Callose Deposition Research Articles

Microsporocytic ARF17 misexpression leads to an excess callose deposition and male sterility in Arabidopsis.

The accurate callose deposition plays important roles in pollen wall formation and pollen fertility. As a direct target of miRNA160, ARF17 participate in the formation of the callose wall. However, the impact of ARF17 misexpression in microsporocytes on callose wall formation and pollen fertility remains unknown. Here, the SDS promoter, which is capable of specifically driving gene expression in microsporocytes, was employed to drive the expression of 5mARF17. The pSDS:5mARF17#3 transgenic line were male sterile. TEM revealed that sporopollenin substance was embedded in a thicker callose layer, which resulted in the complete loss of exine structure and pollen abortion in the pSDS:5mARF17#3 line. Consistently, RT-qPCR revealed an increase in the expression of several Cals genes in pSDS:5mARF17#3. EMSA assay demonstrated that ARF17 could bind to the promoter of Cals4 gene, which further suggest that ARF17 could regulate several Cals genes expression. It is notable that the expression of several exine formation-related genes increased significantly in pSDS:5mARF17#3. In conclusion, our findings highlight that the regulation of miRNA160-ARF17 in microsporocytes modulates the thickness of the callose wall, which is crucial for pollen exine formation and intercellular communication.

Viral diversity and phloem transcriptional changes in grapevine Shiraz disease infected vines

Shiraz disease (SD) is a highly destructive disease of grapevines that is associated with grapevine virus A (GVA) infection of vineyards in Australia and South Africa. However, little is known about the transcriptional modifications in grapevine phloem tissues induced by SD. In this study, we explored the viral diversity and transcriptional changes linked to SD. Vines symptomatic for SD exhibited higher viral abundance and were also shown to be co-infected with both GVA and grapevine leafroll-associated virus (GLRaV-4) strain 9. Differential gene (DE) expression analysis revealed physiological responses of Vitis vinifera to the infection. Similar to other plant pathogen infections, SD upregulated genes associated with the systemic acquired resistance (SAR) mechanism and downregulated genes related to vine immunity. Additionally, upregulated genes suggests that callose deposition and the blocking of phloem sieve elements are likely employed by V. vinifera as a defence strategy to limit the internal spread of SD viruses.

Variability and coexistence of sexual and apomictic embryo sac development pathways in Fragaria x ananassa cv. ‘Camino real’

ABSTRACT While most apomictic plants have a defined type of gametophytic apomixis, the cultivars of strawberry reported various mechanisms of unreduced embryo sac formation. In this study, the main events of megasporogenesis and megagametogenesis during the sexual and apomictic processes of embryo sac formation were analyzed in the cultivar “Camino real”. Ovules at different developmental stages were sectioned and cleared and were observed using light microscopy and confocal laser scanning microscopy. Archesporial cells developed asynchronously and had diverse fates. These cells can alternatively be selected as mother cells, apomeiotic initials, persist in a resting stage, or degenerate. Other cells in the nucellus are able to acquire a functional megaspore identity. Apomixis did not involve callose deposition in their initial cells and derivatives. The initiation of apomixis was variable in time and eventually related to the interruption of the sexual process at pre-meiotic and post-meiotic stages of the ovule development. The occurrence of sexuality, apospory and diplospory of Antennaria and Taraxacum types is conserved across cultivars. The comparison with wild Fragaria species suggests that hybridization did not directly cause the observed phenotypic variation in apomixis mechanisms. An overview of the mode and time of unreduced sac formation can contribute to the development and management of clonal seed production in strawberry. The complex phenotypic variation additionally remains to be fully explained in light of emerging concepts regarding the regulatory mechanisms of plant reproductive development.

