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MAIN CONCLUSION: Four cultivars of Paeonia lactiflora pollen have a different viability after cryopreservation, and that the difference of pollen viability is related to calcium ions and cell wall deposition. Cryopreservation is a vital technique for preserving germplasm resources, offering extensive application prospects. Understanding the factors influencing pollen viability after cryopreservation is crucial for the permanent preservation and exchange of pollen resources. This study investigated pollen from four Paeonia lactiflora cultivars with varying viability after cryopreservation, aiming to determine whether calcium ions (Ca2+) and cell wall deposition affect these viability changes. The results showed that Ca2+-ATPase activity and cytoplasmic Ca2+ of all four cultivars exhibited an increasing trend after cryopreservation; the calmodulin (CaM) content varied with cultivars. Correlation analysis showed that fresh pollen viability was significantly negatively correlated with cytoplasmic Ca2+ content and positively correlated with Ca2+-ATPase activity, while pollen viability after cryopreservation exhibited a significantly negative correlation with cytoplasmic Ca2+ content and a positive correlation with CaM content. The pollen cell wall of the cultivar 'Zi Feng Chao Yang' (ZFCY), which showed increased viability after cryopreservation, contained significantly higher levels of low-temperature tolerance-related phospholipids and proteins compared to other cultivars. Additionally, all cultivars maintained a clear Ca2+ gradient at the tips of pollen tubes after cryopreservation, without significant callose accumulation. These findings suggest that differences in Ca2+ signaling and cell wall components deposition influence changes in pollen viability after cryopreservation, and the Ca2+ gradient and callose at the tip of pollen tubes are not responsible for preventing pollen tube growth.

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Theanine (N-ethyl-γ-glutamine), as a unique non-protein amino acid, plays vital roles in abiotic stress resistance, while its roles in biotic stress resistance are still unclear. Gray mold caused by Botrytis cinerea is a major disease in strawberries. Effects of theanine on the development of gray mold, cell-wall and phenylpropanoid metabolisms in strawberries were investigated in this study. Results showed that 5 mmol L-1 theanine treatment reduced disease incidence and severity of gray mold in strawberries with antifungal activity in vitro. Meanwhile, theanine treatment enhanced the accumulation of phenolic compounds and lignin, especially ellagic acid, cyanidin, and quercetin, which was associated with increased phenylpropanoid pathway related enzyme activities. Moreover, theanine induced callose deposition and suppressed cell- wall disassembling enzymes, accompanied by higher levels of water insoluble pectin, hemicellulose and cellulose. Therefore, theanine treatment could alleviate decay of B. cinerea-inoculated strawberries by regulating phenylpropanoid and cell-wall metabolisms, maintaining higher levels of phenolic compounds and cell-wall components, thereby contributing to disease resistance and cell-wall structure integrity.

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The cytoplasms of most plant cells are connected by membrane-lined cell wall channels, the plasmodesmata (PD). Dynamic regulation of sugar, hormone and protein diffusion through PD is essential for plant development and stress responses. Understanding this regulation requires knowledge of factors and mechanisms that control PD permeability through the modulation of callose levels in the cell wall around PD openings. We investigated PD regulation in leaf epidermis cells in relation to drought stress in Arabidopsis thaliana. Upon finding PD-mediated cell wall permeability decreased by drought stress and the hormone ABA, we tested several PD-associated genes with drought-responsive expression for their involvement in this response. Mutants of NHL12 showed relatively low PD permeability that was unaffected by drought or ABA treatment. Overexpression of NHL12 in Nicotiana benthamiana epidermis cells increased PD permeability. Moreover, we show that NHL12 can potentially interact with the callose synthase-regulator NHL3 and we explored the effect of NHL12 abundance and/or lower interface permeability on ABA signaling genes. Our results indicate that NHL12 is a drought-responsive negative regulator of PD callose levels and, thereby, interface permeability. Results are discussed with regard to PD function during drought stress and the regulation of intercellular transport.

