First Report of 'Candidatus Phytoplasma asteris'-related Strain (16SrI group) Associated with Small Leaves, Leaf Yellowing, and Shoot Proliferation of Morning Glory (Ipomoea biflora) in Taiwan

Ipomoea biflora L., commonly known as morning glory, is an herbaceous vine plant in the Convolvulaceae family and is widespread at low elevations in Taiwan and other East Asian countries. In September 2023, six I. biflora plants exhibiting small leaves, leaf yellowing, and shoot proliferation were observed in a vacant lot in Taiwan Agricultural Research Institute (TARI), Wufeng District, Taichung, Taiwan, representing 100% disease incidence in the area. All the symptomatic morning glory climbed onto Murraya paniculata L. (common jasmine orange) which however showed no similar symptoms. The total DNA (two samples for each plant) from leaf tissues of three symptomatic morning glory plants, two asymptomatic morning glory plants, and one asymptomatic common jasmine orange was isolated by the CTAB method (Fulton et al. 1995) and used for PCR with the universal primers, P1 (Deng and Hiruki 1991)/P7 (Schneider et al. 1995), to amplify a fragment containing partial 16S rDNA. Expected 1.8-kb bands were amplified from DNA extracted from all symptomatic plants, whereas no PCR product was detected from that of the asymptomatic I.biflora and M. paniculata plants. Six PCR products were cloned and sequenced in the Biotechnology Center DNA-sequencing facility at National Chung Hsing University, and one representative sequence was selected and deposited in GenBank. BLAST analysis revealed that the obtained 16S rDNA sequence (PP230905) shared 99.92% identity with the following phytoplasma strains: rapeseed phyllody phytoplasma (CP055264), plumbago auriculata leaf yellowing phytoplasma (MN239503), and aster yellows phytoplasma (MK992774), which all belong to the 16SrI subgroup. The query 16S rDNA sequence shares 99.84% identity with that of the 'Candidatus Phytoplasma asteris' reference strain (M30790), suggesting that the phytoplasma is a 'Ca. Phytoplasma asteris'-related strain. A virtual restriction fragment length polymorphism (RFLP) analysis was conducted using iPhyClassifier tool (Zhao et al. 2009), and the pattern derived from the 16S rDNA fragment of the I. biflora phytoplasma was identical (similarity coefficient 1.00) to the reference pattern of 16SrI, subgroup B (onion yellows phytoplasma OY-M; AP006628). Six total DNA samples from symptomatic plants were used as templates to amplify 842 bp secA sequences with SecAfor1 and SecArev3 primers (Hodgetts et al. 2008), and one representative sequence was deposited in GenBank. The partial secA sequence (PP263636) showed 98.22% identity with that of Trema levigatum witches'-broom phytoplasma (MW032212) that also belongs to the 16SrI group (Wan et al. 2021). Phylogenetic analysis of both 16S rDNA and secA confirmed I. biflora phytoplasma as 16SrI, subgroup B. Taken together, we concluded that the morning glory phytoplasma in this study was a 'Ca. Phytoplasma asteris'-related strain belonging to the 16SrI group. To the best of our knowledge, this is the first report of a phytoplasma-infected I. biflora in Taiwan, suggesting morning glory as a new natural host of 16SrI phytoplasmas, alongside other plants like roselle and citrus (Tseng et al. 2014; Feng et al. 2015).

Direct Organogenesis of Citrus Cultivars from Shoot Tip Nodal Segments

BACKGROUND

Citrus cultivar improvement via conventional breeding strategies is impeded by factors related to its reproductive biology. The orange is a hybrid between pomelo (Citrus maxima) and mandarin (Citrus reticulata). Among various orange cultivars, Valencia oranges have a bit of bitter tang mixed in with their sweetness, as Navel oranges are, the most widely cultivated citrus species, quite sweeter, and also don't contain any seeds. Tangelo mandarin orange cultivar is a hybrid of C. reticulata × C. maxima or × C. paradisi.

OBJECTIVE

The present study was undertaken to optimize the hormonal composition of the media with regard to plant growth regulators for in vitro propagation of sweet orange cultivars from nodal segment explants.

METHODS

The nodal segment explants were collected from three citrus cultivars, Washington Navel, Valencia and Tangelo. Murashige and Skoog (MS) medium supplemented with sucrose and different concentrations of growth regulators was used for shoot proliferation and root induction, and the optimum medium composition was assessed. The patent for Citrus Tissue Culture was obtained from the Office of Research Affairs, Haramaya University.

