CisPG21 and CisCEL16 are involved in the regulation of the degradation of cell walls during secretory cavity cell programmed cell death in the fruits of Citrus sinensis (L.) Osbeck

CgLS mediates limonene synthesis of main essential oil component in secretory cavity cells of Citrus grandis 'Tomentosa' fruits

d-Limonene is the predominant component of essential oil from Exocarpium Citri Grandis, known for its antibacterial, antioxidant, insecticidal, and anti-inflammatory properties. The synthesis, transport, and accumulation of d-limonene in Citrus grandis 'Tomentosa' fruits are regulated by limonene synthase (LS) and its associated regulatory genes. This study addresses the gap in understanding the spatiotemporal cytological changes of LS and its regulatory genes. Through cytochemical techniques, we investigated the distribution of essential oil in the plastids, endoplasmic reticulum, and vacuoles of secretory cavity cells. We identified the LS-encoding gene CgLS via transcriptomics and demonstrated in vitro that CgLS catalyzes the formation of d-limonene from geranyl diphosphate (GPP). Transient overexpression of CgLS increased monoterpene limonene accumulation, while TRV virus-induced gene silencing reduced it. CgLS expression levels and d-limonene content showed spatiotemporal consistency with fruit development, with in situ hybridization revealing predominant expression in secretory cavity cells. Immunocytochemical localization indicated that CgLS is primarily located in the endoplasmic reticulum, plastids, and vacuoles. Our findings suggest that CgLS is translated in the endoplasmic reticulum and transported to plastids and vacuoles where d-limonene synthesis occurs. This study provides comprehensive insights into the characteristics of CgLS and its role in d-limonene synthesis at the tissue, cellular, and molecular levels in C. grandis 'Tomentosa'.

Transcriptome differential expression analysis of defoliation of two different lemon varieties

'Allen Eureka' is a bud variety of Eureka lemon with excellent fruiting traits. However, it suffers from severe winter defoliation that leads to a large loss of organic nutrients and seriously affects the tree's growth and development as well as the yield of the following year, and the mechanism of its response to defoliation is still unclear. In order to investigate the molecular regulatory mechanisms of different leaf abscission periods in lemon, two lemon cultivars ('Allen Eureka' and 'Yunning No. 1') with different defoliation traits were used as materials. The petiole abscission zone (AZ) was collected at three different defoliation stages, namely, the pre-defoliation stage (CQ), the mid-defoliation stage (CZ), and the post-defoliation stage (CH). Transcriptome sequencing was performed to analyze the gene expression differences between these two cultivars. A total of 898, 4,856, and 3,126 differentially expressed genes (DEGs) were obtained in CQ, CZ, and CH, respectively, and the number of DEGs in CZ was the largest. GO analysis revealed that the DEGs between the two cultivars were mainly enriched in processes related to oxidoreductase, hydrolase, DNA binding transcription factor, and transcription regulator activity in the defoliation stages. KEGG analysis showed that the DEGs were concentrated in CZ and involved plant hormone signal transduction, phenylpropanoid biosynthesis, glutathione metabolism, and alpha-linolenic acid metabolism. The expression trends of some DEGs suggested their roles in regulating defoliation in lemon. Eight gene families were obtained by combining DEG clustering analysis and weighted gene co-expression network analysis (WGCNA), including β-glucosidase, AUX/IAA, SAUR, GH3, POD, and WRKY, suggesting that these genes may be involved in the regulation of lemon leaf abscission. The above conclusions enrich the research related to lemon leaf abscission and provide reliable data for the screening of lemon defoliation candidate genes and analysis of defoliation pathways.