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Watermelon (Citrullus lanatus) is the third largest fruit crop in the world in term of production. However, it is susceptible to several viruses. Watermelon vine decline (WVD), caused by whitefly-transmitted squash vein yellowing virus (SqVYV), is a disease that has caused over $60 million in losses in the US and continues to occur regularly in southeastern states. Understanding the molecular mechanisms underlying resistance to SqVYV is important for effective disease management. A time-course transcriptomic analysis was conducted on resistant (392291-VDR) and susceptible (Crimson Sweet) watermelon genotypes inoculated with SqVYV. Significantly higher levels of SqVYV were observed over time in the susceptible compared to the resistant genotype. The plasmodesmata callose binding protein (PDCB) gene, which is responsible for increased callose deposition in the plasmodesmata, was more highly expressed in the resistant genotype than in the susceptible genotype before and after inoculation, suggesting the inhibition of cell-to-cell movement of SqVYV. The potential role of the RNA interference (RNAi) pathway was observed in the resistant genotype based on differential expression of eukaryotic initiation factor (eIF), translin, DICER, ribosome inactivating proteins, RNA-dependent RNA polymerase (RDR), and Argonaute (AGO) genes after inoculation. The significant differential expression of hormone-related genes, including those involved in the ethylene, jasmonic acid, auxin, cytokinin, gibberellin, and salicylic acid signaling pathways, was observed, emphasizing their regulatory roles in the defense response. Genes regulating pectin metabolism, cellulose synthesis, cell growth and development, xenobiotic metabolism, and lignin biosynthesis were overexpressed in the susceptible genotype, suggesting that alterations in cell wall integrity and growth processes result in disease symptom development. These findings will be helpful for further functional studies and the development of SqVYV-resistant watermelon cultivars.

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Fertilization relies on pollen mother cells able to transit from mitosis to meiosis to supply gametes. This process involves remarkable changes at the molecular, cellular and physiological levels including (but not limited to) remodelling of the cell wall. During the meiosis onset, cellulose content at the pollen mother cell walls gradually declines with the concurrent deposition of the polysaccharide callose in anther locules. We aim to understand the biological significance of cellulose-to-callose turnover in pollen mother cells walls using electron microscopic analyses of rice flowers. Our observations indicate that in wild type rice anthers, the mitosis-to-meiosis transition coincides with a gradual reduction in the number of cytoplasmic connections called plasmodesmata. A mutant in the Oryza sativa callose synthase GSL5 (Osgsl5-3), impaired in callose accumulation in premeiotic and meiotic anthers, displayed a greater reduction in plasmodesmata frequency among pollen mother cells and tapetal cells suggesting a role for callose in plasmodesmata maintenance. In addition, a significant increase in extracellular distance between pollen mother cells and impaired premeiotic cell shaping was observed in the Osgsl5-3 mutant. The results suggest that callose-to-cellulose turnover during mitosis-meiosis transition is necessary to maintain cell-to-cell connections and optimal extracellular distance among the central anther locular cells. Findings of this study contribute to our understanding of the regulatory influence of callose metabolism during meiosis initiation in flowering plants.

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Seeds are initiated from the carpel margin meristem (CMM) and high seed yield is top one of breeding objectives for many crops. ß-1,3-glucanases play various roles in plant growth and developmental processes; however, whether it participates in CMM development and seed formation remains largely unknown. Here, we identified a ß-1,3-glucanase gene (GLU19) as a determinant of CMM callose deposition and seed yield in cotton. GLU19 was differentially expressed in carpel tissues between Gossypium barbadense (Gb) and Gossypium hirsutum (Gh). Based on resequencing data, one interspecies-specific InDel in the promoter of GLU19 was further detected. The InDel was involved in the binding site of the CRABS CLAW (CRC) transcription factor, a regulator of carpel development. We found that the CRC binding affinity to the GLU19 promoter of G. barbadense was higher than that of G. hirsutum. Since G. barbadense yields fewer seeds than G. hirsutum, we speculated that stronger CRC binding to the GLU19 promoter activated higher expression of GLU19 which in turn suppressed seed production. Consistent with this hypothesis was that the overexpression of GhGLU19 caused reduced seed number, boll weight and less callose formation in CMM. Conversely, GhGLU19-knockdown (GhGLU19-KD) cotton led to the opposite phenotypes. By crossing GhGLU19-KD lines with several G. hirsutum and G. barbadense cotton accessions, all F1 and F2 plants carrying GhGLU19-KD transgenic loci exhibited higher seed yield than control plants without the locus. The increased seed effect was also found in the down-regulation of Arabidopsis orthologs lines, indicating that this engineering strategy may improve the seed yield in other crops.

