Potential for Seed Transmission of Verticillium longisporum in Oilseed Rape (Brassica napus)

Heterosis of endophytic microbiomes in hybrid rice varieties improves seed germination

Seed endophytic microbiomes are shaped by host and environmental factors and play a crucial role in their host growth and health. Studies have demonstrated that host genotype, including hybridization, affects seed microbiomes. Heterosis features are also observed in root-associated microbiomes. It remains unclear, however, whether heterosis exists in seed endophytic microbiomes and whether hybrid microbiota provide noticeable advantages to host plant growth, especially to seed germination. Here, we investigated the structure of seed endophytic bacterial and fungal communities from three hybrid rice varieties and their respective parents using amplicon sequencing targeting 16S rRNA and ITS2 genes. Heterosis was found in diversity and composition of seed endophytic microbiomes in hybrids, which hosted more diverse communities and significantly higher abundances of plant growth-promoting taxa, such as Pseudomonas and Rhizobium genera compared with their parental lines. Co-occurrence network analysis revealed that there are potentially tighter microbial interactions in the hybrid seeds compared with their parent seeds. Finally, inoculation of seed-cultivable endophytes, isolated from hybrids, resulted in a greater promotion of seed germination compared with those isolated from parent lines. These findings suggest that heterosis exists not only in plant traits but also in seed endophytic microbiota, the latter in turn promotes seed germination, which offers valuable guidance for microbiome-assisted rice breeding.IMPORTANCEGenetic and physiological changes associated with plant hybridization have been studied for many crop species. Still, little is known about the impact of hybridization on the seed microbiota. In this study, we indicate that hybridization has a significant impact on the endophytic bacterial and fungal communities in rice seeds. The seed endophytic microbiomes of hybrids displayed distinct characteristics from those of their parental lines and exhibited potential heterosis features. Furthermore, the inoculation of seed-cultivable endophytes isolated from hybrids exhibited a greater promotion effect on seed germination compared with those isolated from the parents. Our findings make a valuable contribution to the emerging field of microbiome-assisted plant breeding, highlighting the potential for a targeted approach that aims to achieve not only desired plant traits but also plant-beneficial microbial communities on the seeds.

Tissue-Specific Hormone Signalling and Defence Gene Induction in an In Vitro Assembly of the Rapeseed Verticillium Pathosystem

Priming plants with beneficial microbes can establish rapid and robust resistance against numerous pathogens. Here, compelling evidence is provided that the treatment of rapeseed plants with Trichoderma harzianum OMG16 and Bacillus velezensis FZB42 induces defence activation against Verticillium longisporum infection. The relative expressions of the JA biosynthesis genes LOX2 and OPR3, the ET biosynthesis genes ACS2 and ACO4 and the SA biosynthesis and signalling genes ICS1 and PR1 were analysed separately in leaf, stem and root tissues using qRT-PCR. To successfully colonize rapeseed roots, the V. longisporum strain 43 pathogen suppressed the biosynthesis of JA, ET and SA hormones in non-primed plants. Priming led to fast and strong systemic responses of JA, ET and SA biosynthesis and signalling gene expression in each leaf, stem and root tissue. Moreover, the quantification of plant hormones via UHPLC-MS analysis revealed a 1.7- and 2.6-fold increase in endogenous JA and SA in shoots of primed plants, respectively. In roots, endogenous JA and SA levels increased up to 3.9- and 2.3-fold in Vl43-infected primed plants compared to non-primed plants, respectively. Taken together, these data indicate that microbial priming stimulates rapeseed defence responses against Verticillium infection and presumably transduces defence signals from the root to the upper parts of the plant via phytohormone signalling.

From seed to seed: the role of microbial inheritance in the assembly of the plant microbiome

Despite evidence that the microbiome extends host genetic and phenotypic traits, information on how the microbiome is transmitted and maintained across generations remains fragmented. For seed-bearing plants, seeds harbor a distinct microbiome and play a unique role by linking one generation to the next. Studies on microbial inheritance, a process we suggest including both vertical transmission and the subsequent migration of seed microorganisms to the new plant, thus become essential for our understanding of host evolutionary potential and host-microbiome coevolution. We propose dividing the inheritance process into three stages: (i) plant to seed, (ii) seed dormancy, and (iii) seed to seedling. We discuss the factors affecting the assembly of the microbiome during the three stages, highlight future research directions, and emphasize the implications of microbial inheritance for fundamental science and society.