Ca2+- and Zn2+-dependent nucleases co-participate in nuclear DNA degradation during programmed cell death in secretory cavity development in Citrus fruits

Calcium (Ca2+)- and zinc Zn2+-dependent nucleases play pivotal roles in plant nuclear DNA degradation in programmed cell death (PCD). However, the mechanisms by which these two nucleases co-participate in PCD-associated nuclear DNA degradation remain unclear. Here, the spatiotemporal expression patterns of two nucleases (CrCAN and CrENDO1) were analyzed qualitatively and quantitatively during PCD in secretory cavity formation in Citrus reticulata 'Chachi' fruits. Results show that the middle and late initial cell stages and lumen-forming stages are key stages for nuclear degradation during the secretory cavity development. CAN and ENDO1 exhibited potent in vitro DNA degradation activity at pH 8.0 and pH 5.5, respectively. Quantitative real-time reverse-transcription polymerase chain reaction, in situ hybridization assays, the subcellular localization of Ca2+ and Zn2+, and immunocytochemical localization showed that CrCAN was activated at the middle and late initial cell stages, while CrENDO1 was activated at the late initial cell and lumen-forming stages. Furthermore, we used immunocytochemical double-labelling to simultaneously locate CrCAN and CrENDO1. The DNA degradation activity of the two nucleases was verified by simulating the change of intracellular pH in vitro. Our results also showed that CrCAN and CrENDO1 worked respectively and co-participated in nuclear DNA degradation during PCD of secretory cavity cells. In conclusion, we propose the model for the synergistic effect of Ca2+- and Zn2+-dependent nucleases (CrCAN and CrENDO1) in co-participating in nuclear DNA degradation during secretory cavity cell PCD in Citrus fruits. Our findings provide direct experimental evidence for exploring different ion-dependent nucleases involved in nuclear degradation during plant PCD.

Novel Mechanisms Underlying Rubber Accumulation and Programmed Cell Death in Laticiferous Canals of Decaisnea insignis Fruits: Cytological and Transcriptomic Analyses

Natural rubber is one of the most important industrial raw materials, and its biosynthesis is still a fascinating process that is still largely unknown. In this research, we studied Decaisnea insignis, a unique rubber-producing plant that is different from other rubber-producing species due to the presence of lactiferous canals in its pericarp. The present study aims to provide novel insights into the mechanisms underlying rubber accumulation and PCD by subjecting the Decaisnea insignis laticiferous canals to light microscopy, TUNEL assay, and DAPI staining, as well as viability analysis, cellular ultrastructure analysis, and molecular analysis using light microscopy, scanning electron microscopy, immunofluorescence labeling, transmission electron microscopy, and transcriptome sequencing. At the cellular level, the origin of small rubber particles in the laticiferous canals had no morphological correlation with other organelles, and these particles were freely produced in the cytosol. The volume of the rubber particles increased at the sunken and expanding stage, which were identified as having the characteristics of programmed cell death (PCD); meanwhile, plenty of the rubber precursors or rubber particles were engulfed by the vacuoles, indicating a vacuole-mediated autophagy process. The accumulation of rubber particles occurred after the degeneration of protoplasts, suggesting a close association between rubber biosynthesis and PCD. The molecular analysis revealed the expression patterns of key genes involved in rubber biosynthesis. The upstream genes DiIPP, DiFPP, and DiGGPPS showed a decreasing trend during fruit ripening, while DiHRT, which is responsible for rubber particle extension, exhibited the highest expression level during the rubber particle formation. Moreover, the transcription factors related to PCD, DiLSD1, and DiLOL2 showed a negative correlation with the expression pattern of DiHRT, thus exhibiting strict rules of sequential expression during rubber biosynthesis. Additionally, the expression trends of DiXCP1 and DiCEP1, which act as proteases during PCD, were positively correlated with DiGGPPS expression. In conclusion, the findings suggest that the autophagic PCD may play a crucial role in rubber accumulation in D. insignis. Further research is still needed to fully understand the complex regulatory network underlying rubber biosynthesis in plants.

