Pharmacological analysis of transmission activation of two aphid-vectored plant viruses, turnip mosaic virus and cauliflower mosaic virus.

Turnip mosaic virus (TuMV, family Potyviridae) and cauliflower mosaic virus (CaMV, family Caulimoviridae) are transmitted by aphid vectors. They are the only viruses shown so far to undergo transmission activation (TA) immediately preceding plant-to-plant propagation. TA is a recently described phenomenon where viruses respond to the presence of vectors on the host by rapidly and transiently forming transmissible complexes that are efficiently acquired and transmitted. Very little is known about the mechanisms of TA and on whether such mechanisms are alike or distinct in different viral species. We use here a pharmacological approach to initiate the comparison of TA of TuMV and CaMV. Our results show that both viruses rely on calcium signaling and reactive oxygen species (ROS) for TA. However, whereas application of the thiol-reactive compound N-ethylmaleimide (NEM) inhibited, as previously shown, TuMV transmission it did not alter CaMV transmission. On the other hand, sodium azide, which boosts CaMV transmission, strongly inhibited TuMV transmission. Finally, wounding stress inhibited CaMV transmission and increased TuMV transmission. Taken together, the results suggest that transmission activation of TuMV and CaMV depends on initial calcium and ROS signaling that are generated during the plant's immediate responses to aphid manifestation. Interestingly, downstream events in TA of each virus appear to diverge, as shown by the differential effects of NEM, azide and wounding on TuMV and CaMV transmission, suggesting that these two viruses have evolved analogous TA mechanisms.

Turnip Mosaic Virus Is a Second Example of a Virus Using Transmission Activation for Plant-to-Plant Propagation by Aphids.

Cauliflower mosaic virus (CaMV; family Caulimoviridae) responds to the presence of aphid vectors on infected plants by forming specific transmission morphs. This phenomenon, coined transmission activation (TA), controls plant-to-plant propagation of CaMV. A fundamental question is whether other viruses rely on TA. Here, we demonstrate that transmission of the unrelated turnip mosaic virus (TuMV; family Potyviridae) is activated by the reactive oxygen species H2O2 and inhibited by the calcium channel blocker LaCl3 H2O2-triggered TA manifested itself by the induction of intermolecular cysteine bonds between viral helper component protease (HC-Pro) molecules and by the formation of viral transmission complexes, composed of TuMV particles and HC-Pro that mediates vector binding. Consistently, LaCl3 inhibited intermolecular HC-Pro cysteine bonds and HC-Pro interaction with viral particles. These results show that TuMV is a second virus using TA for transmission but using an entirely different mechanism than CaMV. We propose that TuMV TA requires reactive oxygen species (ROS) and calcium signaling and that it is operated by a redox switch.IMPORTANCE Transmission activation, i.e., a viral response to the presence of vectors on infected hosts that regulates virus acquisition and thus transmission, is an only recently described phenomenon. It implies that viruses contribute actively to their transmission, something that has been shown before for many other pathogens but not for viruses. However, transmission activation has been described so far for only one virus, and it was unknown whether other viruses also rely on transmission activation. Here we present evidence that a second virus uses transmission activation, suggesting that it is a general transmission strategy.

Comparative Genomics Identifies a Novel Conserved Protein, HpaT, in Proteobacterial Type III Secretion Systems that Do Not Possess the Putative Translocon Protein HrpF.

Xanthomonas translucens is the causal agent of bacterial leaf streak, the most common bacterial disease of wheat and barley. To cause disease, most xanthomonads depend on a highly conserved type III secretion system, which translocates type III effectors into host plant cells. Mutagenesis of the conserved type III secretion gene hrcT confirmed that the X. translucens type III secretion system is required to cause disease on the host plant barley and to trigger a non-host hypersensitive response (HR) in pepper leaves. Type III effectors are delivered to the host cell by a surface appendage, the Hrp pilus, and a translocon protein complex that inserts into the plant cell plasma membrane. Homologs of the Xanthomonas HrpF protein, including PopF from Ralstonia solanacearum and NolX from rhizobia, are thought to act as a translocon protein. Comparative genomics revealed that X. translucens strains harbor a noncanonical hrp gene cluster, which rather shares features with type III secretion systems from Ralstonia solanacearum, Paraburkholderia andropogonis, Collimonas fungivorans, and Uliginosibacterium gangwonense than other Xanthomonas spp. Surprisingly, none of these bacteria, except R. solanacearum, encode a homolog of the HrpF translocon. Here, we aimed at identifying a candidate translocon from X. translucens. Notably, genomes from strains that lacked hrpF/popF/nolX instead encode another gene, called hpaT, adjacent to and co-regulated with the type III secretion system gene cluster. An insertional mutant in the X. translucens hpaT gene, which is the first gene of a two-gene operon, hpaT-hpaH, was non-pathogenic on barley and did not cause the HR or programmed cell death in non-host pepper similar to the hrcT mutant. The hpaT mutant phenotypes were partially complemented by either hpaT or the downstream gene, hpaH, which has been described as a facilitator of translocation in Xanthomonas oryzae. Interestingly, the hpaT mutant was also complemented by the hrpF gene from Xanthomonas euvesicatoria. These findings reveal that both HpaT and HpaH contribute to the injection of type III effectors into plant cells.

Insect transmission of plant viruses: Multilayered interactions optimize viral propagation.

By serving as vectors of transmission, insects play a key role in the infection cycle of many plant viruses. Viruses use sophisticated transmission strategies to overcome the spatial barrier separating plants and the impediment imposed by the plant cell wall. Interactions among insect vectors, viruses, and host plants mediate transmission by integrating all organizational levels, from molecules to populations. Best-examined on the molecular scale are two basic transmission modes wherein virus-vector interactions have been well characterized. Whereas association of virus particles with specific sites in the vector's mouthparts or in alimentary tract regions immediately posterior to them is required for noncirculative transmission, the cycle of particles through the vector body is necessary for circulative transmission. Virus transmission is also determined by interactions that are associated with changes in vector feeding behaviors and with alterations in plant host's morphology and/or metabolism that favor the attraction or deterrence of vectors. A recent concept in virus-host-vector interactions proposes that when vectors land on infected plants, vector elicitors and effectors "inform" the plants of the confluence of interacting entities and trigger signaling pathways and plant defenses. Simultaneously, the plant responses may also influence virus acquisition and inoculation by vectors. Overall, a picture is emerging where transmission depends on multilayered virus-vector-host interactions that define the route of a virus through the vector, and on the manipulation of the host and the vector. These interactions guarantee virus propagation until one or more of the interactants undergo changes through evolution or are halted by environmental interventions.

Draft Genome Sequence of Xanthomonas translucens pv. graminis Pathotype Strain CFBP 2053.

Strains of Xanthomonas translucens pv. graminis cause bacterial wilt on several forage grasses. A draft genome sequence of pathotype strain CFBP 2053 was generated to facilitate the discovery of new pathogenicity factors and to develop diagnostic tools for the species X. translucens.