Strigolactones affect tomato hormone profile and somatic embryogenesis

Transcriptome analysis reveals that auxin promotes strigolactone-induced adventitious root growth in the hypocotyl of melon seedlings

Introduction

Strigolactone (SL) and auxin are two important phytohormones involved in plant root development, but whether they show synergistic or mutual promotion effects during adventitious root (AR) formation has not been adequately explored.

Methods

In this study, we investigated the mechanisms of GR24 (synthetic SL) and indole-3-acetic acid (IAA; a type of auxin) in the formation of ARs using melon as the study material.

Results

Morphological measurements showed that the AR number, length, superficial area, and volume under the GR24 treatment were 1.60-3.27, 1.58-3.99, 2.06-3.42, and 3.00-6.11 times greater than those of the control group, respectively, at 6-10 days; the GR24+IAA treatment further promoted AR formation in melon seedlings, and the AR number, length, superficial area, and volume under the GR24+IAA treatment were 1.44-1.51, 1.28-1.73, 1.19-1.83, and 1.31-1.87 times greater than those obtained with the GR24 treatment, respectively. Transcriptome analysis revealed 2,742, 3,352, and 2,321 differentially expressed genes (DEGs) identified from the GR24 vs. control, GR24+IAA vs. control, and GR24+IAA vs. GR24 comparisons, respectively. The GR24 treatment and GR24+IAA treatment affected auxin and SL synthesis as well as components of the phytohormone signal transduction pathway, such as auxin, brassinosteroid (BR), ethylene (ETH), cytokinin (CK), gibberellin (GA), and abscisic acid (ABA). The concentrations of auxin, GA, zeatin (ZT), and ABA were evaluated using high-performance liquid chromatography (HPLC). From 6 to 10 days, the auxin, GA, and ZT contents in the GR24 treatment group were increased by 11.48%-15.34%, 11.83%-19.50%, and 22.52%-66.17%, respectively, compared to the control group, and these features were increased by 22.00%-31.20%, 21.29%-25.75%, 51.76%-98.96%, respectively, in the GR24+IAA treatment group compared with the control group. Compared to that in the control, the ABA content decreased by 10.30%-11.83% in the GR24 treatment group and decreased by 18.78%-24.00% in the GR24+IAA treatment group at 6-10 days.

Discussion

Our study revealed an interaction between strigolactone and auxin in the induction of AR formation in melon seedlings by affecting the expression of genes related to plant hormone pathways and contents.

Application of Data Modeling, Instrument Engineering and Nanomaterials in Selected Medid the Scientific Recinal Plant Tissue Culture

At present, most precious compounds are still obtained by plant cultivation such as ginsenosides, glycyrrhizic acid, and paclitaxel, which cannot be easily obtained by artificial synthesis. Plant tissue culture technology is the most commonly used biotechnology tool, which can be used for a variety of studies such as the production of natural compounds, functional gene research, plant micropropagation, plant breeding, and crop improvement. Tissue culture material is a basic and important part of this issue. The formation of different plant tissues and natural products is affected by growth conditions and endogenous substances. The accumulation of secondary metabolites are affected by plant tissue type, culture method, and environmental stress. Multi-domain technologies are developing rapidly, and they have made outstanding contributions to the application of plant tissue culture. The modes of action have their own characteristics, covering the whole process of plant tissue from the induction, culture, and production of natural secondary metabolites. This paper reviews the induction mechanism of different plant tissues and the application of multi-domain technologies such as artificial intelligence, biosensors, bioreactors, multi-omics monitoring, and nanomaterials in plant tissue culture and the production of secondary metabolites. This will help to improve the tissue culture technology of medicinal plants and increase the availability and the yield of natural metabolites.

Synthetic Strigolactone GR24 Improves Arabidopsis Somatic Embryogenesis through Changes in Auxin Responses

