Callose, found in the cell walls of higher plants such as β-1,3-glucan with β-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.

Virus diseases greatly affect oilseed rape (Brassica napus) production. Investigating antiviral genes may lead to the development of disease-resistant varieties of oilseed rape. In this study, we examined the effects of the suppressor of gene silencing 3 in Brassica napus (BnSGS3, a putative antiviral gene) with different genus viruses by constructing BnSGS3-overexpressing (BnSGS3-Ov) and BnSGS3-silenced (BnSGS3-Si) oilseed rape (cv. Zhongshuang No. 6) plants. These three viruses are Oilseed rape mosaic virus (ORMV), Turnip mosaic virus (TuMV) and Cucumber mosaic virus (CMV). The native BnSGS3 expressed in all examined tissues with the highest expression in siliques. All three viruses induced BnSGS3 expression, but ORMV induced a dramatic increase in the BnSGS3-Ov plants, followed by TuMV and CMV. Upon inoculation with three different viruses, transcript abundance of BnSGS3 gene follows: BnSGS3-Ov > non-transgenic plants > BnSGS3-Si. The accumulation quantities of ORMV and TuMV exhibited a similar trend. However, CMV accumulation showed an opposite trend where virus accumulations were negatively correlated with BnSGS3 expression. The results suggest that BnSGS3 selectively inhibits CMV accumulation but promotes ORMV and TuMV accumulation. BnSGS3 should be used in different ways (up- and down-regulation) for breeding virus-resistant oilseed rape varieties.

Alternaria brown spot is an important fungal disease in citrus. The infection of young citrus organs usually coincides with strong rainfall, which causes low efficiency of fungicides and the outbreak of this disease. Here, the microbiomes of the asymptomatic peels, the spot edge, and the center of citrus fruits were compared to reveal the commensal microbes as alternative control methods for the Alternaria pathogen. As the disease severity increased from the asymptomatic peels to the spot edge and the center, the bacterial communities were more severely changed than the fungal communities. Both the bacterial diversity, represented by the Shannon diversity index, and the bacterial composition and structure significantly decreased and altered, respectively. Increased Alternaria, in relative abundance, correlated positively with bacterial genera like Massilia and Sphingomonas, while negatively correlating with bacterial genera like Delftia and Pantoea. In addition, Alternaria fASV1 positively correlated with several top ASVs of 1174_901_12 and Sphingomonas. These results suggest that the bacterial communities respond to Alternaria brown spot by Alternaria–bacteria cross-kingdom interactions; these responsive bacteria are worth testing experimentally.

Pseudofabraea citricarpa (Dermateaceae: Helotiales) is known as a significant pathogen causing Citrus target spot disease and results in profound yield loss. In the present study, the complete mitochondrial genome (mitogenome) determined based on next-generation sequencing technology. The circular mitogenome (56,935 bp) comprised 14 conserved protein-coding genes (PCGs), 16 ORFs, two ribosomal RNA genes (rns and rnl), one non-coding RNA gene (rnpB), one ribosomal protein S3 (rps3) and 28 transfer RNA (tRNA) genes. The overall base composition is as follows: 36.08% A, 35.25% T, 13.04% C, and 15.63% G, with a GC content of 28.70%. The phylogenetic analysis shows that P. citricarpa, belonging to Dermateaceae, forms a separate clade and is sister to Sclerotiniaceae. The mitogenome of P. citricarpa reported in this study provides more molecular data for further research on the evolutionary relationships of Helotiales.

A plant pathogen is a deterministic factor affecting the plant microbiome. However, whether the influence of different pathogen species on the plant microbiome is consistent is not well-known. In this study, mulberry leaves with similar spots caused by two diseases—mulberry brown spot (MBS, caused by Neophloeospora maculans) and mulberry snags rotten leaves disease (MSRL, caused by Boeremia exigua var. mori)—along with asymptomatic leaves, were collected. Their endophytic fungal and bacterial communities were sequenced. The occurrence of MSRL reduced leaf fungal richness and diversity (observed species from 329 to 203; Shannon diversity index from 3.6 to 2.6, p < 0.05; phylogenetic diversity index from 45.8 to 20.6, p < 0.05), along with leaf bacterial diversity; in contrast, the occurrence of MBS had little effect on both fungal and bacterial richness and diversity. Both diseases significantly altered the composition and structure of leaf fungal communities, whereas only MBS did so for bacterial communities. For the dominant genera, the fungal genera, except Cercospora, were differently affected by the occurrence of MBS and MSRL, while the bacterial genera Pseudomonas (from 8.3% to 30.6%, p < 0.05 for MBS; from 23.1% to 28.1% for MSRL), Massilia (0.2% to 7.7%, p < 0.05; 2.1% to 6.1%), Chryseobacterium (0.1% to 0.3%; 2.7% to 7%), Pantoea (2.6% to 3.7%; 0.4% to 1%), and Aureimonas (0.7% to 1.4%; 1.3% to 1.7%) were consistently enriched by both diseases. In addition, Chryseobacterium (LDA score = 4.6, p < 0.01) was detected as a biomarker after the occurrence of MSRL. Intriguingly, there were four times more significant fungi–bacteria positive associations induced by the occurrence of MSRL compared to MBS (10.2% to 2.5%). Taken together, the responses of mulberry leaf microbiome to pathogen infection vary in response to the pathogen species; although several bacterial genera were consistently enriched, whether their enrichment reflects their antagonistic roles in fungal—bacterial inter-kingdom interactions remains to be determined and should be interpreted with further experimental tests.