Micropropagation in the Twenty-First Century

Plant Tissue Culture: Industrial Relevance and Future Directions

Plant tissue culture has evolved in the last decades with several types of cultures being developed to promote a more sustainable food production system. Moreover, these cultures can be applied for the production of relevant metabolites with medicinal potential, thus contributing to nutrition and healthcare. Importantly, plant micropropagation has enabled agricultural expansion and tissue culture has emerged as a promising production alternative for several plants and their metabolites in the food, cosmetic, and pharmaceutical industries. Plant tissue cultures present several advantages over conventional propagation techniques as they are season independent, enabling a continuous supply of the plants/compounds of interest, with the guarantee of high phytosanitary quality. In addition, genetic uniformity is generally maintained, thus reducing chemical variability that can compromise safety and efficacy. Nevertheless, despite their undeniable potential, with many researchers focusing on new strategies to improve production yield in cell cultures, such as with the use of elicitors or resorting to metabolomics engineering, an effective and lucrative large-scale production has yet to be obtained. Indeed, only a few compounds with market value are produced in this regard and several limitations such as contaminations, low culture yield and production costs still need to be overcome in order to take advantage of the full potential of these techniques.

Woodfordia fruticosa (L.) Kurz: in vitro biotechnological interventions and perspectives

Woodfordia fruticosa (L.) Kurz is a woody medicinal shrub (Lythraceae) commonly known as the "fire flame bush." W. fruticosa plant parts either alone or whole plant have a long history of recommended use in the Indian medicine systems of Ayurveda, Unani, and Siddha (AUS). This plant is prominently known for its pharmacological properties, viz., antimicrobial, anti-inflammatory, anti-peptic ulcer, hepatoprotective, immunomodulatory, antitumor, cardioprotective, analgesic, and wound healing activities. Its important phyto-constituents, woodfordin C, woodfordin I, oenothein B, and isoschimacoalin-A, exhibit in vitro or in vivo physiological activities beneficial to human health. As the plant is a rich storehouse of phyto-constituents, it is indiscriminately used in its wild habitats. Moreover, due to very poor seed viability and difficult-to-root qualities, it is placed under IUCN list of endangered plant species. For W. fruticosa, biomass production or to its conservation by in vitro regeneration is the best feasible alternative. Till date, only few important in vitro regeneration methods are reported in W. fruticosa. ISSR molecular markers based clonal fidelity and Agrobacterium-mediated transformation has been demonstrated, indicating that W. fruticosa is amenable to genetic manipulation and genome editing studies. This review presents concise summary of updated reports on W. fruticosa phyto-constituents and their biological activities, while a critical appraisal of biotechnological interventions, shortcomings, and factors influencing such potential areas success was presented. The unexplored gaps addressed here are relevant for W. fruticosa scientific innovations yet to come. In this paper, for the first time, we have presented a simple and reproducible protocol for synthetic seed production in W. fruticosa. KEY POINTS: • Critical and updated records on W. fruticosa phytochemistry and its activities • In vitro propagation and elicitation of secondary metabolites in W. fruticosa • Key bottlenecks, in vitro flowering, value addition, and outlook in W. fruticosa.

A Common Molecular Signature Indicates the Pre-Meristematic State of Plant Calli

In response to different degrees of mechanical injury, certain plant cells re-enter the division cycle to provide cells for tissue replenishment, tissue rejoining, de novo organ formation, and/or wound healing. The intermediate tissue formed by the dividing cells is called a callus. Callus formation can also be induced artificially in vitro by wounding and/or hormone (auxin and cytokinin) treatments. The callus tissue can be maintained in culture, providing starting material for de novo organ or embryo regeneration and thus serving as the basis for many plant biotechnology applications. Due to the biotechnological importance of callus cultures and the scientific interest in the developmental flexibility of somatic plant cells, the initial molecular steps of callus formation have been studied in detail. It was revealed that callus initiation can follow various ways, depending on the organ from which it develops and the inducer, but they converge on a seemingly identical tissue. It is not known, however, if callus is indeed a special tissue with a defined gene expression signature, whether it is a malformed meristem, or a mass of so-called "undifferentiated" cells, as is mostly believed. In this paper, I review the various mechanisms of plant regeneration that may converge on callus initiation. I discuss the role of plant hormones in the detour of callus formation from normal development. Finally, I compare various Arabidopsis gene expression datasets obtained a few days, two weeks, or several years after callus induction and identify 21 genes, including genes of key transcription factors controlling cell division and differentiation in meristematic regions, which were upregulated in all investigated callus samples. I summarize the information available on all 21 genes that point to the pre-meristematic nature of callus tissues underlying their wide regeneration potential.