Somatic embryogenesis in Arabidopsis encompasses an induction phase requiring auxin as the inductive signal to promote cellular dedifferentiation and formation of the embryogenic tissue, and a developmental phase favoring the maturation of the embryos. Strigolactones (SLs) have been categorized as a novel group of plant hormones based on their ability to affect physiological phenomena in plants. The study analyzed the effects of synthetic strigolactone GR24, applied during the induction phase, on auxin response and formation of somatic embryos. The expression level of two SL biosynthetic genes, MOREAXILLARY GROWTH 3 and 4 (MAX3 and MAX4), which are responsible for the conversion of carotene to carotenal, increased during the induction phase of embryogenesis. Arabidopsis mutant studies indicated that the somatic embryo number was inhibited in max3 and max4 mutants, and this effect was reversed by applications of GR24, a synthetic strigolactone, and exacerbated by TIS108, a SL biosynthetic inhibitor. The transcriptional studies revealed that the regulation of GR24 and TIS108 on somatic embryogenesis correlated with changes in expression of AUXIN RESPONSIVE FACTORs 5, 8, 10, and 16, known to be required for the production of the embryogenic tissue, as well as the expression of WUSCHEL (WUS) and Somatic Embryogenesis Receptor-like Kinase 1 (SERK1), which are markers of cell dedifferentiation and embryogenic tissue formation. Collectively, this work demonstrated the novel role of SL in enhancing the embryogenic process in Arabidopsis and its requirement for inducing the expression of genes related to auxin signaling and production of embryogenic tissue.

Petunia PLEIOTROPIC DRUG RESISTANCE 1 Is a Strigolactone Short-Distance Transporter with Long-Distance Outcomes

Phytohormones of the strigolactone (SL) family have been characterized as negative regulators of lateral bud outgrowth and triggers of symbioses between plants and mycorrhizal fungi. SLs and their precursors are synthesized in root tips as well as along shoot and root vasculature; they either move shoot-wards and regulate plant architecture or are exuded from roots into the soil to establish mycorrhizal symbiosis. Owing to the difficulty in quantification of SL in shoot tissues because of low abundance, it is not yet clear how SL distribution in plants is regulated at short- and long-distances from SL biosynthetic and target tissues. To address this question, we grafted wild-type scions and rootstocks from different petunia mutants for SL biosynthesis/transport and investigated SL activity by quantifying lateral bud outgrowth in the main shoot. Based on these results, we show that (i) the previously reported petunia SL transporter PLEIOTROPIC DRUG RESISTANCE 1 (PDR1) directly accounts for short-distance SL transport and (ii) long-distance transport of SLs seems to be partially and not directly dependent on PDR1. These data suggest that the root-to-shoot transport of SLs occurs either via the vasculature bundle through transporters other than PDR1 or involves SL precursors that are not substrates of PDR1.

Efficient and reproducible somatic embryogenesis and micropropagation in tomato via novel structures - Rhizoid Tubers

A dual in vitro regeneration system consisting of indirect organogenesis and somatic embryogenesis (SE), applicable to several varieties of tomato—Solanum lycopersicum (cv. Riogrande, cv. Roma, hybrid 17905 and model cv. M82) has been established. This system is both improved and highly reproducible compared to current methods. Callus initiation, plant regeneration and SE was developed for one-week-old cotyledon explants. Indirect organogenesis via callus induction (CI) was developed for all four varieties of tomato used in this study. One-week-old tomato seedlings were used as a source of cotyledon and hypocotyl segments as explants. The explants were subsequently cultured on Murashige and Skoog (MS) medium supplemented with different combination and concentrations of plant growth regulators (PGRs). Substantial trends in regeneration and propagation response were observed among the varieties and treatments. For commercial varieties cvs. Riogrande and Roma, maximum CI was observed at 2 weeks in CIMT⁹ (0.5 mg/L NAA, 1 mg/L BAP) and CIMT¹² (2 mg/L IAA, 2 mg/L NAA, 2 mg/L BAP, 4 mg/L KIN). However, cv. M82 responded after 4 weeks to a combination of treatments CIMT⁹ (0.5 mg/L NAA + 1 mg/L BAP) and CIMT¹³ (2 mg/L IAA + 2 mg/L NAA + 2 mg/L BAP + 4 mg/L ZEA) for the production of calli. Subsequent shoot and root organogenesis were optimized for all four varieties. Cv. Riogrande, exhibited fastidious in vitro regeneration potential and selected for induction of somatic embryos via SE involving novel structure: rhizoid tubers (RTBs). Numerous fine hair like rhizoids (~23/explants) were first developed from cotyledon and hypocotyl explants cultured on MS medium supplemented with 0.5 or 2 mg/L NAA at pH 4.0 in dark conditions. Further incubation of each rhizoid under light conditions on MS media supplemented with 5 mg/L TDZ or BAP at pH 4.0 led to the formation of a novel structure—rhizoid Tubers (RTBs). Thus, as evident from histology, SE in Riogrande tomato species requires a medium with pH of (4.0) and higher concentration of cytokinins (BAP/TDZ) to form on average 40–45 RTBs from both explants. Histological and morphological studies revealed that RTBs develop through different stages of embryogenesis to multiple plantlets, on MS medium with 5 mg/L TDZ/BAP at normal pH (5.8). The results obtained indicated that the induced somatic embryos of tomato with lower pH are a more efficient mode of propagation than the organogenesis with or without callus formation. The RTBs led to a complete plantlets regeneration in 45 days compared to indirect organogenesis at 60 days.

