic-assisted breeding, next-generation sequencing (NGS)-based genotyping and genome editing tools in improving the coconut. Also, combining these technologies with high-throughput phenotyping approaches and speed breeding could speed up the rate of genetic gain in coconut breeding to solve problems that have been plaguing the industry for decades.

Fruit Biology of Coconut (Cocos nucifera L.)

Coconut (Cocos nucifera L.) is an important perennial crop adapted to a wide range of habitats. Although global coconut demand has increased sharply over the past few years, its production has been decreasing due to palm senility, as well as abiotic and biotic stresses. In fact, replanting efforts are impeded due to the lack of good quality seedlings. In vitro technologies have a great potential; however, their applications may take time to reach a commercial level. Therefore, traditional seed propagation is still critical to help meet the rising demand and its practice needs to be improved. To achieve an improved propagation via seeds, it is important to understand coconut fruit biology and its related issues. This review aims to provide a comprehensive summary of the existing knowledge on coconut fruit morpho-anatomy, germination biology, seed dispersal, distribution, fruit longevity and storage. This will help to identify gaps where future research efforts should be directed to improve traditional seed propagation.

Cloning Coconut via Somatic Embryogenesis: A Review of the Current Status and Future Prospects

Coconut [Cocos nucifera L.] is often called "the tree of life" because of its many uses in the food, beverage, medicinal, and cosmetic industries. Currently, more than 50% of the palms grown throughout the world are senile and need to be replanted immediately to ensure production levels meet the present and increasing demand for coconut products. Mass replanting will not be possible using traditional propagation methods from seed. Recent studies have indicated that in vitro cloning via somatic embryogenesis is the most promising alternative for the large-scale production of new coconut palms. This paper provides a review on the status and prospects for the application of somatic embryogenesis to mass clonal propagation of coconut.

Development of the first axillary in vitro shoot multiplication protocol for coconut palms

The coconut palm or "tree of life" is one of nature's most useful plants and the demand for its fruit is increasing. However, coconut production is threatened by ageing plantations, pests and diseases. Currently, the palm is exclusively propagated via seeds, limiting the amount of planting material. A novel micropropagation method is presented, based on axillary shoot formation. Apical meristems of in vitro coconut seedlings are cultured onto Y3 medium containing 1 µM TDZ. This induces the apical meristem to proliferate through axillary shoots in ~ 27% of the initiated explants. These axillary shoots are seen as white clumps of proliferating tissue and can be multiplied at a large scale or regenerated into rooted in vitro plantlets. This innovative micropropagation method will enable the production of disease-free, high quality in vitro plantlets, which will solve the worldwide scarcity of coconut planting material.

Somatic embryogenesis from leaf tissues of macaw palm [Acrocomia aculeata (Jacq.) Lodd. ex Mart.]

A somatic embryogenesis protocol was developed from the immature leaves of adult plants of the macaw palm. Leaf explants from different regions of the palm heart were used for callus initiation in a modified Y3 medium, supplemented with 2,4-D or Picloram at 450 μM. Calli were separated from the leaf explants at 6-, 9- and 12-month periods and transferred to a fresh culture medium of the same composition. They were multiplied for up to 120 days. Reduced concentrations of 2,4-D and Picloram were used to differentiate somatic embryos. They were then germinated in a medium without plant growth regulators. Morphological and anatomical analyses were conducted at different stages of the embryogenic process. The best results for callus induction were achieved by Picloram, when explants were maintained for up to 9 months on culture medium (64.9%). The farthest portions of the apical meristem were those that provided the biggest calli formation. The formation of the somatic embryos was observed from the calli multiplication phase. Reduction in concentrations of growth regulators failed to promote the formation of complete plants. Picloram at 450 μM promotes high callogenesis in leaf tissues of macaw palm, with a potential for somatic embryo formation.

Somatic embryogenesis as an alternative for in vitro multiplication of Butia odorata from mature zygotic embryos

Butia odorata is a palm native to southern Brazil and Uruguay, not domesticated, much appreciated for its fruits and economic potential. However, the extractivism and the difficulty of propagation have led to the decline of natural populations. The objective of this work was to prove the possibility of induction of somatic embryogenesis in B. odorata. Mature zygotic embryos were induced in two media, MS and Y3, combined with auxin 2,4-D and picloram in five concentrations (2,4-D: 0, 361.99, 452.49, 542.99 and 633.48 μM/L, picloram: 0, 50, 150, 300 and 450 μM/L). The results promising during induction with the formation of embryogenic calli and somatic embryos, however the regeneration of them was not efficient, this may be due to the occurrence of somatic embryos fused during its development. The roots were formed, but the aerial part remained molten, not completing its development. Auxin picloram and Y3 medium provided the most adequate conditions for calogenesis, formation of embryogenic callus and somatic embryos, with concentrations of 150, 300 and 450 μM/L. This is the first description of somatic embryogenesis in B. odorata that will serve as the basis for future research and adjustments of the methodology proposed here.