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Fusarium graminearum, a devastating fungal pathogen, causes great economic losses to crop yields worldwide. The present study investigated the potential of Streptomyces pratensis S10 to alleviate F. graminearum stress in wheat seedlings based on plant growth-promoting and resistance-inducing assays. The bioassays revealed that S10 exhibited multiple plant growth-promoting properties, including the production of siderophores, 1-aminocyclopropane-1-carboxylic acid deaminase (ACC), and indole-3-acetic acid (IAA), phosphate solubilization, and nitrogen fixation. Meanwhile, the pot experiment demonstrated that S10 improved wheat plant development, substantially enhancing wheat height, weight, root activity, and chlorophyll content. Consistently, genome mining identified abundant genes associated with plant growth promotion. S10 induced resistance against F. graminearum in wheat seedlings. The disease incidence and disease index reduced by nearly 52% and 65% in S10 pretreated wheat seedlings, respectively, compared with those infected with F. graminearum only in the non-contact inoculation assay. Moreover, S10 enhanced callose deposition and reactive oxygen species (ROS) accumulation and induced the activities of CAT, SOD, POD, PAL, and PPO. Furthermore, the quantitative real-time PCR (qRT-PCR) results indicated that S10 pretreatment increased the expression of SA- (PR1.1, PR2, PR5, and PAL1) and JA/ET-related genes (PR3, PR4a, PR9, and PDF1.2) in wheat seedlings upon F. graminearum infection. In summary, S. pratensis S10 could be an integrated biological agent and biofertilizer in wheat seedling blight management and plant productivity enhancement.

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White rust disease caused by a biotrophic oomycete Albugo candida is one of the most serious impediments in realizing the production potential of Brassica juncea. Due to the obligate nature of the pathogen, R-gene-based resistance is unstable as the newer virulent races emerge quickly. For this, a deep understanding of the molecular basis of resistance is essential for developing durable resistant varieties. In this study, we selected one susceptible cultivar, 'Pusa Jaikisan' and its single R gene based resistant NIL, 'Pusa Jaikisan WRR as the source of understanding the defense mechanism in B. juncea against A. candida. Comparative histochemical analysis at 12 dpi showed higher callose deposition in the resistant cultivar than in the susceptible which hints towards its possible role in defense mechanism. Based on the biochemical markers observation, total protein was found to have a negative correlation with the resistance. The antioxidant enzymes (POX, CAT, and SOD) and non-enzymatic ROS scavenging compounds such as polyphenols and proline showed a positive correlation with the white rust resistance. Polyphenol Oxidase (PPO) total chlorophyll and total carotenoids were also found to be more abundant in the 'Pusa Jaikisan WRR'. Based on the heat map analysis, PAL was identified to be the comparatively most induced enzyme involved in the defense mechanism. The polyphenol oxidase, total chlorophyll and total carotenoids were also found to show higher activity in the 'Pusa Jaikisan WRR'. Furthermore, to study the defense response of 'Pusa Jaikisan WRR' compared to 'Pusa Jaikisan' against A. candida infection, the gene expression analyses of salicylic acid (SA)-marker PR protein genes (PR1 and PR2) and jasmonic acid (JA)-marker PR protein genes (PR3 and PR12) were done by qRT-PCR. Based on the results, PR2 emerged as the best possible gene for defense against A. candida followed by PR1. PR3 and PR12 also showed positive correlation with the disease resistance which may be due to the JA pathway acting complementary to the SA pathway in case of B. juncea-A. candida interaction. This provides evidence for the JA-SA hormonal crosstalk to be synergistic in case of the white rust resistance.