RESULTS

The results indicate that the highest shoot response was recorded for Washington's navel with maximum shoot proliferation rate (99.75%), shoot number per explant (1.76), shoot length (10.70 cm), leaf number per explants (3.54) after three weeks of culture. In all experiments, no growth was observed for the basal MS medium. Phytohormone combinations of IAA (1.2 mg/L) and kinetin (2.0 mg/L) were found to be the best for shoot proliferation. Among the cultivars, there were significant differences for the highest rooting rate (81.255), root number (2.22), and root length (2.95 cm) variables for Washington Navel. The lowest rooting rate (48.45%), root number (1.47) and root length (2.26 cm) were observed for Valencia. The highest rooting rate (84.90%), root number per microshoot (2.22) and root length (3.05 cm) was on MS medium supplemented with 1.5 mg/L NAA.

CONCLUSION

A comparison of different concentrations of IAA and NAA on root induction of microshoots from nodal segments of citrus cultivars demonstrated NAA was a more effective hormone than IAA.

The Cytokinins BAP and 2-iP Modulate Different Molecular Mechanisms on Shoot Proliferation and Root Development in Lemongrass (Cymbopogon citratus)

The known activities of cytokinins (CKs) are promoting shoot multiplication, root growth inhibition, and delaying senescence. 6-Benzylaminopurine (BAP) has been the most effective CK to induce shoot proliferation in cereal and grasses. Previously, we reported that in lemongrass (Cymbopogon citratus) micropropagation, BAP 10 µM induces high shoot proliferation, while the natural CK 6-(γ,γ-Dimethylallylamino)purine (2-iP) 10 µM shows less pronounced effects and developed rooting. To understand the molecular mechanisms involved, we perform a protein-protein interaction (PPI) network based on the genes of Brachypodium distachyon involved in shoot proliferation/repression, cell cycle, stem cell maintenance, auxin response factors, and CK signaling to analyze the molecular mechanisms in BAP versus 2-iP plants. A different pattern of gene expression was observed between BAP- versus 2-iP-treated plants. In shoots derived from BAP, we found upregulated genes that have already been demonstrated to be involved in de novo shoot proliferation development in several plant species; CK receptors (AHK3, ARR1), stem cell maintenance (STM, REV and CLV3), cell cycle regulation (CDKA-CYCD3 complex), as well as the auxin response factor (ARF5) and CK metabolism (CKX1). In contrast, in the 2-iP culture medium, there was an upregulation of genes involved in shoot repression (BRC1, MAX3), ARR4, a type A-response regulator (RR), and auxin metabolism (SHY2).

Exploring Genetic and Epigenetic Changes in Lingonberry Using Molecular Markers: Implications for Clonal Propagation

Lingonberry (Vaccinium vitis-idaea L.) is an important and valuable horticultural crop due to its high antioxidant properties. Plant tissue culture is an advanced propagation system employed in horticultural crops. However, the progeny derived using this technique may not be true-to-type. In order to obtain the maximum return of any agricultural enterprise, uniformity of planting materials is necessary, which sometimes is not achieved due to genetic and epigenetic instabilities under in vitro culture. Therefore, we analyzed morphological traits and genetic and epigenetic variations under tissue-culture and greenhouse conditions in lingonberry using molecular markers. Leaf length and leaf width under greenhouse conditions and shoot number per explant, shoot height and shoot vigor under in vitro conditions were higher in hybrid H1 compared to the cultivar Erntedank. Clonal fidelity study using one expressed sequence tag (EST)-polymerase chain reaction (PCR), five EST-simple sequence repeat (SSR) and six genomic (G)-SSR markers revealed monomorphic bands in micropropagated shoots and plants in lingonberry hybrid H1 and cultivar Erntedank conforming genetic integrity. Epigenetic variation was studied by quantifying cytosine methylation using a methylation-sensitive amplification polymorphism (MSAP) technique. DNA methylation ranged from 32% in greenhouse-grown hybrid H1 to 44% in cultivar Erntedank under a tissue culture system. Although total methylation was higher in in vitro grown shoots, fully methylated bands were observed more in the greenhouse-grown plants. On the contrary, hemimethylated DNA bands were more prominent in tissue culture conditions as compared to the greenhouse-grown plants. The study conclude that lingonberry maintains its genetic integrity but undergoes variable epigenetic changes during in vitro and ex vitro conditions.

