Cryptic viruses in Black-grass : Investigating their role in plant stress tolerance

Abstract

Cryptic viruses are a group of persistent plant viruses characterised by a lack of disease symptoms, very low virus titer and lifelong persistence in individual hosts. These characteristics highlight a very close relationship between cryptic viruses and their hosts and so, our main hypothesis is that they are mutualistic symbionts of their hosts. Our main aim was to study the effect of viral cryptic infections on the tolerance of plants to abiotic stress. We worked with three cryptic viruses recently discovered in black-grass (Alopecurus myosuroides) populations: Alopecurus myosuroides partitivirus 1 (AMPV1), Alopecurus myosuroides partitivirus 2 (AMPV2) and Alopecurus myosuroides varicosavirus 1 (AMVV1). We started by characterizing these viruses. AMPV1 and AMPV2 are widespread in the studied populations and vertically transmitted. The titer of each virus varies across a wide range, both between and inside all tested populations. This increases population plasticity, the possible negative and positive effects of the viruses being distributed at different “strength levels” across the populations, increasing its survival potential against changing environmental conditions, and thus, potentially improving population fitness. AMVV1 shows variable incidence and titer in the populations and vertical transmission. It is possible that this variability might be due to different levels of resistance to this virus or/and to the efficiency of the transmission. Bioassays were carried out to analyse the viral effect on plant tolerance to drought stress. Although no significant effect on tolerance was observed, different trends were associated with each virus. AMVV1 appears to act as a conditional mutualist, hindering growth under normal conditions but alleviating these negative effects under stress. AMPV1 affects the growth pattern of its host regardless of the environmental conditions, favouring the development of short plants with many tillers. This could increase the plant’s fitness as it might increase its competitiveness in grassland. In contrast, AMPV2 acts as an antagonist, negatively affecting the growth of its host, an effect that is exacerbated under stress