Involvement of Vacuolar Processing Enzyme CgVPE1 in Vacuole Rupture in the Programmed Cell Death during the Development of the Secretory Cavity in Citrus grandis 'Tomentosa' Fruits

Vacuolar processing enzymes (VPEs) with caspase-1-like activity are closely associated with vacuole rupture. The destruction of vacuoles is one of the characteristics of programmed cell death (PCD) in plants. However, whether VPE is involved in the vacuole destruction of cells during secretory cavity formation in Citrus plants remains unclear. This research identified a CgVPE1 gene that encoded the VPE and utilized cytology and molecular biology techniques to explore its temporal and spatial expression characteristics during the PCD process of secretory cavity cells in the Citrus grandis 'Tomentosa' fruit. The results showed that CgVPE1 is an enzyme with VPE and caspase-1-like activity that can self-cleave into a mature enzyme in an acidic environment. CgVPE1 is specifically expressed in the epithelial cells of secretory cavities. In addition, it mainly accumulates in vacuoles before it is ruptured in the secretory cavity cells. The spatial and temporal immunolocalization of CgVPE1 showed a strong relationship with the change in vacuole structure during PCD in secretory cavity cells. In addition, the change in the two types of VPE proteins from proenzymes to mature enzymes was closely related to the change in CgVPE1 localization. Our results indicate that CgVPE1 plays a vital role in PCD, causing vacuole rupture in cells during the development of the secretory cavity in C. grandis 'Tomentosa' fruits.

CgPG21 is involved in the degradation of the cell wall during the secretory cavity formation in Citrus grandis 'Tomentosa' fruits

MAIN CONCLUSION

CgPG21 is mainly located in the cell wall, participates in the intercellular layer degradation of the cell wall during the formation of secretory cavity in the intercellular space-forming and lumen-expanding stages. The secretory cavity is a common structure in Citrus plants and is the main site for synthesis and accumulation of medicinal ingredients. The secretory cavity is formed in lysogenesis, when epithelial cells enter a process of programmed cell death. Pectinases are known to be involved in degradation of the cell wall during the cytolysis of secretory cavity cells, but the changes in cell structure, the dynamic characteristics of cell wall polysaccharides and the related genes regulating cell wall degradation are unclear. In this study, electron microscopy and cell wall polysaccharide-labeling techniques were used to study the main characteristics of cell wall degradation of the secreting cavity of Citrus grandis 'Tomentosa' fruits. At the same time, the full CDS length of the pectinase gene CgPG21 was cloned, encoding a protein composed of 480 amino acids. CgPG21 is mainly located in the cell wall, participates in the degradation of the intercellular layer of the cell wall during the development of the secretory cavity, and plays an important role in the formation of the secretory cavity in the intercellular space-forming and lumen-expanding stages. With the development of secretory cavity, the cell wall polysaccharides of epithelial cells gradually degrade. CgPG21 is mainly involved in the intercellular layer degradation.

Developmental Programmed Cell Death Involved in Ontogenesis of Dictamnus dasycarpus Capitate Glandular Hairs

Plant glandular trichomes have received much attention due to their commercial and biological value. Recent studies have focused on the development of various glands in plants, suggesting that programmed cell death (PCD) may play an important role during the development of plant secretory structures. However, the development processes and cytological characteristics in different types of plant secretory structures differed significantly. This study aims to provide new data on the developmental PCD of the capitate glandular hairs in Dictamnus dasycarpus. Light, scanning, immunofluorescence labeling, and transmission electron microscopy were used to determine the different developmental processes of the capitate glandular hairs from a cytological perspective. Morphologically, the capitate glandular hair originates from one initial epidermal cell and differentiates into a multicellular trichome characterized by two basal cells, two lines of stalk cells, and a multicellular head. It is also histochemically detected by essential oils. TUNEL-positive reactions identified nuclei with diffused fluorescence or an irregular figure by DAPI, and Evans blue staining showed that the head and stalk cells lost their viability. Ultrastructural evidence revealed the developmental process by two possible modes of PCD. Non-autolytic PCD was characterized by buckling cell walls and degenerated nuclei, mitochondria, plastids, multivesicular body (MVB), and end-expanded endoplasmic reticulum in the condensed cytoplasm, which were mainly observed in the head cells. The MVB was detected in the degraded vacuole, a degraded nucleus with condensed chromatin and diffused membrane, and eventual loss of the vacuole membrane integrity exhibited typical evidence of vacuole-mediated autolytic PCD in the stalk cells. Furthermore, protoplasm degeneration coupled with dark oil droplets and numerous micro-dark osmiophilic substances was observed during late stages. The secretion mode of essential oils is also described in this paper.