Swords and shields: the war between Candidatus Liberibacter asiaticus and citrus

Citrus Huanglongbing (HLB) represents a significant threat to the citrus industry, mainly caused by the phloem-limited bacterium Candidatus Liberibacter asiaticus (CLas). In this review, we summarize recent advances in understanding the relationship between citrus and CLas, particularly examining the functions of Sec-dependent effectors (SDEs) and non-classically secreted proteins (ncSPs) in virulence, as well as their targeted interactions with citrus. We further investigate the impact of SDEs on various physiological processes, including systemic acquired resistance (SAR), reactive oxygen species (ROS) accumulation, vesicle trafficking, callose deposition, cell death, autophagy, chlorosis and flowering. Additionally, we focus on the functional research on specific disease-resistant genes in citrus and the molecular mechanisms underlying disease resistance. Finally, we discuss the existing gaps and unresolved questions regarding citrus-CLas interactions, proposing potential solutions to facilitate the development of HLB-resistant citrus varieties.

Mixed DAMP/MAMP oligosaccharides promote both growth and defense against fungal pathogens of cucumber.

Plants recognize a variety of environmental molecules, thereby triggering appropriate responses to biotic or abiotic stresses. Substances containing microbes-associated molecular patterns (MAMPs) and damage-associated molecular patterns (DAMPs) are representative inducers of pathogen resistance and damage repair, thus treatment of healthy plants with such substances can pre-activate plant immunity and cell repair functions. In this study, the effects of DAMP/MAMP oligosaccharides mixture (Oligo-Mix) derived from plant cell wall (cello-oligosaccharide and xylo-oligosaccharide), and fungal cell wall (chitin-oligosaccharide) were examined in cucumber. Treatment of cucumber with Oligo-Mix promoted root germination and plant growth, along with increased chlorophyll contents in the leaves. Oligo-Mix treatment also induced typical defense responses such as MAP kinase activation and callose deposition in leaves. Pretreatment of Oligo-Mix enhanced disease resistance of cucumber leaves against pathogenic fungi Podosphaera xanthii (powdery mildew) and Colletotrichum orbiculare (anthracnose). Oligo-Mix treatment increased the induction of hypersensitive cell death around the infection site of pathogens, which inhibited further infection and the conidial formation of pathogens on the cucumber leaves. RNA-seq analysis revealed that Oligo-Mix treatment upregulated genes associated with plant structural reinforcement, responses to abiotic stresses and plant defense. These results suggested that Oligo-Mix has beneficial effects on growth and disease resistance in cucumber, making it a promising biostimulant for agricultural application.

The effect of silicon supplementation and drought stress on the deposition of callose and chemical components in the cell walls of the Brassica napus roots.

Silicon has an important role in regulating water management in plants. It is deposited in cell walls and creates a mechanical barrier against external factors. The aim of this study was to determine the role of silicon supplementation in the synthesis and distribution of callose in oilseed rape roots and to characterize the modifications of cell wall structure of these organs after exposure to drought stress. Histological and ultrastructural analyses were performed to determine the changes in the distribution of arabinogalactan proteins, pectins, and extensin in roots of Brassica napus growing under drought and supplemented with silicon. Callose deposition and the accumulation of callose synthase protein were assessed, followed by transcriptional analysis of callose synthase genes. The results showed that silicon supplementation under drought conditions alter the direction of cortex cell differentiation, promoting fiber formation and proliferation of callose-depositing cells in the roots of the tested plants. This was reflected in an increase in the level of callose synthase and a decrease in the transcriptional activity of the gene encoding this enzyme, indicating regulation based on negative feedback under drought stress. The changes in abundance and distribution of investigated arabinogalactan proteins, pectins and extensin in roots of Si supplemented plants growing under drought stress were observed, indicating cell walls remodeling. Silicon supplementation in oilseed rape roots induced significant changes in cell wall composition, including increased callose deposition and altered pectins and arabinogalactan proteins distribution. These modifications, along with the formation of fibres in the root cortex, likely contribute to enhanced cell wall strength providing a physical barrier against water loss and mechanical stress, as a probable defence mechanism induced during drought stress.

Unveiling the Virulence Mechanism of Leptosphaeria maculans in the Brassica napus Interaction: The Key Role of Sirodesmin PL in Cell Death Induction.