Dynamic changes in the expression pattern of miRNAs and associated target genes during coconut somatic embryogenesis

Genome-wide analysis of small RNAs identifies somatic embryogenesis- specific miRNAs and their targets and provides novel insights into the mechanisms governing somatic embryogenesis in coconut, a highly in vitro recalcitrant species. Coconut, a major plantation crop of the tropics is recalcitrant to in vitro culture with a very low rate of somatic embryo turnover. Clonal propagation to enhance the production of high yielding, disease-free planting material in coconut has remained a distant reality. To better understand the molecular basis of this recalcitrance and to throw light on the complex regulatory network involved in the transition of coconut somatic cells to embryogenic calli, genome-wide profiling of small RNAs from embryogenic (EC) and non-embryogenic calli (NEC) was undertaken using Illumina Hiseq 2000 platform. We have identified a total of 110 conserved miRNAs (representing 46 known miRNA families) in both types of calli. In addition, 97 novel miRNAs (48 specific to EC, 21 specific to NEC and 28 common to both the libraries) were also identified. Among the conserved miRNAs, 10 were found to be differentially expressed between NEC and EC libraries with a log2 fold change > 2 following RPM-based normalization. miR156f, miR167c, miR169a, miR319a, miR535a, and miR5179 are upregulated and miR160a, miR166a, miR171a, and miR319b are down-regulated in NEC. To confirm the differential expression pattern and their regulatory role in SE, the expression patterns of miRNAs and their putative targets were analyzed using qRT- PCR and most of the analyzed miRNA-target pairs showed inverse correlation during somatic embryogenesis. Selected targets were further validated by RNA ligase mediated rapid amplification of 5' cDNA ends (5'RLM-RACE). Our data suggest that a few conserved miRNAs and species-specific miRNAs act in concert to regulate the process of somatic embryogenesis in coconut. The results of this study provide the first overview into the regulatory landscape of somatic embryogenesis in coconut and possible strategies for fine-tuning or reprogramming to enhance somatic embryo turn over in coconut.

Phytochemical composition and antioxidant activity of coconut cotyledon

Coconut tree (Cocos nucifera L.), a perennial, monocot tree, belonging to the family Arecaceae, is distributed through the tropics. Bioactivities of coconut water, husk fiber, oil, flowers, spadix and mesocarp of coconut fruit are widely reported. However, there is no study on cotyledon of coconut. In this study, carbohydrates, proteins, lipids, phenols, flavonoids, tannins, alkaloids and antioxidants were quantified in hot and cold percolated extracts of coconut cotyledon. Further, the antioxidant activity was studied using 2,2-diphenyl-1-picrylhydrazyl (DPPH); ferric reducing antioxidant power (FRAP); ferric thiocyanate (FTC); thiobarbituric acid (TBA); nitric oxide (NO) radical scavenging and β-carotene bleaching assays. Among the secondary metabolites, only cardiac glycosides were detected. Methanolic extraction by cold percolation extracted high content of secondary metabolites and exhibited significant antioxidant activity in DPPH, FRAP, NO and β-carotene bleaching assays, with EC₅₀ of 0.12, 6.43, 16.21 and 8.09 mg/ml respectively. The chloroform extracts recorded high lipid content and scavenged the radicals in FTC (EC₅₀ 13.31 mg/ml) and TBA (EC₅₀ 9.21 mg/ml) assays. The study recommends extraction of compounds using methanol through cold percolation. The cotyledon of coconut is found to be a potent nutritive source equivalent to the endosperm.

High-Efficiency Somatic Embryogenesis from Seedlings of Koelreuteria paniculata Laxm

