Milena Malisic’s research while affiliated with Max Planck Institute for Plant Breeding Research and other places

Carbon concentrating mechanisms enhance the carboxylase efficiency of the central photosynthetic enzyme rubisco by providing supra-atmospheric concentrations of CO2 in its surrounding. In the C4 photosynthesis pathway, this feat is realised by combinatory changes to leaf biochemistry and anatomy. In contrast to the C4 pathway, carbon concentration can also be achieved by the photorespiratory glycine shuttle which requires fewer and less complex modifications. Plants displaying CO2 compensation points between 10 to 40 ppm are often considered to utilize such a photorespiratory shuttle and are termed 'C3-C4 intermediates'. In the present study, we perform a physiological, biochemical and anatomical survey of a large number of Brassicaceae species to better understand the C3-C4 intermediate phenotype, including its basic components and its plasticity. Our phylogenetic analysis suggested that C3-C4 metabolism evolved up to five times independently in the Brassicaceae. The efficiency of the pathway showed considerable variation between tested plant species. Centripetal accumulation of organelles in the bundle sheath was consistently observed in all C3-C4 classified taxa indicating a crucial role of anatomical features for CO2 concentrating pathways. Leaf metabolite patterns were strongly influenced by the individual species, but accumulation of photorespiratory shuttle metabolites glycine and serine was generally observed. Analysis of PEPC activities and metabolite composition suggests that C4-like shuttles have not evolved in the investigated Brassicaceae. Convergent evolution of the photorespiratory shuttle indicates that it represents a distinct and fit photosynthesis type.

Soil-dwelling microbes are the principal inoculum for the root microbiota, but our understanding of microbe-microbe interactions in microbiota establishment remains fragmentary. We tested 39,204 binary interbacterial interactions for inhibitory activities in vitro, allowing us to identify taxonomic signatures in bacterial inhibition profiles. Using genetic and metabolomic approaches, we identified the antimicrobial 2,4-diacetylphloroglucinol (DAPG) and the iron chelator pyoverdine as exometabolites whose combined functions explain most of the inhibitory activity of the strongly antagonistic Pseudomonas brassicacearum R401. Microbiota reconstitution with a core of Arabidopsis thaliana root commensals in the presence of wild-type or mutant strains revealed a root niche-specific cofunction of these exometabolites as root competence determinants and drivers of predictable changes in the root-associated community. In natural environments, both the corresponding biosynthetic operons are enriched in roots, a pattern likely linked to their role as iron sinks, indicating that these cofunctioning exometabolites are adaptive traits contributing to pseudomonad pervasiveness throughout the root microbiota.

Carbon concentrating mechanisms enhance the carboxylase efficiency of the central photosynthetic enzyme rubisco by providing supra-atmospheric concentrations of CO 2 in its surrounding. In the C 4 photosynthesis pathway, this is achieved by combinatory changes to leaf biochemistry and anatomy. Carbon concentration by the photorespiratory glycine shuttle requires fewer and less complex modifications. It could represent an early step during evolution from C 3 to C 4 photosynthesis and an inspiration for engineering approaches. Plants displaying CO 2 compensation points between 10 to 40 ppm are therefore often termed ‘C 3 –C 4 intermediates’. In the present study, we perform a physiological, biochemical and anatomical survey of a large number of Brassicaceae species to better understand the C 3 -C 4 intermediate phenotype. Our phylogenetic analysis suggested that C 3 -C 4 metabolism evolved up to five times independently in the Brassicaceae. The efficiency of the pathways showed considerable variation between the species but also within species. Centripetal accumulation of organelles in the bundle sheath was consistently observed in all C 3 -C 4 classified accessions indicating a crucial role of anatomical features for CO 2 concentrating pathways. Leaf metabolite patterns were strongly influenced by the individual plant accessions, but accumulation of photorespiratory shuttle metabolites glycine and serine was generally observed. Analysis of PEPC activities suggests that C 4 -like shuttles have not evolve in the investigated Brassicaceae. Highlight Our physiological, biochemical and anatomical survey of Brassicaceae revels multiple evolution of C 3 -C 4 intermediacy connected to variation in photorespiratory carbon recapturing efficiency and a distinct C 3 -C 4 bundle sheath anatomy.