Successful In Vitro Shoot Multiplication of Quercus robur L. Trees Aged up to 800 Years

The conservation of the genetic resources of old trees is crucial to their ecological role but is extremely difficult, especially for oak species (Quercus spp.) displaying recalcitrance in seed and vegetative propagation methods. Our study aimed to assess the regenerative potential of Quercus robur trees of different ages (up to 800 years) during micropropagation. We also aimed to determine how in vitro conditions can influence in vitro regeneration responses. Lignified branches collected from 67 selected trees were cultivated ex vitro in culture pots at 25 °C to obtain epicormic shoots (explant sources). The explants were cultivated on an agar medium supplemented with 0.8 mg L^-1 6-benzylaminopurine (BAP) for at least 21 months. In a second experiment, two different shoot multiplication conditions (temporary immersion-RITA^® bioreactor and agar medium) and two culture medium formulations (Woody Plant Medium and modified Quoirin and Lepoivre medium) were tested. The results showed that the mean length of the epicormic shoots obtained in a pot culture was a function of donor age and was similar among the group of younger trees (ca. 20-200 years), and varied between older trees (ca. 300-800 years). The efficiency of in vitro shoot multiplication strictly depended on the genotype. A sustainable in vitro culture (defined as survival after 6 months) was only possible for half of the tested old donor trees, even when they survived the first month of in vitro growth. A continuous monthly increase in the number of in vitro cultured shoots was reported in younger oaks and in some old oaks. We found a significant effect of the culture system and the macro- and micronutrient composition on in vitro shoot growth. This is the first report demonstrating that the in vitro culture can be successfully applied to the propagation of even 800-year-old pedunculate oak trees.

Low-cost and automated phenotyping system "Phenomenon" for multi-sensor in situ monitoring in plant in vitro culture

BACKGROUND

The current development of sensor technologies towards ever more cost-effective and powerful systems is steadily increasing the application of low-cost sensors in different horticultural sectors. In plant in vitro culture, as a fundamental technique for plant breeding and plant propagation, the majority of evaluation methods to describe the performance of these cultures are based on destructive approaches, limiting data to unique endpoint measurements. Therefore, a non-destructive phenotyping system capable of automated, continuous and objective quantification of in vitro plant traits is desirable.

RESULTS

An automated low-cost multi-sensor system acquiring phenotypic data of plant in vitro cultures was developed and evaluated. Unique hardware and software components were selected to construct a xyz-scanning system with an adequate accuracy for consistent data acquisition. Relevant plant growth predictors, such as projected area of explants and average canopy height were determined employing multi-sensory imaging and various developmental processes could be monitored and documented. The validation of the RGB image segmentation pipeline using a random forest classifier revealed very strong correlation with manual pixel annotation. Depth imaging by a laser distance sensor of plant in vitro cultures enabled the description of the dynamic behavior of the average canopy height, the maximum plant height, but also the culture media height and volume. Projected plant area in depth data by RANSAC (random sample consensus) segmentation approach well matched the projected plant area by RGB image processing pipeline. In addition, a successful proof of concept for in situ spectral fluorescence monitoring was achieved and challenges of thermal imaging were documented. Potential use cases for the digital quantification of key performance parameters in research and commercial application are discussed.

CONCLUSION

The technical realization of "Phenomenon" allows phenotyping of plant in vitro cultures under highly challenging conditions and enables multi-sensory monitoring through closed vessels, ensuring the aseptic status of the cultures. Automated sensor application in plant tissue culture promises great potential for a non-destructive growth analysis enhancing commercial propagation as well as enabling research with novel digital parameters recorded over time.