Leptosphaeria maculans is the causal agent of blackleg disease in Brassica napus, leading to substantial yield losses. Sirodesmin PL, the principal toxin produced by L. maculans, has been implicated in the infective process in plants. However, the precise molecular and physiological mechanisms governing its effects remain elusive. This study investigates the changes induced by Sirodesmin PL at the transcriptomic, physiological, and morphological levels in B. napus cotyledons. Sirodesmin PL treatment upregulates genes associated with plant defense processes, including response to chitin, sulfur compound biosynthesis, toxin metabolism, oxidative stress response, and jasmonic acid/ethylene synthesis and signaling. Validation of these transcriptomic changes is evidenced by several typical defense response processes, such as the accumulation of reactive oxygen species (ROS) and callose deposition. Concomitantly, oxidized Sirodesmin PL induces concentration- and exposure duration-dependent cell death. This cellular death is likely attributed to diminished activity of photosystem II and a reduction in the number of chloroplasts per cell. In agreement, a down-regulation of genes associated with the photosynthesis process is observed following Sirodesmin PL treatment. Thus, it is plausible that L. maculans exploits Sirodesmin PL as a virulence factor to instigate cell death in B. napus during its necrotrophic stage, favoring the infective process.

A Virulence Factor from Sclerotinia sclerotiorum Targets the Host Chloroplast Proteins to Promote Infection.

Chloroplasts are not only places for photosynthesis, but also participate in plant immunity and are important targets of pathogens. Pathogens secrete chloroplast-targeted proteins (CTPs) that disrupt host immunity and promote infection. Sclerotinia sclerotiorum (Lib.) de Bary is a phytopathogenic fungus with a broad host range. However, little is known about the pathogenic mechanisms underlying this wide host range. In this study, we investigated the role of Chloroplast-Targeted Protein 1 (SsCTP1) secreted by S. sclerotiorum in pathogenesis, which inhibits plant immunity and promotes pathogen infections. SsCTP1 was highly up-regulated during the early stages of S. sclerotiorum infection in various hosts, and its transient expression in Nicotiana benthamiana revealed that it was predominantly localized within chloroplasts. Mutants with SsCTP1 deletion exhibited a similar growth rate and colony morphology to the wild type, but significantly reduced pathogenicity in various hosts. Moreover, SsCTP1 inhibited chitin-induced callose deposition and defense gene expression, and enhanced sensitivity to S. sclerotiorum in N. benthamiana. Similarly, transgenic Arabidopsis thaliana overexpressing SsCTP1 displayed an increased susceptibility to S. sclerotiorum. Furthermore, two host proteins that interact with SsCTP1, Coproporphyrinogen-III oxidase (GmCPX), and shikimate kinase 2 (GmSKL2) were identified by screening the soybean cDNA library, and these interactions were confirmed in vivo. Importantly, the silencing of NbCPX by virus-induced gene silencing enhanced N. benthamiana resistance to S. sclerotiorum. Our results indicate that SsCTP1 is an important pathogenic factor that contributes to the wide host range of S. sclerotiorum and may inhibit plant immunity by targeting the chloroplast proteins GmCPX and GmSKL2, which are ubiquitous in host plants.

A viral protein activates the MAPK pathway to promote viral infection by downregulating callose deposition in plants

Mitogen-activated protein kinase (MAPK) cascades are evolutionarily conserved in both plants and animals and play critical roles in activating innate immunity to defend against various pathogens. However, the role of MAPK cascades in positively regulating or enhancing viral infections in plants is unclear. In this study, we investigate the involvement of MAPK cascades in infection by the positive-strand RNA virus tomato chlorosis virus (ToCV). Our findings reveal that ToCV infection activates MAPK cascades, promoting virus spread within plants. Specifically, ToCV P7, a pathogenicity determinant protein, localizes to the plasma membrane and recruits NbMPK3/6 from the nucleus. Subsequently, P7 is directly phosphorylated on serine 59 by NbMPK3/6. Phosphorylated P7 interacts with NbREM1.1 and inhibits its ability to induce callose deposition at plasmodesmata. These results demonstrate that NbMPK3/6 directly phosphorylate ToCV P7, modulating antiviral defence mechanisms by downregulating callose deposition at plasmodesmata and thereby enhancing ToCV transmission in N. benthamiana. This study sheds light on the intricate arms race between host defence and viral counter-defence strategies.

A rice rhizosphere plant growth-promoting Streptomyces corchorusii isolate antagonizes Magnaporthe oryzae and elicits defense responses in rice.