Plant pathogenic and beneficial fungi have evolved several strategies to evade immunity and cope with host-derived hydrolytic enzymes and oxidative stress in the apoplast, the extracellular space of plant tissues. Fungal hyphae are surrounded by an inner insoluble cell wall (CW) layer and an outer soluble extracellular polysaccharide (EPS) matrix. Here we show by proteomics and glycomics that these two layers have distinct protein and carbohydrate signatures, and hence likely have different biological functions. The barley (Hordeum vulgare) β-1,3-endoglucanase HvBGLUII, which belongs to the widely distributed apoplastic glycoside hydrolase 17 family (GH17), releases a conserved β-1,3;1,6-glucan decasaccharide (β-GD) from the EPS matrices of fungi with different lifestyles and taxonomic positions. This low molecular weight β-GD does not activate plant immunity, is resilient to further enzymatic hydrolysis by β-1,3-endoglucanases due to the presence of three β-1,6-linked glucose branches and can scavenge reactive oxygen species. Exogenous application of β-GD leads to enhanced fungal colonization in barley, confirming its role in the fungal counterdefensive strategy to subvert host immunity. Our data highlight the hitherto undescribed capacity of this often-overlooked EPS matrix from plant-associated fungi to act as an outer protective barrier important for fungal accommodation within the hostile environment at the apoplastic plant-microbe interface.

Plant pathogenic and beneficial fungi have evolved several strategies to evade immunity and cope with host-derived hydrolytic enzymes and oxidative stress in the apoplast, the extracellular space of plant tissues. Fungal hyphae are surrounded by an inner, insoluble cell wall (CW) layer and an outer, soluble extracellular polysaccharide (EPS) matrix. Here we show by proteomics and glycomics that these two layers have distinct protein and carbohydrate signatures, implicating different biological functions. The barley ( Hordeum vulgare ) β-1,3-endoglucanase Hv BGLUII, which belongs to the widely distributed apoplastic glycoside hydrolase 17 family (GH17), releases a conserved β-1,3;1,6-glucan decasaccharide (β-GD) from the EPS matrices of fungi with different lifestyles and taxonomic positions. This low molecular weight β-GD does not activate plant immunity, is resilient to further enzymatic hydrolysis by β-1,3-endoglucanases due to the presence of three β-1,6-linked glucose branches and can scavenge reactive oxygen species. Additionally, exogenous application of β-GD leads to enhanced fungal colonization in barley. Our data highlights the hitherto undescribed capacity of this often overseen fungal EPS layer to act as an outer protective barrier important for fungal accommodation within the hostile environment at the apoplastic plant-microbe interface. Significance Here we identify and characterize a conserved β-1,3;1,6-glucan decasaccharide with antioxidant activity released from the fungal extracellular polysaccharide (EPS) matrix by the activity of a plant apoplastic endoglucanase. In addition, we provide a quantitative proteomic analysis of the fungal EPS and cell wall (CW) layers. HIGHLIGHTS The fungal extracellular polysaccharide (EPS) matrix and the cell wall (CW) are specific layers with distinct protein and carbohydrate signatures A conserved β-1,3;1,6-glucan decasaccharide (β-GD) is released from the EPS matrices of different fungi by the activity of the barley β-1,3-endoglucanase BGLUII, a member of the widely distributed apoplastic GH17 family The β-GD efficiently scavenges reactive oxygen species (ROS) and enhances fungal colonization The immunomodulatory potential as microbe-associated molecular pattern (MAMP) as well as the biochemical activity as ROS scavenger of soluble low molecular weight β-glucans are defined by the presence of β-1,6-glucose branches

To defend against microbial invaders but also to establish symbiotic programs, plants need to detect the presence of microbes through the perception of molecular signatures characteristic of a whole class of microbes. Among these molecular signatures, extracellular glycans represent a structurally complex and diverse group of biomolecules that has a pivotal role in the molecular dialogue between plants and microbes. Secreted glycans and glycoconjugates like symbiotic lipochitooligosaccharides or immunosuppressive cyclic β-glucans act as microbial messengers that prepare the ground for host colonization. On the other hand, microbial cell-surface glycans are important indicators of microbial presence. They are conserved structures normally exposed and thus accessible for plant hydrolytic enzymes and cell-surface receptor proteins. While the immunogenic potential of bacterial cell-surface glycoconjugates such as lipopolysaccharides and peptidoglycan has been intensively studied in the past years, perception of cell-surface glycans from filamentous microbes such as fungi or oomycetes is still largely unexplored. To date, only few studies have focused on the role of fungal-derived cell-surface glycans other than chitin, highlighting a knowledge gap that needs to be addressed. The objective of this review is to give an overview on the biological functions and perception of microbial extracellular glycans, primarily focusing on their recognition and their contribution to plant-microbe interactions.