Effect of Plant Age, Temperature, and Vector Load on 'Candidatus Liberibacter solanacearum' in Planta Titer and Shoot Proliferation Symptoms in Carrot

A decade ago, shoot proliferation symptoms (i.e., witches' broom) in carrots were believed to be the cause of 'Candidatus Phytoplasma' and Spiroplasma infection, yet in recent years this association appeared to have weakened, and a closer association was found with the yet-unculturable, psyllid-transmitted Gram-negative bacterium 'Candidatus Liberibacter solanacearum'. In Israel, carrots are grown throughout the year, yet shoot proliferation symptoms tend to appear only in mature plants and mostly in late spring to early summer. We hypothesized that factors such as plant age, temperature, and vector load, which vary during the year, have a critical effect on symptom development and examined these factors under controlled conditions. Here we show that young carrot seedlings are as prone as older plants to develop shoot proliferation symptoms after 'Ca. L. solanacearum' inoculation. Surprisingly, we found that the local 'Ca. L. solanacearum' haplotype was extremely sensitive to constant temperature of 30°C, which led to a significant reduction in bacterial growth and symptom development compared with 18°C, which was very conducive to symptom development. We have also found that inoculations with 10 or 20 psyllids per plant results in faster symptom development compared with inoculations with two psyllids per plant; however, the difference between vector loads in disease progress rate was not significant. These data provide important insights to the effects of plant age, growth temperature, and vector load on 'Ca. L. solanacearum' and its associated symptoms and further strengthen the notion that 'Ca. L. solanacearum' is the main responsible agent for carrot witches' broom in Israel.[Formula: see text] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

Protocols for Adventitious Regeneration of Amelanchier alnifolia var. cusickii and Lonicera kamtschatica 'Jugana'

The aim of this work was to assess the regeneration capacity of Amelanchier alnifolia var. cusickii and Lonicera kamtschatica cv. ‘Jugana’ from different types of explants under various hormonal treatments. The whole leaves, petioles, and internodal segments of in vitro plants were examined as explants. Several plant growth regulators (cytokinins and auxins) were evaluated for their ability to induce adventitious regeneration. Direct and indirect organogenesis was achieved under certain culture conditions in both species. The frequency of shoot regeneration was strongly dependent on concentrations of plant growth regulators in the induction media (L.kamtschatica ‘Jugana’) or concentrations of plant growth regulators in the induction media and type of explant (A. alnifolia var. cusickii). Results showed that leaves were not suitable explants for A. alnifolia var. cusickii. Both species were able to regenerate shoots from internodal segments and petioles. The highest induction of shoots was obtained on Murashige and Skoog (MS) medium enriched with 2 mg/L thidiazuron (TDZ) and 0.5 mg/L indole-3-butyric acid (IBA) for Amelanchier alnifolia and with 1 mg/L TDZ and 0.2 mg/L indole-3-acetic acid (IAA) for L. kamtschatica ‘Jugana’. Obtained adventitious shoots were further proliferated in order to investigate their multiplication capacity. The multiplication of shoots was successful in all cultivars, with the best results reported in A. alnifolia var. cusickii (7.07 shoots/explant on average).

Applications of Cytokinins in Horticultural Fruit Crops: Trends and Future Prospects

Cytokinins (CKs) are a chemically diverse class of plant growth regulators, exhibiting wide-ranging actions on plant growth and development, hence their exploitation in agriculture for crop improvement and management. Their coordinated regulatory effects and cross-talk interactions with other phytohormones and signaling networks are highly sophisticated, eliciting and controlling varied biological processes at the cellular to organismal levels. In this review, we briefly introduce the mode of action and general molecular biological effects of naturally occurring CKs before highlighting the great variability in the response of fruit crops to CK-based innovations. We present a comprehensive compilation of research linked to the application of CKs in non-model crop species in different phases of fruit production and management. By doing so, it is clear that the effects of CKs on fruit set, development, maturation, and ripening are not necessarily generic, even for cultivars within the same species, illustrating the magnitude of yet unknown intricate biochemical and genetic mechanisms regulating these processes in different fruit crops. Current approaches using genomic-to-metabolomic analysis are providing new insights into the in planta mechanisms of CKs, pinpointing the underlying CK-derived actions that may serve as potential targets for improving crop-specific traits and the development of new solutions for the preharvest and postharvest management of fruit crops. Where information is available, CK molecular biology is discussed in the context of its present and future implications in the applications of CKs to fruits of horticultural significance.