A Novel Algicidal Bacterium and Its Effects against the Toxic Dinoflagellate Karenia mikimotoi (Dinophyceae)

The toxic dinoflagellate Karenia mikimotoi is a harmful algal bloom-forming species in coastal areas around the world. It produces ichthyotoxins and hemolytic toxins, with deleterious effects on marine ecosystems. In this study, the bacterium Pseudoalteromonas sp. FDHY-MZ2, with high algicidal efficiency against K. mikimotoi, was isolated from a bloom event. Based on the results, it completely lysed K. mikimotoi cells within 24 h 0.5% (vol/vol), with the algicidal activity of the supernatant of the bacterium culture. Algal cell wall fragmentation occurred, leading to cell death. There was a marked decline in various photochemical traits. When treated with the supernatant, cellulase, pheophorbide a oxygenase (PAO) and cyclin B genes were significantly increased, suggesting induced cell wall deterioration, chloroplast degradation and cell cycle regulation of K. mikimotoi cells. In addition, the expression levels of reactive oxygen species (ROS) scavenging gene was significantly inhibited, indicating that the ROS removal system was damaged. The bacterial culture was dried to obtain the spray-dried powder, which showed algicidal activity rates of 92.2 and 100% against a laboratory K. mikimotoi culture and a field microcosm of Karlodinium sp. bloom within 24 h with the addition of 0.04% mass fraction powder. Our results demonstrate that FDHY-MZ2 is a suitable strain for K. mikimotoi and Karlodinium sp. blooms management. In addition, this study provides a new strategy for the anthropogenic control of harmful algal bloom-forming species in situ. IMPORTANCE K. mikimotoi is a noxious algal bloom-forming species that cause damaging of the aquaculture industry and great financial losses. Bacterium with algicidal activity is an ideal agency to inhibit the growth of harmful algae. In this approach application, the bacterium with high algicidal activity is required and the final management material is ideal for easy-to-use. The algicidal characteristics are also needed to understand the effects of the bacterium for managing strategy exploration. In this study, we isolated a novel algicidal bacterium with extremely high lysis efficiency for K. mikimotoi. The algicidal characteristics of the bacterium as well as the chemical and molecular response of K. mikimotoi with the strain challenge were examined. Finally, the algicidal powder was explored for application. The results demonstrate that FDHY-MZ2 is suitable for K. mikimotoi and Karlodinium sp. blooms controlling, and this study provides a new strategy for algicidal bacterium application.

The study of schizogenous formation of secretory ducts in Ferula ferulaeoides (Steud.) Korov

The secretory ducts of Ferula ferulaeoides (Steud.) Korov. are the main tissue of synthesis, secretion, and accumulation of resin. The formation of secretory ducts is closely related to the harvest and quality of resin, but the lumen formation mode and corresponding mechanism have not been thoroughly studied. This study of F. ferulaeoides investigated the microstructure and ultrastructure of the secretory ducts from a developmental point of view. Stem samples were analyzed by light microscopy, transmission electron microscopy, and fluorescence microscopy. The data results showed (1) the walls of secretory cells were intact during the development of secretory ducts in F. ferulaeoides; (2) the plastids and endoplasmic reticulum of secretory cells participated in the synthesis of resin; (3) pectinase was involved in the degradation of the middle lamella; and (4) no features of programmed cell death during the formation of secretory ducts. The results suggested that the formation of F. ferulaeoides' secretory ducts was schizogenous, and pectinase was involved in its formation. These data may be beneficial to further explore the formation of secretory duct in other species of Ferula L. and the formation mechanism of schizogenous secretory structures.

