Synergy between an emerging monopartite begomovirus and a DNA-B component

Abstract

In recent decades, a legion of monopartite begomoviruses transmitted by the whitefly Bemisia tabaci has emerged as serious threats to vegetable crops in Africa. Recent studies in Burkina Faso (West Africa) reported the predominance of pepper yellow vein Mali virus (PepYVMLV) and its frequent association with a previously unknown DNA-B component. To understand the role of this DNA-B component in the emergence of PepYVMLV, we assessed biological traits related to virulence, virus accumulation, location in the tissue and transmission. We demonstrate that the DNA-B component is not required for systemic movement and symptom development of PepYVMLV (non-strict association), but that its association produces more severe symptoms including growth arrest and plant death. The increased virulence is associated with a higher viral DNA accumulation in plant tissues, an increase in the number of contaminated nuclei of the phloem parenchyma and in the transmission rate by B. tabaci. Our results suggest that the association of a DNA-B component with the otherwise monopartite PepYVMLV is a key factor of its emergence.

Figure 1

Disease symptoms in (a) Nicotiana benthamiana and (b) tomato (Solanum lycopersicum) plants agroinoculated with mock, PepYVMLV in single (DNA-A) or mixed (DNA-A and -B) infection at 29 days post inoculation. Distinct symptoms of leaf crumpling with yellowing and stunting were observed between single (mild symptoms) and mixed (very severe symptoms) infections on both N. benthamiana and tomato plants.

Figure 2

(a) Kinetics of estimated symptom severity of tomato (yellow) leaf curl disease following agroinoculation of tomato plants with PepYVMLV DNA-A and TYLCV-IL DNA-A in single (green and black lines) or mixed (red and blue lines) infection with DNA-B, respectively. Each point represents the average for one experiment (n = 30). The symptom severity scale (left scale bar) ranges from 1 (no symptoms) to 10 (plant death). (b) Mean height of tomato plants after agroinfection with PepYVMLV DNA-A or TYLCV-IL in single or mixed infection with DNA-B at 32 days post inoculation. For each dot, vertical bars represent 95% confidence intervals. Identical letters on top of the bars indicate groups with non-significant differences in height.

Figure 3

SYBR Green real-time PCR quantifications. (a) Average accumulation of PepYVMLV in single (DNA-A) or mixed (DNA-A and -B) infection of tomato plants (Solanum lycopersicum) at 15, 22 and 29 days post inoculation (dpi). (b) The bottom panels represent the linear correlation between PepYVMLV DNA-A and -B normalized loads. Vertical bars represent 95% confidence intervals. Identical letters on top of the bars indicate groups with non-significant differences in height.

Figure 4

Location of PepYVMLV DNA-A in single (a–d) or mixed (e–h) infection with DNA-B in cross sections of infected tomato petioles. DNA-A (b,f) and DNA-B (c,g) components revealed by red and green FISH probes, respectively. Nuclei are stained with DAPI blue. Images (d,h) in which the three channels (blue, red, and green) are merged enable identification of the components located together in the cell nuclei. Phloem (ph) and xylem (xy) bundles are indicated.