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Callose, found in the cell walls of higher plants such as ß-1,3-glucan with ß-1,6 branches, is pivotal for both plant development and responses to biotic and abiotic stressors. Plasmodesmata (PD), membranous channels linking the cytoplasm, plasma membrane, and endoplasmic reticulum of adjacent cells, facilitate molecular transport, crucial for developmental and physiological processes. The regulation of both the structural and transport functions of PD is intricate. The accumulation of callose in the PD neck is particularly significant for the regulation of PD permeability. This callose deposition, occurring at a specific site of pathogenic incursion, decelerates the invasion and proliferation of pathogens by reducing the PD pore size. Scholarly investigations over the past two decades have illuminated pathogen-induced callose deposition and the ensuing PD regulation. This gradual understanding reveals the complex regulatory interactions governing defense-related callose accumulation and protein-mediated PD regulation, underscoring its role in plant defense. This review systematically outlines callose accumulation mechanisms and enzymatic regulation in plant defense and discusses PD's varied participation against viral, fungal, and bacterial infestations. It scrutinizes callose-induced structural changes in PD, highlighting their implications for plant immunity. This review emphasizes dynamic callose calibration in PD constrictions and elucidates the implications and potential challenges of this intricate defense mechanism, integral to the plant's immune system.

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Quercus aliena, a native Chinese tree species, is significant in industry and landscaping. However, it is traditionally propagated by seeds with many limitations, such as pest infestations, seed yield and quality. Thus, this study firstly introduces a somatic embryogenesis (SE) system for Q. aliena, enhancing its cultivation prospects. Thereinto, the development stage of zygotic embryo had a significant effect on SE, only immature embryos in 10-11 weeks after full bloom (WAF), rich in endogenous abscisic acid (ABA), could induce SE. Exogenous application ABA had positive roles in the early development process of both primary and secondary SE, while its antagonist had opposite roles. Transcriptome analysis showed that transcription regulation occupied the major position. Mfuzz cluster and WGCNA co-expression analysis showed that 24 candidate genes were involved in the SE process. The expression of the 24 genes were also affected by exogenous ABA signals, among which QaLEC2, QaCALS11 and QaSSRP1 occupied the important roles. Additionally, the callose content were also affected by exogenous ABA signals, which had significantly positive correlations with the expression of QaLEC2 and QaCALS11. This study not only established an efficient reproduction system for Q. aliena, but also revealed the difference in embryogenic ability of zygotic embryos from the aspects of transcriptome and endogenous hormone content, and lay a foundation for clarifying the molecular mechanism of SE, and provided a reference for exploring the vital roles of ABA in SE.

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Garcinia dulcis (Roxb.) Kurz (Clusiaceae) is a medicinal plant native to Southeastern Asia, with a peculiar, precocious pollenkitt production in early microspore development. We aimed to find out whether different secretory activities of the tapetum or a premature sporoderm development provides additional evidence for our recent hypothesis for the precocious pollenkitt production. Histology, histochemistry and ultrastructure of tapetum and sporoderm development during pollenkitt secretion in Garcinia dulcis were conducted, based on light and electron microscopy analysis. The results showed that Garcinia dulcis possesses normal pollen development. The presence of two different pollen coating types, precocious pollenkitt (L1) and common pollenkitt (L2), in the anther tapetum indicate that they are produced in two different active stages of the secretory tapetum. The precocious pollenkitt production and transport to the locule takes place in early active tapetal cells at early tetrad to early microspore stage and is ongoing until late microspore stage. The production of the second type of pollenkitt (L2) starts shortly after the first active tapetum stage together with the formation of sporopollenin precursors. The sporoderm formation was completed at late microspore stage, when the tapetal cell walls start to disintegrate. Orbicules are lining the inner tapetum wall at middle to late microspore stage. ER (during early microspore stage) and plastids (during late microspore stage) were the two main sources of pollenkitt, which finally fused to pollenkitt droplets when the tapetal cells degenerated at mature bicellular pollen stage.