Cloning and expression analysis of LoCCD8 during IAA-induced bulbils outgrowth in lily (Oriental Hybrid 'Sorbonne')

Aerial bulbils, which resemble tiny bulbs, develop from axillary buds and facilitate rapid propagation of Lilium. In most species of lily, bulbils are perpetually dormant and little is known about induction of these vegetative structures. Herein, we proposed that strigolactones (SLs) may regulate the induction of bulbils in Lilium. We tested this hypothesis by isolating and investigating the expression patterns of 2 copies of the carotenoid cleavage dioxygenase8 (CCD8) gene in lily-LoCCD8a and LoCCD8b-with regard to biosynthesis of SLs. Expression analyses revealed that LoCCD8a principally is expressed during vegetative growth, whereas LoCCD8b mainly is expressed during reproductive growth. The maximum quantity of LoCCD8a transcripts was observed in the basal plate in most developmental stages, which suggests that SLs may originate from underground parts, especially the basal plate, and move upward. The effects of treatments with indole-3-acetic acid or SL analog (GR24) on outgrowth of bulbils and expression of LoCCD8 genes suggested that SLs function downstream of auxin to inhibit the outgrowth of bulbil. The expression patterns of LoCCD8a and LoCCD8b at sprouting and bulblet weighting stages also implied that SLs may function in nutrient redistribution. Our findings are expected to promote the utilization of bulbils as vegetative propagules for commercial practice.

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.

Transcriptome analysis of tomato (Solanum lycopersicum L.) shoots reveals a crosstalk between auxin and strigolactone

Auxin and strigolactone (SL) are two important phytohormones involved in shoot branching and morphology. Tomato (Solanum lycopersicum L.), a member of the Solanaceae family, is one of the most popular food crops with high economic value in the world. To seek a better understanding of the responses to exogenous hormones, transcriptome analyses of the tomato shoots treated with exogenous auxin and SL, separately or together, were performed. A total of 2326, 260 and 1379 differential expressed genes (DEGs) were identified under the IAA, GR24 and IAA+GR24 treatments, respectively. Network analysis pointed out two enriched interaction clusters, including “ethylene biosynthesis” and “photosynthesis”. Several ethylene biosynthesis and metabolism-related genes were up-regulated under both IAA and IAA+GR24 treatments, suggesting their involvement in the regulation of ethylene biosynthesis. Besides, auxin-SLs-triggered the expression of several CAB genes may lead to systemic increases in the induction of photosynthesis. Several auxin-activated metabolic pathways could be reduced by the GR24 treatment, indicated that the crosstalk between auxin and SLs may be involved in the metabolic regulation of tomato. Further analysis showed that SLs affect the responses of tomato shoots to auxin by inducing the expression of a series of auxin downstream genes. On the other hand, auxin regulated the biosynthesis of SLs by affecting the genes in the “Carotenoid biosynthesis” pathway. Our data will give us an opportunity to reveal the crosstalk between auxin and SLs in the shoots of tomato.

Strigolactones cross the kingdoms: plants, fungi, and bacteria in the arbuscular mycorrhizal symbiosis

Strigolactones (SLs) first evolved as regulators of simple developmental processes in very ancient plant lineages, and then assumed new roles to sustain the increasing biological complexity of land plants. Their versatility is also shown by the fact that during evolution they have been exploited, once released in the rhizosphere, as a communication system towards plant-interacting organisms even belonging to different kingdoms. Here, we reviewed the impact of SLs on soil microbes, paying particular attention to arbuscular mycorrhizal fungi (AMF). SLs induce several responses in AMF, including spore germination, hyphal branching, mitochondrial metabolism, transcriptional reprogramming, and production of chitin oligosaccharides which, in turn, stimulate early symbiotic responses in the host plant. In the specific case study of the AMF Gigaspora margarita, SLs are also perceived, directly or indirectly, by the well-characterized population of endobacteria, with an increase of bacterial divisions and the activation of specific transcriptional responses. The dynamics of SLs during AM root colonization were also surveyed. Although not essential for the establishment of this mutualistic association, SLs act as positive regulators as they are relevant to achieve the full extent of colonization. This possibly occurs through a complex crosstalk with other hormones such as auxin, abscisic acid, and gibberellins.