Translational regulation in pathogenic and beneficial plant-microbe interactions.

Plants are surrounded by a vast diversity of microorganisms. Limiting pathogenic microorganisms is crucial for plant survival. On the other hand, the interaction of plants with beneficial microorganisms promotes their growth or allows them to overcome nutrient deficiencies. Balancing the number and nature of these interactions is crucial for plant growth and development, and thus, for crop productivity in agriculture. Plants use sophisticated mechanisms to recognize pathogenic and beneficial microorganisms and genetic programs related to immunity or symbiosis. Although most research has focused on characterizing changes in the transcriptome during plant-microbe interactions, the application of techniques such as Translating Ribosome Affinity Purification (TRAP) and Ribosome profiling allowed examining the dynamic association of RNAs to the translational machinery, highlighting the importance of the translational level of control of gene expression in both pathogenic and beneficial interactions. These studies revealed that the transcriptional and the translational responses are not always correlated, and that translational control operates at cell-specific level. In addition, translational control is governed by cis-elements present in the 5'mRNA leader of regulated mRNAs, e.g. upstream open reading frames (uORFs) and sequence-specific motifs. In this review, we summarize and discuss the recent advances made in the field of translational control during pathogenic and beneficial plant-microbe interactions.

TRAP-SEQ of Eukaryotic Translatomes Applied to the Detection of Polysome-Associated Long Noncoding RNAs.

Translating ribosome affinity purification (TRAP) technology allows the isolation of polysomal complexes and the RNAs associated with at least one 80S ribosome. TRAP consists of the stabilization and affinity purification of polysomes containing a tagged version of a ribosomal protein. Quantitative assessment of the TRAP RNA is achieved by direct sequencing (TRAP-SEQ), which provides accurate quantitation of ribosome-associated RNAs, including long noncoding RNAs (lncRNAs). Here we present an updated procedure for TRAP-SEQ, as well as a primary analysis guide for identification of ribosome-associated lncRNAs. This methodology enables the study of dynamic association of lncRNAs by assessing rapid changes in their transcript levels in polysomes at organ or cell-type level, during development, or in response to endogenous or exogenous stimuli.

Reprogramming of Root Cells during Nitrogen-Fixing Symbiosis Involves Dynamic Polysome Association of Coding and Noncoding RNAs.

Translational control is a widespread mechanism that allows the cell to rapidly modulate gene expression in order to provide flexibility and adaptability to eukaryotic organisms. We applied translating ribosome affinity purification combined with RNA sequencing to characterize translational regulation of mRNAs at early stages of the nitrogen-fixing symbiosis established between Medicago truncatula and Sinorhizobium meliloti Our analysis revealed a poor correlation between transcriptional and translational changes and identified hundreds of regulated protein-coding and long noncoding RNAs (lncRNAs), some of which are regulated in specific cell types. We demonstrated that a short variant of the lncRNA Trans-acting small interference RNA3 (TAS3) increased its association to the translational machinery in response to rhizobia. Functional analysis revealed that this short variant of TAS3 might act as a target mimic that captures microRNA390, contributing to reduce trans acting small interference Auxin Response Factor production and modulating nodule formation and rhizobial infection. The analysis of alternative transcript variants identified a translationally upregulated mRNA encoding subunit 3 of the SUPERKILLER complex (SKI3), which participates in mRNA decay. Knockdown of SKI3 decreased nodule initiation and development, as well as the survival of bacteria within nodules. Our results highlight the importance of translational control and mRNA decay pathways for the successful establishment of the nitrogen-fixing symbiosis.

Small GTPases in plant biotic interactions.

The superfamily of small monomeric GTPases originated in a common ancestor of eukaryotic multicellular organisms and, since then, it has evolved independently in each lineage to cope with the environmental challenges imposed by their different life styles. Members of the small GTPase family function in the control of vesicle trafficking, cytoskeleton rearrangements and signaling during crucial biological processes, such as cell growth and responses to environmental cues. In this review, we discuss the emerging roles of these small GTPases in the pathogenic and symbiotic interactions established by plants with microorganisms present in their nearest environment, in which membrane trafficking is crucial along the different steps of the interaction, from recognition and signal transduction to nutrient exchange.

The monomeric GTPase RabA2 is required for progression and maintenance of membrane integrity of infection threads during root nodule symbiosis.

KEY MESSAGE: Progression of the infection canal that conducts rhizobia to the nodule primordium requires a functional Rab GTPase located in Golgi/trans-Golgi that also participate in root hair polar growth. Common bean (Phaseolus vulgaris) symbiotically associates with its partner Rhizobium etli, resulting in the formation of root nitrogen-fixing nodules. Compatible bacteria can reach cortical cells in a tightly regulated infection process, in which the specific recognition of signal molecules is a key step to select the symbiotic partner. In this work, we show that RabA2, a monomeric GTPase from common bean, is required for the progression of the infection canal, referred to as the infection thread (IT), toward the cortical cells. Expression of miss-regulated mutant variants of RabA2 resulted in an increased number of abortive infection events, including bursting of ITs and a reduction in the number of nodules. Nodules formed in these plants were small and contained infected cells with disrupted symbiosome membranes, indicating either early senescence of these cells or defects in the formation of the symbiosome membrane during bacterial release. RabA2 localized to mobile vesicles around the IT, but mutations that affect GTP hydrolysis or GTP/GDP exchange modified this localization. Colocalization of RabA2 with ArfA1 and a Golgi marker indicates that RabA2 localizes in Golgi stacks and the trans-Golgi network. Our results suggest that RabA2 is part of the vesicle transport events required to maintain the integrity of the membrane during IT progression.