The mycorrhiza-dependent defensin MtDefMd1 of Medicago truncatula acts during the late restructuring stages of arbuscule-containing cells

Abstract

Different symbiotic and pathogenic plant-microbe interactions involve the production of cysteine-rich antimicrobial defensins. In Medicago truncatula, the expression of four MtDefMd genes, encoding arbuscular mycorrhiza-dependent defensins containing an N-terminal signal peptide and exhibiting some differences to non-symbiotic defensins, raised over the time of fungal colonization. Whereas the MtDefMd1 and MtDefMd2 promoters were inactive in cells containing young arbuscules, cells with fully developed arbuscules displayed different levels of promoter activities, indicating an up-regulation towards later stages of arbuscule formation. MtDefMd1 and MtDefMd2 expression was absent or strongly down-regulated in mycorrhized ram1-1 and pt4-2 mutants, known for defects in arbuscule branching or premature arbuscule degeneration, respectively. A ~97% knock-down of MtDefMd1/MtDefMd2 expression did not significantly affect arbuscule size. Although overexpression of MtDefMd1 in arbuscule-containing cells led to an up-regulation of MtRam1, encoding a key transcriptional regulator of arbuscule formation, no morphological changes were evident. Co-localization of an MtDefMd1-mGFP6 fusion with additional, subcellular markers revealed that this defensin is associated with arbuscules in later stages of their life-cycle. MtDefMd1-mGFP6 was detected in cells with older arbuscules about to collapse, and ultimately in vacuolar compartments. Comparisons with mycorrhized roots expressing a tonoplast marker indicated that MtDefMd1 acts during late restructuring processes of arbuscule-containing cells, upon their transition into a post-symbiotic state.

Important innate differences in determining symbiotic responsiveness in host and non-hosts of arbuscular mycorrhiza

“…Mycorrhizae also increase host plant resistance to pathogens and tolerance to abiotic and biotic stress 2 , 3 . In return, the plant host supplies the fungal partner with essential carbon sources such as hexoses 4 , 5 and lipids 6 – 8 to facilitate AM fungal growth and reproduction.…”

Lipo‐chitooligosaccharide signalling blocks a rapid pathogen‐induced ROS burst without impeding immunity

“…However, host plant responses to symbionts and pathogen MAMPs overlap (Nakagawa et al, 2011), and pathogens utilize components of the plant genetic systems controlling symbiotic interactions to colonize root tissues (Rey & Jacquet, 2018). Meanwhile, plant immunity is involved in fine-tuning interactions with beneficial microbes (Berrabah et al, 2014Gourion et al, 2015;Uhe et al, 2018). LysM Receptor Like Kinases (LysM-RLKs) such as Chitin Elicitor Receptor Kinase 1 (OsCERK1) sense both pathogen-and symbiont-derived signals (Zhang et al, 2015).…”

Physiological and transcriptomic response ofMedicago truncatulato colonization with high and low benefit arbuscular mycorrhizal fungi

“…Defensins are known to inhibit pathogen growth (Maróti et al 2015), so it is tempting to speculate that Ga165 could temper host defense responses to avoid detection and sanctioning. However, the defensin DefMd1 when overexpressed upregulates the expression of the GRAS transcription factor RAM1 , which plays a critical role in arbuscular branching (Park et al, 2015; Uhe et al, 2018). RAM1 was highly upregulated in both Ga165 and Ri09 colonized roots ( Supplemental Table S3 ) suggesting that defensins might play a role in AM symbiosis.…”