Rice blast, caused by Magnaporthe oryzae, is one of the most devastating diseases of rice (Oryza sativa L.). The aim of this study was to investigate the biocontrol potential of rice rhizosphere actinomycetes against M. oryzae Guy 11, and elucidate the antagonistic mechanisms. An isolate characterized as a Streptomyces corchorusii strain (Sc75) using the 16S rRNA gene exhibited superior antifungal activity. Sc75 had an inhibitory effect of 69.25%±0.15% against M. oryzae and broad antifungal activity on other fungal plant pathogens in the dual culture assay. Its cell-free culture filtrate inhibited fungal growth and reduced mycelial mass. Also, the ethyl acetate crude extract completely inhibited conidia germination and appressoria formation on the hydrophobic coverslips and detached leaf at a concentration of 20mg/ml. Its volatile organic compounds (VOCs) suppressed fungal growth by 98.42%. GC-MS analysis of the VOCs identified butanoic acid, 2-methyl-, methyl ester; di-tert-butyl peroxide; furan, 2-pentyl-; and undecanoic acid, 10-methyl-, methyl ester as the main components. In the greenhouse experiment, the disease severity was reduced and growth promotion was evident. Molecular investigation revealed that Sc75 upregulated defense-related genes involved in the synthesis of jasmonic acid, salicylic acid signaling pathway, and led to callose deposition and ROS production in the leaves. Finally, Sc75 produced hydrolytic enzymes, siderophore, indole acetic acid, gibberellic acid, phosphate solubilization, and 1-aminocyclopropane-1-carboxylate deaminase. The rice rhizosphere soil harbors actinomycetes that can be explored as biocontrol agents against fungal pathogens such as M. oryzae. The isolate Sc75 had superior antifungal activity against M. oryzae and other selected plant pathogenic fungi. It showed remarkable antagonistic activity through direct antibiosis, production of VOCs, antifungal metabolites in the culture filtrates and crude extracts, and produced enzymes. In addition, the isolate promoted plant growth, reduced rice blast disease index in the greenhouse experiment, and elicited defense-related responses. Sc75 is a promising candidate for future exploration as a biofungicide and a biofertilizer.

Phytosulfokine downregulates defense-related WRKY transcription factors and attenuates pathogen-associated molecular pattern-triggered immunity.

Phytosulfokine (PSK) is a plant growth-promoting peptide hormone that is perceived by its cell surface receptors PSKR1 and PSKR2 in Arabidopsis. Plants lacking the PSK receptors show phenotypes consistent with PSK signaling repressing some plant defenses. To gain further insight into the PSK signaling mechanism, comprehensive transcriptional profiling of Arabidopsis treated with PSK was performed, and the effects of PSK treatment on plant defense readouts were monitored. Our study indicates that PSK's major effect is to downregulate defense-related genes; it has a more modest effect on the induction of growth-related genes. WRKY transcription factors (TFs) emerged as key regulators of PSK-responsive genes, sharing commonality with a pathogen-associated molecular pattern (PAMP) responses, flagellin 22 (flg22), but exhibiting opposite regulatory directions. These PSK-induced transcriptional changes were accompanied by biochemical and physiological changes that reduced PAMP responses, notably mitogen-activated protein kinase (MPK) phosphorylation (previously implicated in WRKY activation) and the cell wall modification of callose deposition. Comparison with previous studies using other growth stimuli (the sulfated plant peptide containing sulfated tyrosine [PSY] and Pseudomonas simiae strain WCS417) also reveals WRKY TFs' overrepresentations in these pathways, suggesting a possible shared mechanism involving WRKY TFs for plant growth-defense trade-off.

The fungal effector AaAlta1 inhibits PATHOGENESIS-RELATED PROTEIN10-2-mediated callose deposition and defense responses in apple.

