Transcriptomic and Ultrastructural Signatures of K+-Induced Aggregation in Phytophthora parasitica Zoospores

Abstract

<jats:p>Most pathogenic oomycetes of the genus Phytophthora spread in water films as flagellated zoospores. Zoospores perceive and produce signals attracting other zoospores, resulting in autoaggregation in vitro or biofilm formation on plant surface. The mechanisms underlying intercellular communication and consequent attraction, adhesion and aggregation are largely unknown. In Phytophthora parasitica, the perception of a K+ gradient induces coordinated motion and aggregation. To define cellular and molecular events associated with oomycete aggregation, we combined transcriptomic and ultrastructural analyses. Results indicate involvement of electroception in K+ sensing. They establish that the transcriptome repertoire required for swimming and aggregation is already fully functional at zoospore release. At the time points analyzed, aggregates are mainly constituted of zoospores. They produce vesicular and fibrillary material discharged at cell-to-cell contacts. Consistently, the signature of transcriptome dynamics during transition to aggregates is an upregulation of genes potentially related to vesicular trafficking. Moreover, transcriptomic and functional analyses show a strong enhancement of carbonic anhydrase activity, indicating that pH homeostasis may contribute to aggregation by acting on both zoospore movement and adhesion. This study poses the molecular and cellular bases of aggregative behavior within oomycetes and expands the current knowledge of ion perception-mediated dissemination of propagules in the rhizosphere.</jats:p>

Phytophthora zoospores: From perception of environmental signals to inoculum formation on the host-root surface

“…The availability of gene sequences and mRNA-level quantification data generated by the different Phytophthora genome projects has had a massive impact on the definition and the classification of the molecular repertoire at different stages of the Phytophthora life cycle, including zoospores [2] . Several studies have indicated the occurrence and the importance of putative pumps, ligand-gated channels, tyrosine kinase-like and G-protein signaling [32] , [66] , [67] in Phytophthora . Nevertheless, to date, very little is known about the major classes of receptor and signaling pathways involved in environment perception.…”

“…However, the capacity of Phytophthora species to release 10 5 -10 6 zoospores per infected plant in controlled conditions [16] , [31] is indicative of their ability to induce radical change in the explored soil environment. For example, the 30-fold upregulation of a gene encoding a secreted alpha carbonic anhydrase (α-CA), observed in P. parasitica zoospores upon aggregation establishment [32] , together with the oomycete’s ability to produce a high-density inoculum in proximity of the target [16] , suggests a potential and transitory role of this zoospore enzyme in non-photosynthetic CO 2 fixation. A recent study showed that soil CA activity varied with the diversity of microbial communities and CA gene expression patterns [33] .…”

“…The increased cellular density leads to bioconvection, with plume formation and downward migration, and consequent rapid aggregation [15] . Investigations at the cellular and molecular level suggested that this behavior could be regulated by cell-to-to cell signaling and cation transport because Ca 2+ and K + channels were found to be involved in K + electroception and a remarkable K + -induced enhancement of alpha carbonic anhydrase (α-CA) activity [32] .…”

Coordination of two opposite flagella allows high-speed swimming and active turning of individual zoospores

“…For instance, potassium, which is uptaken by roots in the soil, reduces zoospore swimming speed, causes immediate directional changes and also results in perpetual circle trajectories [17, 18]. Bassani et al provide transcriptomic studies showing that potassium induces zoospore aggregation, which facilitates the advantages for zoospores to attack the host-root [19]. Experimental evidence has demonstrated that zoospore-zoospore interaction can lead to “pattern swimming”, a microbial bioconvection happened without the appearance of chemical or electrical signals [20, 21].…”

“…Bassani et al . provide transcriptomic studies showing that potassium induces zoospore aggregation, which facilitates the advantages for zoospores to attack the host-root [19].…”