Efficient Tissue Culture Protocol for Magnolia lucida (Magnoliaceae) and Confirmation of Genetic Stability of the Regenerated Plants

Magnolia lucida (Magnoliaceae) is classified as an endangered species by the International Union for Conservation of Nature. It has high commercial value owing to its attractive tree shape and flowers. We adopted an excellent genotype of M. lucida as the parent material and established a mini-cut orchard through grafting to provide trunk shoots explants over the long-term. Optimal sterilization was achieved using a combination of 75% ethanol for 30 s, one percent benzalkonium bromide for five minutes, and 0.1% mercuric chloride for five minutes. Modified Murashige and Skoog medium (ML) was the optimal medium for the growth of M. lucida. Addition of one mg/L of 6-benzyl adenine (BA) and 0.05 mg/L of α-naphthaleneacetic acid (NAA) to the medium increased the shoot induction rate to 95.56%, and the ML medium containing 0.4 mg/L BA and 0.04 mg/L NAA achieved the maximum multiplication rate (284.56%). Dark treatment for seven days, followed by continuous light treatment could better resolve the challenge of difficult rooting in M. lucida plants. Using random amplified polymorphic DNA and inter simple sequence repeat markers, we confirmed the genetic uniformity and stability of the regenerated plants. Our protocol should be helpful for the propagation and conservation of this endangered plant.

Efficient Micropropagation Protocol for the Conservation of the Endangered Aloe peglerae, an Ornamental and Medicinal Species

A number of Aloe species are facing an extremely high risk of extinction due to habitat loss and over-exploitation for medicinal and ornamental trade. The last global assessment of Aloe peglerae Schönland (in 2003) ranked its global conservation status as ‘endangered’ with a decreasing population trend. In the National Red List of South African Plants, the extremely rapid decline of this species has resulted in its conservation status being elevated from ‘endangered’ to ‘critically endangered’ based on recent or new field information. This dramatic decline necessitates the development of a simple, rapid and efficient micropropagation protocol as a conservation measure. An in vitro propagation protocol was therefore established with the regeneration of 12 shoots per shoot-tip explant within 8 weeks using Murashige and Skoog (MS) medium supplemented with 2.5 µM meta-topolin riboside (an aromatic cytokinin). The rooting of the shoots with a 100% frequency on half-strength MS medium without any plant growth resulted in additional six shoots produced per cultured shoot. The resultant plantlets were successfully acclimatized with a 100% survival frequency after 6 weeks. Overall, the developed protocol can result in the production of 3906 transplantable shoots that are ready for rooting per annum from a single shoot-tip explant. It is simple and efficient for seedling production in the ex situ cultivation and conservation of the endangered A. peglerae.

Phytoplasmal infection derails genetically preprogrammed meristem fate and alters plant architecture

In the life cycle of higher plants, it is the fate of meristem cells that determines the pattern of growth and development, and therefore plant morphotype and fertility. Floral transition, the turning point from vegetative growth to reproductive development, is achieved via genetically programmed sequential changes in meristem fate from vegetative to inflorescence, and to floral, leading to flower formation and eventual seed production. The transition is rarely reversible once initiated. In this communication, we report that a bacterial infection can derail the genetically programmed fate of meristem cells, thereby drastically altering the growth pattern of the host plant. We identified four characteristic symptoms in tomato plants infected with a cell wall-less bacterium, phytoplasma. The symptoms are a manifestation of the pathogen-induced alterations of growth pattern, whereas each symptom corresponds to a distinct phase in the derailment of shoot apical meristem fate. The phases include premature floral meristem termination, suppressed floral meristem initiation, delayed conversion of vegetative meristem to inflorescence meristem, and repetitive initiation and outgrowth of lateral vegetative meristems. We further found that the pathogen-induced alterations of growth pattern were correlated with transcriptional reprogramming of key meristem switching genes. Our findings open an avenue toward understanding pathological alterations in patterns of plant growth and development, thus aiding identification of molecular targets for disease control and symptom alleviation. The findings also provide insights for understanding stem cell pluripotency and raise a tantalizing possibility for using phytoplasma as a tool to dissect the course of normal plant development and to modify plant morphogenesis by manipulating meristem fate.