Compatibility between Legumes and Rhizobia for the Establishment of a Successful Nitrogen-Fixing Symbiosis.

The root nodule symbiosis established between legumes and rhizobia is an exquisite biological interaction responsible for fixing a significant amount of nitrogen in terrestrial ecosystems. The success of this interaction depends on the recognition of the right partner by the plant within the richest microbial ecosystems on Earth, the soil. Recent metagenomic studies of the soil biome have revealed its complexity, which includes microorganisms that affect plant fitness and growth in a beneficial, harmful, or neutral manner. In this complex scenario, understanding the molecular mechanisms by which legumes recognize and discriminate rhizobia from pathogens, but also between distinct rhizobia species and strains that differ in their symbiotic performance, is a considerable challenge. In this work, we will review how plants are able to recognize and select symbiotic partners from a vast diversity of surrounding bacteria. We will also analyze recent advances that contribute to understand changes in plant gene expression associated with the outcome of the symbiotic interaction. These aspects of nitrogen-fixing symbiosis should contribute to translate the knowledge generated in basic laboratory research into biotechnological advances to improve the efficiency of the nitrogen-fixing symbiosis in agronomic systems.

Translating Ribosome Affinity Purification (TRAP) followed by RNA sequencing technology (TRAP-SEQ) for quantitative assessment of plant translatomes.

Translating Ribosome Affinity Purification (TRAP) is a technology to isolate the population of mRNAs associated with at least one 80S ribosome, referred as the translatome. TRAP is based on the expression of an epitope-tagged version of a ribosomal protein and the affinity purification of ribosomes and associated mRNAs using antibodies conjugated to agarose beads. Quantitative assessment of the translatome is achieved by direct RNA sequencing (RNA-SEQ), which provides accurate quantitation of ribosome-associated mRNAs and reveals alternatively spliced isoforms. Here we present a detailed procedure for TRAP, as well as a guide for preparation of RNA-SEQ libraries (TRAP-SEQ) and a primary data analysis. This methodology enables the study of translational dynamic by assessing rapid changes in translatomes, at organ or cell-type level, during development or in response to endogenous or exogenous stimuli.

A C subunit of the plant nuclear factor NF-Y required for rhizobial infection and nodule development affects partner selection in the common bean-Rhizobium etli symbiosis.

Legume plants are able to interact symbiotically with soil bacteria to form nitrogen-fixing root nodules. Although specific recognition between rhizobia and legume species has been extensively characterized, plant molecular determinants that govern the preferential colonization by different strains within a single rhizobium species have received little attention. We found that the C subunit of the heterotrimeric nuclear factor NF-Y from common bean (Phaseolus vulgaris) NF-YC1 plays a key role in the improved nodulation seen by more efficient strains of rhizobia. Reduction of NF-YC1 transcript levels by RNA interference (RNAi) in Agrobacterium rhizogenes-induced hairy roots leads to the arrest of nodule development and defects in the infection process with either high or low efficiency strains. Induction of three G2/M transition cell cycle genes in response to rhizobia was impaired or attenuated in NF-YC1 RNAi roots, suggesting that this transcription factor might promote nodule development by activating cortical cell divisions. Furthermore, overexpression of this gene has a positive impact on nodulation efficiency and selection of Rhizobium etli strains that are naturally less efficient and bad competitors. Our findings suggest that this transcription factor might be part of a mechanism that links nodule organogenesis with an early molecular dialogue that selectively discriminates between high- and low-quality symbiotic partners, which holds important implications for optimizing legume performance.

A bZIP transcription factor from Phytophthora interacts with a protein kinase and is required for zoospore motility and plant infection.

Zoospores are critical in the disease cycle of Phytophthora infestans, a member of the oomycete group of fungus-like microbes and the cause of potato late blight. A protein kinase induced during zoosporogenesis, Pipkz1, was shown to interact in the yeast two-hybrid system with a putative bZIP transcription factor. This interaction was confirmed in vitro using a pull-down assay. The transcription factor gene, Pibzp1, was single copy and expressed in all tissues. Transformants of P. infestans stably silenced for Pibzp1 were generated using plasmids expressing its coding region in sense or antisense orientations. A protoplast transformation method induced silencing more efficiently than transformation by an electroporation scheme. Wild-type and silenced strains exhibited no differences in hyphal growth or morphology, mating, sporangia production or zoospore release. However, zoospores from the mutants spun in tight circles, instead of exhibiting the normal pattern of straight swimming punctuated by turns. Zoospore encystment was unaffected by silencing, but cysts germinated more efficiently than controls. Germinated cysts from the mutants failed to develop appressoria and were unable to infect plants; however, they could colonize wounded tissue. These phenotypes indicate that Pibzp1 is a key regulator of several stages of the zoospore-mediated infection pathway.

The spores of Phytophthora: weapons of the plant destroyer.

Members of the genus Phytophthora are among the most serious threats to agriculture and food production, causing devastating diseases in hundreds of plant hosts. These fungus-like eukaryotes, which are taxonomically classified as oomycetes, generate asexual and sexual spores with characteristics that greatly contribute to their pathogenic success. The spores include survival and dispersal structures, and potent infectious propagules capable of actively locating hosts. Genetic tools and genomic resources developed over the past decade are now allowing detailed analysis of these important stages in the Phytophthora life cycle.