Research Highlights: In the current study, we established a method for plant regeneration via somatic embryogenesis (SE) in Koelreuteria paniculata Laxm. for the first time. Background and Objectives: K. paniculata is an important ornamental and medicinal plant in China. However, the plant has difficulty with asexual reproduction, which imposes a limitation on large-scale propagation. Materials and Methods: Embryogenic calluses were induced from stems of aseptic seedlings on induction media. The effects of different media types and concentrations of N6-benzyladenine (BA), α-naphthaleneacetic acid (NAA), and 2,4-dichlorophenoxyacetic acid (2,4-D) on callus induction were examined. Embryogenic calluses were then transferred to Driver-Kuniyuki Walnut (DKW) media containing NAA (0.1–0.2 mg L−1) or 2,4-D (0.5–2.0 mg L−1) to develop somatic embryos. Cotyledon embryos were cultured on DKW media containing NAA (0.1–0.2 mg L−1) until maturation, and were then transferred to 1/2 DKW medium supplemented with 1.0 mg L−1 indole-3-butyric acid (IBA) to produce complete plants. The effects of IBA and NAA on rhizogenesis were then examined by clonal culture. Results: The maximum callus induction frequency (80.25%) was obtained on DKW medium supplemented by 0.5 mg L−1 BA, 0.25 mg L−1 NAA, and 1.5 mg L−1 2,4-D. NAA had a more pronounced effect on somatic embryo growth than did 2,4-D, with a maximum SE frequency (54.75%) observed with 0.1 mg L−1 NAA added to DKW medium. For clonal culture, the highest rooting rate (52%) was observed on 1/4 DKW medium containing 1.5 mg L−1 IBA. Histology studies confirmed the presence of embryogenic calluses and somatic embryos in different stages. Conclusions: This protocol provides a novel method for large-scale propagation of K. paniculata, and creates opportunities for genetic engineering in this species.

Addition of ionophore A23187 increases the efficiency of Cocos nucifera somatic embryogenesis

The present study reports the effect of treatment of coconut embryogenic structure explants (derived from embryogenic callus) with the calcium ionophore A23187 (0, 1, 5, 10 µM) to promote somatic embryogenesis under in vitro conditions. The results showed no significant increase in the percentage of explants forming embryogenic callus, but with 1 µM ionophore there were significant increases in the formation of embryogenic structures per callus (2.8-fold), of somatic embryos per callus (1.5-fold) and also a greater absolute number (1.5-fold) of developing plantlets per callus. The ionophore treatment also promoted a change of pattern of the expression of the CnSERK gene during embryogenic callus formation. It is proposed that if the use of ionophore A23187 treatment is coupled with an embryogenic callus multiplication process there could be a potentially greater increase in the efficiency of the formation of somatic embryos and plantlets of coconut.

Biology, propagation and utilization of elite coconut varieties (makapuno and aromatics)

Coconut farming is not only a vital agricultural industry for all tropical countries possessing humid coasts and lowlands, but is also a robust income provider for millions of smallholder farmers worldwide. However, due to its longevity, the security of production of this crop suffers significantly from episodes of natural disasters, including cyclone and tsunami, devastating pest and disease outbreaks, while also affected by price competition for the principal products, especially the oil. In order to reduce these pressures, high-value coconut varieties (makapuno and aromatics) have been introduced in some regions, on a limited scale, but with positive outcomes. Even though these two varieties produce fruit with delicious solid or flavoursome liquid endosperm, their distinct biochemical and cellular features unfortunately prevent their in situ germination. In fact, embryo rescue and culture have been developed historically to nurture the embryo under in vitro conditions, enabling effective propagation. In an attempt to provide a comprehensive review featuring these elite coconut varieties, this paper firstly introduces their food values and nutritional qualities, and then discusses the present knowledge of their biology and genetics. Further possibilities for coconut in general are also highlighted, through the use of advanced tissue culture techniques and efficient seedling management for sustainable production of these highly distinct and commercially attractive varieties of coconut.

Coconut Lethal Yellowing Diseases: A Phytoplasma Threat to Palms of Global Economic and Social Significance

The recent discovery of Bogia coconut syndrome in Papua New Guinea (PNG) is the first report of a lethal yellowing disease (LYD) in Oceania. Numerous outbreaks of LYDs of coconut have been recorded in the Caribbean and Africa since the late Nineteenth century and have caused the death of millions of palms across several continents during the Twentieth century. Despite the severity of economic losses, it was only in the 1970s that the causes of LYDs were identified as phytoplasmas, a group of insect-transmitted bacteria associated with diseases in many other economically important crop species. Since the development of polymerase chain reaction (PCR) technology, knowledge of LYDs epidemiology, ecology and vectors has grown rapidly. There is no economically viable treatment for LYDs and vector-based management is hampered by the fact that vectors have been positively identified in very few cases despite many attempted transmission trials. Some varieties and hybrids of coconut palm are known to be less susceptible to LYD but none are completely resistant. Optimal and current management of LYD is through strict quarantine, prompt detection and destruction of symptomatic palms, and replanting with less susceptible varieties or crop species. Advances in technology such as loop mediated isothermal amplification (LAMP) for detection and tracking of phytoplasma DNA in plants and insects, remote sensing for identifying symptomatic palms, and the advent of clustered regularly interspaced short palindromic repeats (CRISPR)-based tools for gene editing and plant breeding are likely to allow rapid progress in taxonomy as well as understanding and managing LYD phytoplasma pathosystems.