Cell wall‐derived mixed‐linked β‐1,3/1,4‐glucans trigger immune responses and disease resistance in plants

Abstract

Pattern-triggered immunity (PTI) is activated in plants upon recognition by pattern recognition receptors (PRRs) of damage-and microbe-associated molecular patterns (DAMPs and MAMPs) derived from plants or microorganisms, respectively. To understand better the plant mechanisms involved in the perception of carbohydrate-based structures recognized as DAMPs/MAMPs, we have studied the ability of mixed-linked b-1,3/1,4-glucans (MLGs), present in some plant and microbial cell walls, to trigger immune responses and disease resistance in plants. A range of MLG structures were tested for their capacity to induce PTI hallmarks, such as cytoplasmic Ca 2+ elevations, reactive oxygen species production, phosphorylation of mitogen-activated protein kinases and gene transcriptional reprogramming. These analyses revealed that MLG oligosaccharides are perceived by Arabidopsis thaliana and identified a trisaccharide, b-D-cellobiosyl-(1,3)-b-D-glucose (MLG43), as the smallest MLG structure triggering strong PTI responses. These MLG43-mediated PTI responses are partially dependent on LysM PRRs CERK1, LYK4 and LYK5, as they were weaker in cerk1 and lyk4 lyk5 mutants than in wild-type plants. Cross-elicitation experiments between MLG43 and the carbohydrate MAMP chitohexaose [b-1,4-D-(GlcNAc) 6 ], which is also perceived by these LysM PRRs, indicated that the mechanism of MLG43 recognition could differ from that of chitohexaose, which is fully impaired in cerk1 and lyk4 lyk5 plants. MLG43 treatment confers enhanced disease resistance in A. thaliana to the oomycete Hyaloperonospora arabidopsidis and in tomato and pepper to different bacterial and fungal pathogens. Our data support the classification of MLGs as a group of carbohydrate-based molecular patterns that are perceived by plants and trigger immune responses and disease resistance.

Mixed Linkage β-1,3/1,4-Glucan Oligosaccharides Induce Defense Responses in Hordeum vulgare and Arabidopsis thaliana

“…Whether or not this also holds true for glucose trimers and cellodextrins, which have recently been shown to have comparatively higher elicitor activities ( Locci et al, 2019 ) remains to be shown. However, our data are in conflict with the observations of Rebaque et al (2021) , who suggest that MLG perception is at least partially dependent on CERK1, LYK4, and LYK5. It is hard to rationalize this discrepancy, but the direct comparison of β-1,3/1,4-MLG oligosaccharide-induced MAPK activation in the wildtype and corresponding mutant genotypes that we conducted on the same membrane revealed no significant difference.…”

“…In contrast to monocot species, dicot plants such as A. thaliana do not contain β-1,3/1,4-MLGs in their cell walls (Zablackis et al, 1995;Burton and Fincher, 2009). However, the experimental data of this work and a recently published study (Rebaque et al, 2021) clearly demonstrate that Arabidopsis plants can detect β-1,3/1,4-MLG oligosaccharides and react with canonical PTI responses. As β-1,3/1,4-MLG oligosaccharides are not present in Arabidopsis, they are likely to function as a MAMP in this species.…”

“…As β-1,3/1,4-MLG oligosaccharides are not present in Arabidopsis, they are likely to function as a MAMP in this species. Interestingly, β-1,3/1,4-MLG polysaccharides have previously been identified in the plant pathogenic fungus Rhynchosporium secalis , the oomycete Hyaloperonospera arabidopsidis and the endosymbiotic bacterium Sinorhizobium meliloti ( Pettolino et al, 2009 ; Pérez-Mendoza et al, 2015 ; Rebaque et al, 2021 ). Microbial cell walls can contain between 50–60% glucans ( Bowman and Free, 2006 ), however, to what extent β-1,3/1,4-MLGs contribute to their chemical structure remains largely elusive.…”

“…Besides A. thaliana , also the crop plants pepper and tomato can detect β-1,3/1,4-MLG oligosaccharides ( Rebaque et al, 2021 ), which suggests that the responsible perception machinery is evolutionary highly conserved among dicot plants. Our study supports this conclusion and goes beyond in demonstrating an invariable intraspecific conservation in the dicot species Arabidopsis as well as potential evolutionary maintenance in the monocot lineage.…”

“…β-1,3/1,4-MLGs are found in the cell walls of most members of the monocot order Poales including the crop plant barley ( Hordeum vulgare ) as well as in evolutionary older plant lineages such as brown algae, liverworts and Equisetum spp., but are absent in dicot plants such as Arabidopsis thaliana ( Zablackis et al, 1995 ; Popper and Fry, 2003 ; Fry et al, 2008 ; Sørensen et al, 2008 ; Burton and Fincher, 2009 ; Salmeán et al, 2017 ). Interestingly, β-1,3/1,4-MLGs have also been identified in lichen, namely Cetaria islandica , as well as in fungal, oomycete and bacterial species ( Honegger and Haisch, 2001 ; Pettolino et al, 2009 ; Pérez-Mendoza et al, 2015 ; Rebaque et al, 2021 ). Thus, it is tempting to speculate that β-1,3/1,4-MLGs are more widespread cell wall components than previously thought.…”