A primary cell wall cellulose-dependent defense mechanism against vascular pathogens revealed by time-resolved dual transcriptomics

Abstract

Background Cell walls (CWs) are protein-rich polysaccharide matrices essential for plant growth and environmental acclimation. The CW constitutes the first physical barrier as well as a primary source of nutrients for microbes interacting with plants, such as the vascular pathogen Fusarium oxysporum (Fo). Fo colonizes roots, advancing through the plant primary CWs towards the vasculature, where it grows causing devastation in many crops. The pathogenicity of Fo and other vascular microbes relies on their capacity to reach and colonize the xylem. However, little is known about the root-microbe interaction before the pathogen reaches the vasculature and the role of the plant CW during this process. Results Using the pathosystem Arabidopsis-Fo5176, we show dynamic transcriptional changes in both fungus and root during their interaction. One of the earliest plant responses to Fo5176 was the downregulation of primary CW synthesis genes. We observed enhanced resistance to Fo5176 in Arabidopsis mutants impaired in primary CW cellulose synthesis. We confirmed that Arabidopsis roots deposit lignin in response to Fo5176 infection, but we show that lignin-deficient mutants were as susceptible as wildtype plants to Fo5176. Genetic impairment of jasmonic acid biosynthesis and signaling did not alter Arabidopsis response to Fo5176, whereas impairment of ethylene signaling did increase vasculature colonization by Fo5176. Abolishing ethylene signaling attenuated the observed resistance while maintaining the dwarfism observed in primary CW cellulose-deficient mutants. Conclusions Our study provides significant insights on the dynamic root-vascular pathogen interaction at the transcriptome level and the vital role of primary CW cellulose during defense response to these pathogens. These findings represent an essential resource for the generation of plant resistance to Fo that can be transferred to other vascular pathosystems.

Impairment of the cellulose degradation machinery enhances Fusarium oxysporum virulence but limits its reproductive fitness

“…However, both pathogenic fungi and mutualistic mycorrhizae recruit the fatty acid biosynthesis program to facilitate host invasion. As reported for these microbes ( 34 , 73 ), we observed an up-regulation of the expression of these fatty acid biosynthesis genes in Fo during Arabidopsis root infection. In addition, cutinases from plant pathogens were shown to cleave suberin in vitro ( 74 ), a plant compound that prevents the spread of microbial pathogens ( 75 ).…”

“…To understand the role of Fo cellulases in root infection, we aimed to obtain an Fo5176 mutant impaired in cellulose degradation. Considering the large number of putative cellulases expressed by Fo5176 during root colonization ( 34 ) and their high functional redundancy, we chose to target the transcription factor CLR1, a master regulator of fungal cellulase expression previously identified in N. crassa ( 38 ). Using an in silico analysis, we found CLR1 orthologs in various Fusarium genomes, sharing a protein sequence identity above 70% and having the exact same sequence as the DNA binding domain identified in N. crassa ( Fig.…”

“…Microbes need to loosen and digest the plant CWs to colonize their hosts. As the major and most recalcitrant CW component, cellulose is a key target for a plethora of transcriptionally coordinated microbial CWMPs (15,34,38,39,(63)(64)(65)(66). In this work, we studied the function of cellulases in the life cycle of a plant fungal pathogen.…”

“…Our previously reported transcriptomic data indicated that the days 2 and 3 of the root-Fo5176 interaction are critical time points for disease development ( 34 ). The drastic decrease of sucrose levels detected in infected roots at these time points ( Fig.…”

“…Cellulose is the most abundant and most recalcitrant component of the CW that provides mechanical strength to the plant cells because of its paracrystalline structure (7). Consequently, its synthesis is tightly regulated in response to intruders, including Fo (32)(33)(34), making cellulose one of the main targets of microbial CWMPs. Pathogens secrete a broad range of cellulolytic enzymes [glycosyl hydrolases (GHs) and lytic polysaccharide monooxygenases (AAs)] (17), suggesting that this family of enzymes should be essential for infection.…”