Apple leaf spot, caused by Alternaria alternata f. sp mali (ALT), poses a substantial threat to the global apple (Malus × domestica Borkh.) industry. Fungal effectors promote pathogen infestation and survival by interfering with plant immune responses. In our study, we investigated the secretion of effector proteins by the virulent ALT7 strain. Using mass spectrometry, we identified the effector AaAlta1, which belongs to the Alt a 1 protein family (AA1s). Further analysis confirmed that ALT7 secretes AaAlta1. AaAlta1 knockdown mutants displayed reduced pathogenicity in apple tissue culture seedlings, while overexpression strains exhibited enhanced pathogenicity compared to the wild-type ALT7 strain. Using immunoprecipitation followed by mass spectrometry, we isolated pathogenesis-related protein 10-2 (PR10-2) as an interaction partner of AaAlta1 in apple. Knockdown mutants of AaAlta1 showed increased PR10-2-mediated callose deposition in apple, a critical plant defense response. The enhanced defense responses in apple substantially reduced their susceptibility to infection by these ALT7 mutants. Our findings delineate an infection strategy whereby ALT7 secretes AaAlta1 to suppress PR10-2, thereby circumventing the apple defense system.

Integrating gene expression analysis and ecophysiological responses to water deficit in leaves of tomato plants

Soil water deficit (WD) significantly impacts plant survival and crop yields. Many gaps remain in our understanding of the synergistic coordination between molecular and ecophysiological responses delaying substantial drought-induced effects on plant growth. To investigate this synergism in tomato leaves, we combined molecular, ecophysiological, and anatomical methods to examine gene expression patterns and physio-anatomical characteristics during a progressing WD experiment. Four sampling points were selected for transcriptomic analysis based on the key ecophysiological responses of the tomato leaves: 4 and 5 days after WD (d-WD), corresponding to 10% and 90% decrease in leaf stomatal conductance; 8 d-WD, the leaf wilting point; and 10 d-WD, when air embolism blocks 12% of the leaf xylem water transport. At 4 d-WD, upregulated genes were mostly linked to ABA-independent responses, with larger-scale ABA-dependent responses occurring at 5 d-WD. At 8 d-WD, we observed an upregulation of heat shock transcription factors, and two days later (10 d-WD), we found a strong upregulation of oxidative stress transcription factors. Finally, we found that young leaves present a stronger dehydration tolerance than mature leaves at the same drought intensity level, presumably because young leaves upregulate genes related to increased callose deposition resulting in limiting water loss to the phloem, and related to increased cell rigidity by modifying cell wall structures. This combined dataset will serve as a framework for future studies that aim to obtain a more holistic WD plant response at the molecular, ecophysiological and anatomical level.

Comparative analysis of stomatal characteristics and Erysiphe necator (Schw.) Burrill resistance in diverse Vitis sp.

Powdery mildew caused by Erysiphe necator poses a major challenge for grapevine cultivation. This study investigates how stomatal and structural traits influence resistance to this pathogen across diverse Vitis genotypes. Microscopic analysis revealed significant variations in stomatal characteristics. The sunken stomata were observed in V. parviflora, V. jacquemontii, V. rupestris x V. berlandieri (110 Richter) and V. rupestris (St. George) with lower stomatal density. Genotypes with raised stomata had larger stomatal complex areas. Following inoculation with E. necator (accession No. 52218), the Vitis genotypes showed a distinct resistance response. Susceptible V. vinifera genotypes had high pathogen penetration rates, with 71 % of infection attempts forming haustoria. In contrast, V. parviflora, V. jacquemontii, and hybrids such as V. rupestris x V. berlandieri and V. riparia x V. cinerea exhibited programmed cell death (PCD)-mediated resistance, arresting up to 55 per cent of penetration attempts limiting hyphal growth. Cryo-SEM images further indicated sparse fungal growth on the resistant genotypes. The genotype V. parviflora possessed dense trichomes and long wax stripes along the epidermal cells covering leaf veins on the adaxial leaf surface, thus causing a physical barrier against the pathogen. Comparative analyses showed that callose deposition and epicuticular wax significantly contributed to early-stage pathogen defence, while reactive oxygen species and rapid PCD activation triggered hypersensitive responses, enhancing wax deposition and active PCD responses are critical for Vitis sp. resistance to powdery mildew, offering valuable insights for breeding programmes.

Defense-related callose synthase PMR4 promotes root hair callose deposition and adaptation to phosphate deficiency in Arabidopsis thaliana.