Zn2+-Dependent Nuclease Is Involved in Nuclear Degradation during the Programmed Cell Death of Secretory Cavity Formation in Citrus grandis 'Tomentosa' Fruits

Zn2+- and Ca2+-dependent nucleases exhibit activity toward dsDNA in the four classes of cation-dependent nucleases in plants. Programmed cell death (PCD) is involved in the degradation of cells during schizolysigenous secretory cavity formation in Citrus fruits. Recently, the Ca2+-dependent DNase CgCAN was proven to play a key role in nuclear DNA degradation during the PCD of secretory cavity formation in Citrus grandis ‘Tomentosa’ fruits. However, whether Zn2+-dependent nuclease plays a role in the PCD of secretory cells remains poorly understood. Here, we identified a Zn2+-dependent nuclease gene, CgENDO1, from Citrus grandis ‘Tomentosa’, the function of which was studied using Zn2+ ions cytochemical localization, DNase activity assays, in situ hybridization, and protein immunolocalization. The full-length cDNA of CgENDO1 contains an open reading frame of 906 bp that encodes a protein 301 amino acids in length with a S1/P1-like functional domain. CgENDO1 degrades linear double-stranded DNA at acidic and neutral pH. CgENDO1 is mainly expressed in the late stage of nuclear degradation of secretory cells. Further spatiotemporal expression patterns of CgENDO1 showed that CgENDO1 is initially located on the endoplasmic reticulum and then moves into intracellular vesicles and nuclei. During the late stage of nuclear degradation, it was concentrated in the area of nuclear degradation involved in nuclear DNA degradation. Our results suggest that the Zn2+-dependent nuclease CgENDO1 plays a direct role in the late degradation stage of the nuclear DNA in the PCD of secretory cavity cells of Citrus grandis ‘Tomentosa’ fruits.

CgPBA1 may be involved in nuclear degradation during secretory cavity formation by programmed cell death in Citrus grandis 'Tomentosa' fruits

Caspase-3 is the crucial executor caspase of apoptosis in mammalian cells, which is essential for chromatin condensation and DNA fragmentation. Although plants have no caspase-3 homologs, PBA1 acts as a plant caspase-3-like enzyme in plant programmed cell death (PCD). PCD occurs during the formation of secretory cavities in Citrus fruits; hence, secretory cavities could be utilized as a new cell biology model for investigating the regulatory mechanisms of plant PCD. To further study the association between PBA1 and PCD during secretory cavity development in Citrus fruits, CgPBA1 was identified in the fruit of Citrus grandis 'Tomentosa'. The temporal and spatial expression of CgPBA1 during secretory cavity development were analyzed using quantitative real-time PCR and in situ hybridization, and the morphological changes in the apoptotic cell nuclei were observed using TUNEL assay and ultra-thin section technology. The results revealed that the full-length cDNA of CgPBA1 contains a 711 bp ORF that encodes a putative protein containing 236 amino acid with a proteasome-β-6 functional domain that belongs to the Ntn hydrolase super family. CgPBA1 was predominantly expressed in the secretory cavities; its expression changes coincided with the morphological changes and DNA fragmentation in apoptotic cell nuclei. The green fluorescent fusion protein of CgPBA1 is also located in the nucleus of tobacco epidermal cells. Based on previous research and the findings of the present study, we speculate that CgPBA1 is a highly functional conserved protein in plants, and it might be involved in nuclear degradation during PCD for secretory cavity formation in C. grandis 'Tomentosa' fruits.