Factors influencing reversion from virus infection in sweetpotato

Abstract

Viruses limit sweetpotato (Ipomoea batatas) production worldwide. Many sweetpotato landraces in East Africa are, however, largely virus-free. Moreover, some plants infected by the prevalent Sweet potato feathery mottle virus (SPFMV) may be able to revert to virus-free status. In this study, we analysed reversion from SPFMV, Sweet potato virus C, Sweet potato mild mottle virus, Sweet potato chlorotic stunt virus (SPCSV) and Sweet potato leaf curl Uganda virus using the indicator plant I. setosa and PCR/reverse-transcriptase PCR. We also investigated environmental factors (temperature and soil nutrients) that may influence reversion from virus infection. We tested reversion in the East African cultivars New Kawogo, NASPOT 1 and NASPOT 11, and the United States cultivars Resisto and Beauregard. Reverted plants were asymptomatic and virus was undetectable in assayed parts of the plant. After graft inoculation, only the East African cultivars mostly reverted at a high rate and from most viruses though cultivar Beauregard fully reverted following sap inoculation with Sweet potato virus C. None of the tested cultivars fully reverted from single or double infections involving SPCSV, and reversion was only observed in co-infections involving potyviruses. Root sprouts derived from SPFMV-reverted plants were also virus free. Reversion generally increased with increasing temperature and by improved soil nutrition. Overall, these results indicate variation in reversion by cultivar and that the natural ability of sweetpotato plants to revert from viruses is malleable, which has implications for both breeding and virus control.

Keywords: East African cultivars; USA cultivars; sweetpotato; sweetpotato viruses; virus reversion.

FIGURE 1

Ipomoea setosa leaves showing symptoms commonly induced by sweetpotato viruses. (a) Healthy leaf from an uninfected control plant; (b) vein chlorosis induced by Sweet potato mild mottle virus (SPMMV); (c) feathery mottling induced by Sweet potato feathery mottle virus (SPFMV); (d) leaf-curling induced by Sweet potato leaf curl Uganda virus (SPLCUV); (e) leaf chlorosis induced by Sweet potato chlorotic stunt virus (SPCSV); (f) severe mottling, reduced leaf size and leaf distortion due to sweetpotato viral disease (SPVD) induced by dual infection of SPCSV and SPFMV

FIGURE 2

Gels of PCR products showing sweetpotato reversion from infections with different viruses. Each plant was inoculated individually and confirmed to be infected using PCR or RT-PCR prior to the experiment. A plant was considered to have reverted if the virus was completely eliminated from infected plants. This was verified by testing the same plant weekly for 3 weeks. In A1, 6 out of 10 plants of cv. New Kawogo had reverted from SPLCUV by 2 weeks post-inoculation (wpi). In A3, 8 out of 10 plants of cv. New Kawogo had reverted from SPLCUV by 10 wpi. In B1, 5 out of 10 plants of cv. New Kawogo had reverted from SPFMV by 4 wpi. In B3, 10 of 10 plants of cv. New Kawogo had reverted from SPFMV by 10 wpi. In C1, all plants of cv. New Kawogo failed to revert from SPCSV by 4 wpi and similar observations were made in C3 at 10 wpi. D1 is a representative gel for reversion from SPVC in cv. Beauregard following sap inoculation. In Week 1, SPVC was not detected, but was observed in Week 2. In Week 3, the cv. Beauregard plants had reverted. Plates A2, A4, B2, C2 and C4 are the host cytochrome C oxidase reference genes. Plates D2 is host actin reference gene for the respective samples. Lanes: La = 1-kbp ladder, Lb = 100-bp ladder. Lanes P, N and C are positive, negative and no-template controls, respectively

FIGURE 3

Gels of PCR products showing reversion from Sweet potato leaf curl Uganda virus (SPLCUV) when cvs. New Kawogo and Resisto were grown at different temperatures. In A1, 3 out of six plants of cv. New Kawogo had reverted by the fourth week when grown at 30°C. Only one out of six plants of cv. New Kawogo (Lanes 7–12) had reverted by the fourth week when grown at 20°C. In B1, four out of six cv. Resisto plants (Lanes 1–6) had reverted by the fourth week when grown at 30°C and when grown at 20°C, two out of six cv. Resisto plants (Lanes 7–12) had reverted by the fourth week. Plate B2 is host cytochrome C oxidase reference gene. Lanes labelled La and Lb are 1-kbp and 100-bp ladders, respectively. Lanes P, N and C are positive, negative and no-template controls respectively