Session 7
Authors: Sureshkumar Sridevi , Kim Jihyun , Acosta Melissa Uribe , Capador-Barreto Hernan D. 
Published: July 2023

The molecular mechanisms of trinucleotide expansions- a role for SUMO proteases

Arabidopsis wild strain Bur-0 exhibits a temperature-sensitive phenotype known as irregularly impaired leaves (iil), which is caused by the expansion of GAA/TTC intronic trinucleotide repeat at the IIL1 locus. This results in transcriptional downregulation and similar to the intronic repeat expansion seen in the human genetic disorder Friedreich ataxia. By conducting a genetic suppressor screening, we identified FUG1, an uncharacterized SUMO protease that plays a key role in suppressing IIL1 expression and cause epigenetic silencing. Using Bio-chemical assays we discovered that FUG1 protease physically interacts with histone reader AL3, which in turn interacts with the PRC1 complex component LHP1 to cause a chromatin state switch from the active chromatin into inactive chromatin and promote gene silencing at the IIL1 locus. Our findings reveal novel genes and mechanisms of epigenetic silencing caused by expanded repeats which has implications not only for plants but also for human genetic disorders.

Phytophthora capsici RXLR effector Pc12 causes cell death without host defense by inhibiting vesicle trafficking through small GTPase Rab protein

Phytophthora capsici causes blight disease on over 50 crop species including tomato and pepper with annually multi-billion-dollar losses in crop production. P. capsici secret hundreds of effectors into a host cell to manipulate host immunity for successful colonization. However, even with the numerous studies on the roles of RXLR effectors, the molecular mechanisms of effector-induced necrotic host cell death remain elusive. Here, we report a novel P. capsici effector, Pc12, that causes cell death in the Solanaceae family without induction of defense-related gene expression and association with downstream signaling components of effector-triggered immunity. Pc12 enhances the colonization of P. capsici upon transient expression in Nicotiana benthamiana. Moreover, its homologs from other Phytophthora spp. having structural similarity with Pc12, especially in the C-termini, also induce cell death in N. benthamiana. Proteomics and microscopy assay reveal that Pc12 targets small GTPase Rab protein and perturbs vesicle trafficking between ER and Golgi apparatus, suggesting that the expression of Pc12 results in cell death under severe ER stress. Together, our results imply a new effector family functions in their pathogen growth via ER-mediated cell death.

Role of structural and chemical plant defense components on root-microbiome assembly

Plant roots interact in soil with pathogenic and beneficial bacteria. Structural (cutin, suberin, callose, lignin) and chemical (camalexin, glucosinolates, coumarins) defense components act as gatekeepers of microbial colonization. They have been thoroughly characterized by restricting pathogens colonization, however, their role in the assembly of a beneficial root microbiome or in the selection of commensals in the rhizosphere remains underexplored. Here, we studied the microbiome composition in 16 soil-grown Arabidopsis mutants affected in structural or chemical defense components and in different compartments (soil-rhizosphere-root) using 16S rRNA sequencing. Compared to wild type (WT) plants, cutin, suberin or the export of aliphatic glucosinolates mutants displayed significantly altered microbiomes in all compartments, while mutants for indolic glucosinolates or coumarin biosynthesis showed differences only in the rhizosphere. In general structural defenses played a higher role in the endosphere microbiome assembly while chemical defenses a higher role in the rhizosphere, showing that plant mechanisms of microbial selection are compartment specific. When looking at specific bacterial taxa changed in the root compartment compared to WT, most bacteria belonged to the Oxalobacteraceae, while other bacterial families had more mutant-specific patterns and will be targeted in future analyses. Ultimately, we envision to unravel novel plant mechanisms of microbiome recruitment.

Multiple soil-borne pathogens contribute to disease and mortality of Virola nobilis seedlings

According to the Janzen-Connell hypothesis, conspecific negative density dependence (CNDD) maintains the exuberant plant diversity observed in tropical rainforests. Virola nobilis (syn. Virola surinamensis), a tree species native to Central America is a model species to study CNDD: Seedling mortality is high under conspecific V. nobilis trees, but toucans and monkeys disperse some seeds far from their mother tree, where survival is higher. Although it is generally accepted that natural enemies are responsible for CNDD in V. nobilis, it is unknown which microbes are driving it. Therefore, in this study we have characterized fungal pathogens infecting V. nobilis seedlings in Barro Colorado Island, in the Republic of Panama. We sowed seeds under conspecific trees and isolated fungi from symptomatic seedlings. Using infection assays in shade-houses we confirmed pathogenicity for several species of fungi in the Nectriaceae family. Isolates of Calonectria were among the most frequently isolated fungi and caused high mortality on V. nobilis seedlings. Additionally, we have characterized the fungal community using ITS metabarcoding from soil near V. nobilis trees and show that abundance of putative pathogens varies significantly between trees. In all, our results demonstrate the effect of several soil fungal species in seedling mortality and consequently the demographic dynamics of V. nobilis. Moreover, these results enable future studies to understand how plant-microbe interactions shape diversity in tropical forests.