Plants acquire phosphorus (P) primarily as inorganic phosphate (Pi) from the soil. Under Pi deficiency, plants induce an array of physiological and morphological responses, termed phosphate starvation response (PSR), thereby increasing Pi acquisition and use efficiency. However, the mechanisms by which plants adapt to Pi deficiency remain to be elucidated. Here, we report that deposition of a β-1,3-glucan polymer called callose is induced in Arabidopsis thaliana root hairs under Pi deficiency, in a manner independent of PSR-regulating PHR1/PHL1 transcription factors and LPR1/LPR2 ferroxidases. Genetic studies revealed PMR4 (GSL5) callose synthase being required for the callose deposition in Pi-depleted root hairs. Loss of PMR4 also reduces Pi acquisition in shoots and plant growth under low Pi conditions. The defects are not recovered by simultaneous disruption of SID2, mediating defense-associated salicylic acid (SA) biosynthesis, excluding SA defense activation from the cause of the observed pmr4 phenotypes. Grafting experiments and characterization of plants expressing PMR4 specifically in root hair cells suggest that a PMR4 pool in the cell type contributes to shoot growth under Pi deficiency. Our findings thus suggest an important role for PMR4 in plant adaptation to Pi deficiency.

Salivary effector DcE1 suppresses plant defense and facilitates the successful feeding of Asian citrus psyllid, Diaphorina citri.

Piercing-sucking insects secrete diverse repertoires of effectors into their hosts to weaken host defenses and promote infestation. The Asian citrus psyllid, Diaphorina citri Kuwayama, is the most destructive insect pest in citrus orchards because of its role as a vector for the huanglongbing pathogen, Candidatus Liberibacter asiaticus (CLas). However, specific effector proteins and their functions in D. citri remain unclear. We demonstrate that DcE1, a salivary protein gene from D. citri, is predominantly expressed in the salivary gland tissues and is delivered into host plants during feeding. Transient expression in tobacco leaves revealed that DcE1 was subcellularly localized in the cytoplasm and plasma membrane, where it inhibited BAX- and INF1-induced cell death, suppressed callose deposition, and activated the salicylic acid pathway by upregulating the expression of endo-β-1,3-glucanase NtBGL2 and regulatory protein NtNPR1. Further, DcE1 knockdown by double-stranded RNA (dsRNA) injection decreased the survival rates of D. citri and interrupted D. citri phloem-feeding on host plants. These results indicate that DcE1 is a novel effector that promotes plant susceptibility and enables D. citri feeding. These findings enhance our understanding of D. citri-plant interactions and offer a potential new target gene for the development of citrus protection strategies. © 2024 Society of Chemical Industry.

Turnip mosaic virus selectively subverts a PR-5 thaumatin-like, plasmodesmal protein to promote viral infection.

Pathogenesis-related (PR) proteins are induced by abiotic and biotic stresses and generally considered as part of the plant defense mechanism. However, it remains yet largely unclear if and how they are involved in virus infection. Our recent quantitative, comparative proteomic study identified three PR-5 family proteins that are significantly differentially accumulated in the plasmodesmata (PD)-enriched fraction isolated from Nicotiana benthamiana leaves infected by turnip mosaic virus (TuMV). In this study, we employed the TuMV-Arabidopsis pathosystem to characterize the involvement of two Arabidopsis orthologs, AtOSM34 and AtOLP of the three N. benthamiana PR-5-like proteins. We show that AtOSM34 and AtOLP are PD-localized proteins and their expression is up- and downregulated in response to TuMV infection, respectively. Deficiency or overexpression of AtOLP does not affect viral RNA accumulation. Knockdown of AtOSM34 inhibits TuMV infection, whereas its overexpression promotes viral infection. We further demonstrate that AtOSM34 functions as a proviral factor through diminishing PD callose deposition to promote viral intercellular movement, targeting the viral replication complex to enhance viral replication, and suppressing the ROS-mediated antiviral response. Taken together, these data suggest that TuMV has evolved the ability to selectively upregulate and subvert AtOSM34, a PR-5 family protein to assist its infection.