Orchid Micropropagation Using Conventional Semi-Solid and Temporary Immersion Systems: A Review

Orchids, with their astonishingly stunning flowers, dominate the international floricultural market. They are considered prized assets for commercial applications in pharmaceutical and floricultural industries as they possess high therapeutic properties and superior ornamental values. The alarming depletion of orchid resources due to excessive unregulated commercial collection and mass habitat destruction makes orchid conservation measures an extreme priority. Conventional propagation methods cannot produce adequate number of orchids, which would meet the requirement of these ornamental plants for commercial and conservational purposes. In vitro orchid propagation using semi-solid media offers an outstanding prospect of rapidly producing quality plants on a large scale. However, the semi-solid (SS) system has shortcomings with low multiplication rates and high production costs. Orchid micropropagation using a temporary immersion system (TIS) overcomes the limitations of the SS system by reducing production costs and making scaleup and full automation possible for mass plant production. The current review highlights different aspects of in vitro orchid propagation using SS and TIS and their benefits and drawbacks on rapid plant generation.

Chrysanthemum: A Comprehensive Review on Recent Developments on In Vitro Regeneration

Chrysanthemum is a flowering plant grown worldwide and is one of the most popular ornamental plants. Chrysanthemums are usually cultivated using root suckers and shoot cuttings. This conventional technique is relatively slow. In addition, as cuttings are gained regularly from mother plants, there is a chance of viral infection and degeneration, which raises the production cost. The hurdles mentioned above have been managed by applying in vitro propagation techniques, which can enhance reproduction rates through in vitro culture and use very small explants, which are impossible with the conventional approach. Usually, it is difficult to get true-to-type plants as the parents with good quality, but clonal propagation of a designated elite species makes it possible. Hence, this review highlights recent studies of the in vitro propagation of Chrysanthemum included; the appropriate explant sources, medium compositions, alternative disinfection of culture media, plant growth regulators (PGRs), different mutagenesis applications, acclimatization efficiency, and alternative light sources to overcome the shortcomings of conventional propagation techniques.

Genetic diversity and volatile oil components variation in Achillea fragrantissima wild accessions and their regenerated genotypes

Background

Wild medicinal plants are suffering natural environmental stresses and habitat destruction. The genetic diversity evaluation of wild accessions and their in vitro raised genotypes using molecular markers, as well as the estimation of substances of pharmaceutical value in wild plants and their regenerated genotypes are convenient approaches to test the genetic fidelity of regenerated plants as a source of substances of pharmaceutical value. In this study, the genetic diversity of 12 accessions of the medicinal plant Achillea fragrantissima, representing five sites in the mountains of South Sinai, Egypt, were estimated by the inter simple sequence repeats (ISSR) fingerprinting and their volatile oil components were identified using gas chromatography-mass spectrometry (GC-MS) analysis. The same accessions were regenerated in vitro and the genetic diversity and volatile oil components of propagated genotypes were determined and compared to their wild parents.

Results

Clustering and principal component analyses indicated that the wild accessions and their regenerated genotypes were genetically differentiated, but the regenerated plants are relatively more diverse compared to their wild parents. However, genetic variation between wild accessions is inherited to their in vitro propagated genotypes indicating genotypic differentiation of the examined accessions. The number of volatile oil compounds in the wild A. fragrantissima accessions was 31 compounds while in the in vitro propagated plants only 24 compounds were detected. Four major compounds are common to both wild and regenerated plants; these are artemisia ketone, alpha-thujone, dodecane, and piperitone.

Conclusions

Genome profiling and essential oil components analysis showed variations in A. fragrantissima accessions from different populations. Genetic differences between wild and regenerated genotypes were analyzed and validated with the final conclusion that in vitro conditions elicited higher genetic variation that is associated with reduced amount and diversity in the essential oil components.

Supplementary Information

The online version contains supplementary material available at 10.1186/s43141-021-00267-3.

A novel temporary immersion bioreactor system for large scale multiplication of banana (Rasthali AAB-Silk)