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Resumen: La apropiada organogénesis de las plantas, durante su ciclo de vida, propicia su desarrollo y la adaptación a diferentes condiciones am bientales. Diversas fitohormonas regulan el desarrollo vegetal, pero la auxina denominada ácido indol-3-acético (AIA) es una de las más importantes. El AIA se sintetiza en la parte aérea de la planta y se moviliza a los tejidos demandantes por un transporte rápido que utiliza el floema y por el transporte polar de auxinas (TPA). Recientemente, se ha demostrado que las auxinas también se movilizan mediante el transporte simplástico (TS) a través de los plasmodesmos (PD), cuya apertura o cierre está regulada respectivamente por la degradación o la deposición de la callosa. El objetivo del presente trabajo fue profundizar en los avances sobre la participación del transporte simplástico de las auxinas durante el desarrollo vegetal, así como la degradación o deposición de la callosa, en el cierre o apertura de los PD, para regular el desarrollo de algunos órganos de Arabidopsis thaliana. La intervención de las proteínas PDLP5 es determinante para la deposición de la callosa en los PD, lo que regula la distribución de la auxina e impacta en la formación radicular, especialmente en las raíces laterales. La participación del TS es importante para desarrollar la actividad de las auxinas, lo cual favorece la formación radicular, necesaria en la mejora de absorción de nutrientes de las plantas. Este conocimiento puede ser utilizado para mejorar las plantas de interés agronómico.

Abstract: The appropriate organogenesis of plants during their life cycle promotes their development and adaptation to different environmental conditions. Various phytohormones regulate plant development but auxin, called Indole-3-Acetic Acid (IAA), is one of the most important. IAA is synthesized in the aerial part of plant and is mobilized to the demanding tissues by a rapid transport using the phloem and by the polar auxin transport (PAT). Recently, it has been shown that auxins also are mobilized by a symplastic transport (ST) through plasmodesmata (PD), which opening or closing is regulated by the callose degradation or deposition respectively. The objective of the present work was to deepen the analysis on the participation of symplastic transport of auxins during plant development, as well in the callose degradation or deposition, in the closing or opening of the PD, that regulates the development of some organs of Arabidopsis thaliana. The intervention of PDLP5 proteins is decisive for the callose deposition in the PD, which regulates the auxin distribution and impacts root formation, especially at the lateral roots. The participation of TS is important to develop the auxin activity, which favors root formation, necessary for the improvement plant nutrient absorption. This knowledge can be used to improve development plants of agronomic interest.

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Callose, a ß-1,3-glucan plant cell wall polymer, regulates symplasmic channel size at plasmodesmata (PD) and plays a crucial role in a variety of plant processes. However, elucidating the molecular mechanism of PD callose homeostasis is limited. We screened and identified an Arabidopsis mutant plant with excessive callose deposition at PD and found that the mutated gene was α1-COP, a member of the coat protein I (COPI) coatomer complex. We report that loss of function of α1-COP elevates the callose accumulation at PD by affecting subcellular protein localization of callose degradation enzyme PdBG2. This process is linked to the functions of ERH1, an inositol phosphoryl ceramide synthase, and glucosylceramide synthase through physical interactions with the α1-COP protein. Additionally, the loss of function of α1-COP alters the subcellular localization of ERH1 and GCS proteins, resulting in a reduction of GlcCers and GlcHCers molecules, which are key sphingolipid (SL) species for lipid raft formation. Our findings suggest that α1-COP protein, together with SL modifiers controlling lipid raft compositions, regulates the subcellular localization of GPI-anchored PDBG2 proteins, and hence the callose turnover at PD and symplasmic movement of biomolecules. Our findings provide the first key clue to link the COPI-mediated intracellular trafficking pathway to the callose-mediated intercellular signaling pathway through PD.