Fusarium sacchari Effector FsMEP1 Contributes to Virulence by Disturbing Localization of Thiamine Thiazole Synthase ScTHI2 from Sugarcane.

Fusarium sacchari is a significant pathogenic fungus that causes sugarcane Pokkah Boeng. Proteins secreted by pathogenic fungi can be delivered into hosts to suppress plant immunity and establish infection. However, there is still much to be discovered regarding F. sacchari's secreted effectors in overcoming plant immunity. In this paper, we characterize a novel effector called FsMEP1, which is essential for the virulence of F. sacchari. FsMEP1 contains a conserved zinc-binding motif sequence, HEXXH, and is highly expressed during host infection. Using the Agrobacterium tumefaciens-mediated transient expression system, it was confirmed that FsMEP1 could suppress Bcl-2-associated X protein (BAX)-triggered cell death, callose deposition, and ROS explosion in Nicotiana benthamiana. Furthermore, the deletion of FsMEP1 demonstrated its requirement for contributing to the pathogenicity of F. sacchari in sugarcane. Further analysis revealed that FsMEP1 could interact with the sugarcane thiamine thiazole synthase ScTHI2 and disrupt its normal localization, thereby inhibiting the synthesis of thiamine and the defense responses mediated by ScTHI2. Based on these findings, we propose that ScTHI2 represents a potential molecular target for improving sugarcane resistance to Pokkah Boeng disease.

The SA-WRKY70-PR-Callose Axis Mediates Plant Defense Against Whitefly Eggs.

The molecular mechanisms of plant responses to phytophagous insect eggs are poorly understood, despite their importance in insect-plant interactions. This study investigates the plant defense mechanisms triggered by the eggs of whitefly Bemisia tabaci, a globally significant agricultural pest. A transcriptome comparison of tobacco plants with and without eggs revealed that whitefly eggs may activate the response of defense-related genes, including those involved in the salicylic acid (SA) signaling pathway. SA levels are induced by eggs, resulting in a reduction in egg hatching, which suggests that SA plays a key role in plant resistance to whitefly eggs. Employing Agrobacterium-mediated transient expression, virus-induced gene silencing assays, DNA-protein interaction studies, and bioassays, we elucidate the regulatory mechanisms involved. Pathogenesis-related proteins NtPR1-L1 and NtPR5-L2, downstream of the SA pathway, also affect whitefly egg hatching. The SA-regulated transcription factor NtWRKY70a directly binds to the NtPR1-L1 promoter, enhancing its expression. Moreover, NtPR1-L1 promotes callose deposition, which may impede the eggs' access to water and nutrients. This study establishes the SA-WRKY70-PR-callose axis as a key mechanism linking plant responses and defenses against whitefly eggs, providing new insights into the molecular interactions between plants and insect eggs.

Transcriptomic profiling of 'Candidatus Liberibacter asiaticus' in different citrus tissues reveals novel insights into Huanglongbing pathogenesis.

'Candidatus Liberibacter asiaticus' (Las) is a gram-negative bacterial pathogen associated with citrus huanglongbing (HLB) or greening disease. Las is transmitted by the Asian citrus psyllid (ACP) where it colonizes the phloem tissue, resulting in substantial economic losses to citrus industry worldwide. Despite extensive efforts, effective management strategies against HLB remain elusive, necessitating a deeper understanding of the pathogen's biology. Las undergoes cell-to-cell movement through phloem flow and colonizes different tissues in which Las may have varying interactions with the host. Here, we investigate the transcriptomic landscape of Las in citrus seed coat vasculatures, enabling a complete gene expression profiling of Las genome and revealing unique transcriptomic patterns compared to previous studies using midrib tissues. Comparative transcriptomics between seed coat, midrib and ACP identified specific responses and metabolic states of Las in different host tissue. Two Las virulence factors that exhibit higher expression in seed coat can suppress callose deposition. Therefore, they may contribute to unclogging sieve plate pores during Las colonization in seed coat vasculature. Furthermore, analysis of regulatory elements uncovers a potential role of LuxR-type transcription factors in regulating Liberibacter effector gene expression during plant colonization. Together, this work provides novel insights into the pathogenesis of the devastating citrus HLB.