Musa sp. cultivar Rasthali (Silk AAB) is a choice variety of the Asian sub-continent. Its production and sustenance are threatened by Fusarium wilt, which affects the livelihoods of small and marginal farmers. The use of quality planting material is one of the strategies to manage the disease. Availability of quality planting material for varieties other than Grand Naine is limited. Large-scale micropropagation using existing technologies is laborious and expensive. Temporary immersion bioreactor system is emerging as a potential advancement in the micropropagation industry. In this study, a cost-effective temporary immersion bioreactor (TIB) system has been developed and an efficient micropropagation method has been standardized. Explants cultured in TIB with 250 ml of culture medium in a 2-min immersion frequency of 6 h were found to be efficient for shoot proliferation and rooting. Its efficacy has been compared with the semisolid culture method. At the end of the 6th subculture, 1496 ± 110 shoots per explant were obtained in TIB. Chlorophyll, carotenoid, stomatal index, and the number of closed stomata were examined to determine the physiological functions of the plants grown in TIB and compared with semisolid grown plantlets. Plantlets grown in TIB were genetically stable and were confirmed using inter-simple sequence repeat (ISSR) markers. The multiplication of shoots in TIB was 2.7-fold higher than the semisolid culture method, which is suitable for large-scale production of planting material for commercial applications.

Genetic Stability, Phenolic, Flavonoid, Ferulic Acid Contents, and Antioxidant Activity of Micropropagated Lycium schweinfurthii Plants

Lycium schweinfurthii is a Mediterranean wild shrub rich in plant secondary metabolites. In vitro propagation of this plant may support the production of valuable dietary supplements for humanity, introduction of it to the world market, and opportunities for further studies. The presented study aimed to introduce an efficient and reproducible protocol for in vitro micropropagation of L. schweinfurthii and assess the genetic stability of micropropagated plants (MiPs) as well as to estimate phenolic, flavonoid, ferulic acid contents, and the antioxidant activity in leaves of micropropagated plants. Two DNA-based techniques, random amplified polymorphic DNA (RAPD) and inter-simple sequence repeats (ISSR), and one biochemical technique, sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), were used to assess the genetic stability in MiPs. Spectrophotometric analysis was performed to estimate total phenolic and flavonoid contents and antioxidant activity of MiPs leaves, while ferulic acid content was estimated using high-performance thin-layer chromatography (HPTLC). Sufficient shoot proliferation was achieved at MS (Murashige and Skoog) medium supplemented with 0.4 mg L^-1 kinetin and rooted successfully on half-strength MS medium fortified with 0.4 mg L^-1 Indole-3-butyric acid (IBA). The Jaccard's similarity coefficients detected in MiPs reached 52%, 55%, and 82% in the RAPD, ISSR, and SDS-PAGE analyses, respectively. In the dried leaves of MiPs, the phenolic, flavonoid, and ferulic acid contents of 11.53 mg gallic acid equivalent, 12.99 mg catechin equivalent, and 45.52 mg were estimated per gram, respectively. However, an IC₅₀ of 0.43, and 1.99 mg mL^-1 of MiP dried leaves' methanolic extract was required to scavenge half of the DPPH, and ABTS free radicals, respectively. The study presented a successful protocol for in vitro propagation of a valued promising plant source of phenolic compounds.

Hemidesmus indicus L. Br.: critical assessment of in vitro biotechnological advancements and perspectives

Hemidesmus indicus (L.) R. Br. ex Schult., commonly known as Indian sarsaparilla or Anantamul, is an ethnopharmacologically important medicinal plant from the family Apocynaceae. Biosynthesis of an array of bioactive compounds such as methoxybenzaldehydes and their derivatives has been accountable for its intensive medicinal attributes. Low seed setting, over-exploitation from natural habitat and a need for selection and preservation of elite germplasms yielding higher levels of bioactive compounds have led researchers to enquire the standardization of micropropagation techniques and quantitative estimation of phytochemicals from H. indicus. The present work aims to have a comprehensive account on micropropagation in H. indicus, a comparative estimation of biotic and abiotic factors, viz. elicitors and precursors playing a crucial role in H. indicus tissue culture, a critical appraisal of applied protocols to nullify embellished claims and a discussion on future perspectives. The review also highlights the comparative effect of different plant growth regulators as well as their combined role in rhizogenesis, callogenesis, base callus formation, callusing and somatic embryo-mediated indirect organogenesis, outcome of explant selection, contribution of abiotic (temperature, photoperiod, moisture, inorganic molecules) factors, role of carbon source, application of sterilization techniques and associated success rates, role of factors essential for acclimatization and secondary metabolite production, synthetic seed production and genetic transformation techniques. The compiled information along with individual study details, respective outcome and variability of reports will aid in proper assessment of cited standardized tissue culture protocols in H. indicus especially in relation to secondary metabolite production. Moreover, the assessment of applicability of different methodologies in this aspect will aid in the selection of high-yielding germplasms or chemotypes which is not only profitable for industrial application but also important for basic and applied preclinical and clinical research studies. KEY POINTS: • Critical and updated assessment on in vitro biotechnology in Hemidesmus indicus. • Biotechnological advancement via Agrobacterium-mediated transformation. • Key shortcomings and future research directions Graphical abstract Effect of biotic and abiotic factors on Micropropagation and secondary metabolite synthesis in Hemidesmus indicus.