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Nitrogen (N) and silicon (Si) are mineral elements that have shown a reduction in the damage caused by tan spot (Pyrenophora tritici-repentis (Ptr)) in wheat. However, the effects of these elements were studied separately, and the N and Si interaction effect on wheat resistance to tan spot remains elusive. Histocytological and biochemical defense responses against Ptr in wheat leaves treated with Si (+Si) at low (LN) and high N (HN) inputs were investigated. Soil amendment with Si reduced the tan spot severity in 18% due to the increase in the leaf Si concentration (around 30%), but it was affected by the N level used. The superoxide dismutase (SOD) activity was higher in +Si plants and inoculated with Ptr, leading to early and higher H2O2 and callose accumulation in wheat leaf. Interestedly, phenylalanine ammonia-lyase (PAL) activity was induced by the Si supplying, being negatively affected by the HN rate. Meanwhile, catalase (CAT), and peroxidase (POX) activities showed differential response patterns according to the Si and N rates used. Tan spot severity was reduced by both elements, but their interaction does not evidence synergic effects in this disease's control. Wheat plants from -Si and HN and +Si and LN treatments recorded lower tan spot severity.

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BACKGROUND: Soybean mosaic disease caused by soybean mosaic virus (SMV) is one of the most devastating and widespread diseases in soybean producing areas worldwide. The WRKY transcription factors (TFs) are widely involved in plant development and stress responses. However, the roles of the GmWRKY TFs in resistance to SMV are largely unclear. RESULTS: Here, 185 GmWRKYs were characterized in soybean (Glycine max), among which 60 GmWRKY genes were differentially expressed during SMV infection according to the transcriptome data. The transcriptome data and RT-qPCR results showed that the expression of GmWRKY164 decreased after imidazole treatment and had higher expression levels in the incompatible combination between soybean cultivar variety Jidou 7 and SMV strain N3. Remarkably, the silencing of GmWRKY164 reduced callose deposition and enhanced virus spread during SMV infection. In addition, the transcript levels of the GmGSL7c were dramatically lower upon the silencing of GmWRKY164. Furthermore, EMSA and ChIP-qPCR revealed that GmWRKY164 can directly bind to the promoter of GmGSL7c, which contains the W-box element. CONCLUSION: Our findings suggest that GmWRKY164 plays a positive role in resistance to SMV infection by regulating the expression of GmGSL7c, resulting in the deposition of callose and the inhibition of viral movement, which provides guidance for future studies in understanding virus-resistance mechanisms in soybean.

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Garlic cultivars are predominantly characterized by their sterility and reliance on asexual reproduction, which have traditionally prevented the use of hybrid breeding for cultivar improvement in garlic. Our investigation has revealed a notable exception in the garlic line G398, which demonstrates the ability to produce fertile pollen. Notably, at the seventh stage of anther development, callose degradation in the sterile line G390 was impeded, while G398 exhibited normal callose degradation. Transcriptome profiling revealed an enhanced expression of the callose-degrading gene, AsaNRF1, in the mature flower buds of the fertile line G398 compared to the sterile line G390. An insertion in the promoter of AsaNRF1 in G390 was identified, which led to its reduced expression at the tetrad stage and consequently delayed callose degradation, potentially resulting in the male sterility of G390. A discriminatory marker was developed to distinguish between fertile G398 and sterile G390, facilitating the assessment of male fertility in garlic germplasm resources. This study introduces a practical approach to harnessing garlic hybridization, which can further facilitate the breeding of new cultivars and the creation of novel male-fertile garlic germplasm using modern molecular biology methods.

