How to know when it is time for transmission ?

Cauliflower mosaic virus (CaMV) uses, like many plant viruses, aphids for vector transmission and the non-circulative (or non-persistant) mode as the transmission strategy: virus particles (virions) are transported from one host to another attached to the interior of the vector's mouth-parts, which are in the case of aphids the needle-like stylets. For CaMV transmission, a transmissible complex must form that binds to a stylet receptor. This complex is composed of 2 components: the virus particle and the helper component P2, a viral protein that functions as a molecular linker and binds the virus particle to the stylet receptor. P2 and virus particles are largely separated in infected plant cells, with most virus particles localizing to virus factories (VF) and all of P2 sequestered in a single cytoplasmic viral inclusion, the transmission body (TB). This setup forces the vector to reunite P2 and virus particles during feeding activity, i.e. to amass P2 from TB and virus from VF while the insects test-puncture parenchyma cells. How this is achieved, was an enigma because aphids take only minute amounts of cytoplasm during the very short (seconds range) test probes and because TB is hardly accessible to the vector. This is due to the fact that a TB as found in the cytoplasm is too big to pass through the stylets. We solved this mystery recently (http://elife.elifesciences.org/content/2/e00183) and uncovered an intriguing phenomenon: the TB dissolves the very moment that aphids insert their stylets into the tissue, and all P2 associates with cortical microtubules throughout the cell. Simultaneously, virus particles are recruited onto microtubules. In this configuration, P2 and virus particles are brought close together and in addition they are distributed homogeneously over the entire cell. This enables efficient acquisition by the vector and hence transmission even after short probing. Remarkably, TB reform after vector departure to be ready for a second round of transmission. These results open many issues that will be discussed. First of all, the mechanistic details behind the TB reaction are unknown and we have begun unravelling them. Second and more important from a general point of view, our findings suggest that plants perceive aphid activity from the moment of stylet insertion. An objective for the future is to identify the aphid elicitor and the involved pathways. Finally, the results prove that CaMV interferes with the very early plant-aphid interactions to organize transmission. This shows that CaMV is able to respond to cues from the outside world and another question of primordial importance is whether other viruses can do similar, either to achieve transmission or to accomplish other steps of the infection cycle.