Shoot tip necrosis of in vitro plant cultures: a reappraisal of possible causes and solutions

Shoot tip necrosis is a physiological condition that negatively impacts the growth and development of in vitro plant shoot cultures across a wide range of species. Shoot tip necrosis is a physiological condition and disorder that can arise in plantlets or shoots in vitro that results in death of the shoot tip. This condition, which can spread basipetally and affect the emergence of axillary shoots from buds lower down the stem, is due to the cessation of apical dominance. STN can occur at both shoot multiplication and rooting stages. One of the most common factors that cause STN is nutrient deficiency or imbalance. Moreover, the presence or absence of plant growth regulators (auxins or cytokinins) at specific developmental stages may impact STN. The cytokinin to auxin ratio within an in vitro plant can be modified by varying the concentration of cytokinins used in the culture medium. The supply of nutrients to in vitro shoots or plantlets might also affect their hormonal balance, thus modifying the occurrence of STN. High relative humidity within culture vessels and hyperhydricity are associated with STN. An adequate supply of calcium as the divalent cation (Ca²⁺) can hinder STN by inhibiting the accumulation of phenolic compounds and thus programmed cell death. Moreover, the level of Ca²⁺ affects auxin transport and ethylene production, and higher ethylene production, which can occur as a result of high relative humidity in or poor ventilation of the in vitro culture vessel, induces STN. High relative humidity can decrease the mobility of Ca²⁺ within a plant, resulting in Ca²⁺ deficiency and STN. STN of in vitro shoots or plantlets can be halted or reversed by altering the basal medium, mainly the concentration of Ca²⁺, adjusting the levels of auxins or cytokinins, or modifying culture conditions. This review examines the literature related to STN, seeks to discover the associated factors and relations between them, proposes practical solutions, and attempts to better understand the mechanism(s) underlying this condition in vitro.

Transcriptome Profiling Provides Molecular Insights into Auxin-Induced Adventitious Root Formation in Sugarcane (Saccharum spp. Interspecific Hybrids) Microshoots

Adventitious root (AR) formation was enhanced following the treatment of sugarcane microshoots with indole-3-butyric acid (IBA) and 1-naphthalene acetic acid (NAA) combined, suggesting that auxin is a positive regulator of sugarcane microshoot AR formation. The transcriptome profile identified 1737 and 1268 differentially expressed genes (DEGs) in the basal tissues (5 mm) of sugarcane microshoots treated with IBA+NAA compared to nontreated control on the 3rd and 7th days post-auxin or water treatment (days post-treatment—dpt), respectively. To understand the molecular changes, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed. This analysis showed that DEGs associated with the pathways were associated with plant hormone signaling, flavonoid and phenylpropanoid biosyntheses, cell cycle, and cell wall modification, and transcription factors could be involved in sugarcane microshoot AR formation. Furthermore, qRT–PCR analysis was used to validate the expression patterns of nine genes associated with root formation and growth, and the results were consistent with the RNA-seq results. Finally, a hypothetical hormonal regulatory working model of sugarcane microshoot AR formation is proposed. Our results provide valuable insights into the molecular processes associated with auxin-induced AR formation in sugarcane.

Micropropagation of Hardy Hibiscus (Hibiscus Moscheutos L.)

Hardy hibiscus (Hibiscus x moscheutos L.) is gaining popularity as a landscape plant and the accelerated increase in the volume of planting material is of particular relevance. The work investigated the effect of the concentration of 6-BAP (0.5, 1.0, 1.5 mg / L) on the shoot formation of hibiscus of the following varieties: cv. Cranberry Crush, Fantasia, Fireball, Jazzberry Jam. It was revealed that the concentration of 6-BAP 0.5 mg / L is optimal for the proliferation of microshoots of Hibiscus x moscheutos L. With the increase in the concentration of 6-BAP, the number of formed microshoots decreases.