Role of seed infection for the near and far distance dissemination of wheat blast caused by Magnaporthe oryzae pathotype Triticum

Magnaporthe oryzae pathotype Triticum (MoT) is a devastating fungal phytopathogen causing wheat blast disease which threatens wheat production particularly in warmer climate zones. Effective disease control is hampered by the limited knowledge on the life cycle, epidemiology, and pathogenicity of MoT. Since MoT mainly infects and colonizes the inflorescences of wheat, infection, invasion routes and colonization of MoT on wheat ears and in wheat seeds were investigated in order to assess potential seed transmission pathways. MoT was spray inoculated on two wheat cultivars (Sumai 3, susceptible and Milan, resistant) at three ear maturity stages [full ear emergence, growth stage (GS) 59; mid flowering, GS 65; and end of flowering, GS 69]. Incidence of MoT on Sumai 3 seeds was 100% and 20-25% on Milan. MoT sporulation rate on Sumai 3 contaminated seeds was more than 15 times higher than on Milan. Repeated washes of seed samples for removing paraffin fixation hampers seed microscopy. To overcome the damage of seed samples, we used hand-sectioned seed samples instead of paraffin-fixed microtome samples to facilitate microscopy. The colonization of MoT within various seed tissues was followed by light and confocal laser scanning microscopy (CLSM). Invasion of MoT in seeds predominantly occurred in the caryopsis germ region, but entry via other seed parts was also observed, confirming the potential of intense colonization of MoT in wheat grains. Fungal spread in wheat plants growing from MoT infected seeds was monitored through plating, microscopic and molecular techniques. Under greenhouse conditions, no spread of MoT from infected seeds to seedlings later than GS 21 or to ears was detected, neither in Milan nor in Sumai 3. We therefore conclude, that MoT may not systemically contaminate inflorescences and seeds in neither susceptible nor resistant wheat cultivars. However, initial blast symptoms, only found on seedlings of Sumai 3 but not Milan, resulted in the formation of new conidia, which may serve as inoculum source for plant-to-plant dissemination by airborne infection of plant stands in the field (short distance spread). Ultimately the inoculum may infect young inflorescences in the field and contaminate seeds. Our findings again stress the risk of long-distance dissemination of wheat blast across continents through MoT-contaminated seeds. This underlines the importance of mandatory use of healthy seeds in strategies to control any further spread of wheat blast.

Use of red, far-red, and near-infrared light in imaging of yeasts and filamentous fungi

Abstract

While phototoxicity can be a useful therapeutic modality not only for eliminating malignant cells but also in treating fungal infections, mycologists aiming to observe morphological changes or molecular events in fungi, especially when long observation periods or high light fluxes are warranted, encounter problems owed to altered regulatory pathways or even cell death caused by various photosensing mechanisms. Consequently, the ever expanding repertoire of visible fluorescent protein toolboxes and high-resolution microscopy methods designed to investigate fungi in vitro and in vivo need to comply with an additional requirement: to decrease the unwanted side effects of illumination. In addition to optimizing exposure, an obvious solution is red-shifted illumination, which, however, does not come without compromises. This review summarizes the interactions of fungi with light and the various molecular biology and technology approaches developed for exploring their functions on the molecular, cellular, and in vivo microscopic levels, and outlines the progress towards reducing phototoxicity through applying far-red and near-infrared light.

Key points

• Fungal biological processes alter upon illumination, also under the microscope

• Red shifted fluorescent protein toolboxes decrease interference by illumination

• Innovations like two-photon, lightsheet, and near IR microscopy reduce phototoxicity

Inheritance of seed and rhizosphere microbial communities through plant-soil feedback and soil memory

Since the discovery of the role of microbes in the phytobiome, microbial communities (microbiota) have been identified and characterized based on host species, development, distribution, and condition. The microbiota in the plant rhizosphere is believed to have been established prior to seed germination and innate immune development. However, the microbiota in seeds has received little attention. Although our knowledge of the distribution of microbiota in plant seeds and rhizosphere is currently limited, the impact of these microbiota is likely to be greater than expected. This minireview suggests a new function of microbial inheritance from the seed to root and from the first generation of plants to the next. Surprisingly, recruitment and accumulation of microbiota by biotic and abiotic stresses affect plant immunity in the next generation through plant-soil feedback and soil memory. To illustrate this process, we propose a new term called 'microbiota-induced soil inheritance (MISI).' A comprehensive understanding of MISI will provide novel insights into plant-microbe interactions and plant immunity inheritance.