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Plasmodesmata (PDs) are intercellular organelles carrying multiple membranous nanochannels that allow the trafficking of cellular signalling molecules. The channel regulation of PDs occurs dynamically and is required in various developmental and physiological processes. It is well known that callose is a critical component in regulating PD permeability or symplasmic connectivity, but the understanding of the signalling pathways and mechanisms of its regulation is limited. Here, we used the reverse genetic approach to investigate the role of C-type lectin receptor-like kinase 1 (CLRLK1) in the aspect of PD callose-modulated symplasmic continuity. Here, we found that loss-of-function mutations in CLRLK1 resulted in excessive PD callose deposits and reduced symplasmic continuity, resulting in an accelerated gravitropic response. The protein interactome study also found that CLRLK1 interacted with actin depolymerizing factor 3 (ADF3) in vitro and in plants. Moreover, mutations in ADF3 result in elevated PD callose deposits and faster gravitropic response. Our results indicate that CLRLK1 and ADF3 negatively regulate PD callose accumulation, contributing to fine-tuning symplasmic opening apertures. Overall, our studies identified two key components involved in the deposits of PD callose and provided new insights into how symplasmic connectivity is maintained by the control of PD callose homoeostasis.

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This paper reports on a study investigating the viability and senescence of plum ovules when exposed to different constant temperatures over two years. The research was conducted on the primary and secondary ovules of four plum cultivars: 'Mallard', 'Edda', 'Jubileum', and 'Reeves'. The results show that the first indication of ovule viability loss was callose accumulation, which was detected using the fluorescent dye aniline blue. All cultivars had viable ovules, in different percentages, at 8 °C on the twelfth day after anthesis. However, at higher temperatures, distinct patterns emerged, indicating the adaptability of each cultivar at certain temperatures. The first indication of callose accumulation became visible at the chalazal pole. After anthesis, the ovule's ability to remain viable gradually reduced, followed by callose deposition throughout the ovary. The cultivars 'Edda' and 'Reeves', from 6 days after anthesis onward, in both years, showed the highest percentage of nonviable ovules. In contrast, the 'Jubileum' cultivar demonstrated the highest percentage of viable ovules. The loss of viability of secondary ovules followed a similar pattern to that of the primary ovules in all cultivars. This research provides valuable insights into embryological processes, which can help in the following breeding programs, and to cultivate plum cultivars in Western Norway's climate conditions.

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One of the early changes upon tuber induction is the switch from apoplastic to symplastic unloading. Whether and how this change in unloading mode contributes to sink strength has remained unclear. In addition, developing tubers also change from energy to storage-based sucrose metabolism. Here, we investigated the coordination between changes in unloading mode and sucrose metabolism and their relative role in tuber sink strength by looking into callose and sucrose metabolism gene expression combined with a model of apoplastic and symplastic unloading. Gene expression analysis suggests that callose deposition in tubers is decreased by lower callose synthase expression. Furthermore, changes in callose and sucrose metabolism are strongly correlated, indicating a well-coordinated developmental switch. Modelling indicates that symplastic unloading is not the most efficient unloading mode per se. Instead, it is the concurrent metabolic switch that provides the physiological conditions necessary to potentiate symplastic transport and thereby enhance tuber sink strength .

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The microdomains of plasmodesmata, specialized cell-wall channels responsible for communications between neighboring cells, are composed of various plasmodesmata-located proteins (PDLPs) and lipids. Here, we found that, among all PDLP or homologous proteins in Arabidopsis thaliana genome, PDLP5 and PDLP7 possessed a C-terminal sphingolipid-binding motif, with the latter being the only member that was significantly upregulated upon turnip mosaic virus and cucumber mosaic virus infections. pdlp7 mutant plants exhibited significantly reduced callose deposition, larger plasmodesmata diameters, and faster viral transmission. These plants exhibited increased glucosidase activity but no change in callose synthase activity. PDLP7 interacted specifically with glucan endo-1,3-ß-glucosidase 10 (BG10). Consistently, higher levels of callose deposition and slower virus transmission in bg10 mutants were observed. The interaction between PDLP7 and BG10 was found to depend on the presence of the Gnk2-homologous 1 (GnK2-1) domain at the N terminus of PDLP7 with Asp-35, Cys-42, Gln-44, and Leu-116 being essential. In vitro supplementation of callose was able to change the conformation of the GnK2-1 domain. Our data suggest that the GnK2-1 domain of PDLP7, in conjunction with callose and BG10, plays a key role in plasmodesmata opening and closure, which is necessary for intercellular movement of various molecules.