Plant Pathology, University of California, Davis, CA 95616 USA

Background and objectives
The Arlington line of cowpea (Vigna unguiculata) is 'operationally immune' to cowpea mosaic comovirus (CPMV). The 'operation' is inoculation with a concentration of CPMV that is 100x the concentration sufficient to uniformly infect cowpeas of susceptible lines, resulting in no symptoms or detectable accumulation of virions in the putative operationally immune plants [1]. When CPMV and cowpea severe mosaic comovirus (CPSMV) were co-inoculated to Arlington cowpea at a 1:1 mass ratio, the number of CPSMV-induced local lesions was reduced to less than 40% of the value for leaves inoculated with CPSMV alone [2]. The suppression of CPSMV-induced local lesion formation was greater with increasing ratios of CPMV to CPSMV. Virion accumulation also was reduced according to ELISA for CPSMV capsid antigen [3]. This 'concurrent protection' [4] is distinguished from cross-protection and acquired resistance phenomena because the protecting virus, CPMV, apparently does not complete its infection cycle in the inoculated plant. 'Concurrent' refers to the observation that even a few minutes between inoculation of CPMV and subsequent inoculation of the challenging virus, CPSMV, significantly reduced the protective effect compared to that observed after co-inoculation of the two viruses. Co-inoculation of partially purified, genomic RNA1-containing CPMV ribonucleoprotein component B, but not component M (RNA2), with CPSMV, reduced CPSMV accumulation suggesting that the interference by CPMV may map to RNA1 [5]. The Blackeye 5 cowpea is susceptible to both CPMV and CPSMV, both producing local lesions. Protection was assessed by ELISA or by hybridization of probe to CPSMV RNA. CPMV was only marginally protective against CPSMV in Blackeye 5 cowpea except at high ratios of CPMV to CPSMV, e.g., 17:1 [4, 5]. The goal of this research is to understand the mechanisms of Arlington cowpea-derived resistance against CPMV and of concurrent protection.

Results and conclusions
The phenomenon of concurrent protection, as mediated by CPMV, is not limited to the Arlington cowpea and CPSMV as the challenging virus. Homozygous Cpa/Cpa (Arlington-derived operational immunity against CPMV) lines were derived from a back-cross series initiated by a Blackeye 5 cross to Arlington, with Blackeye 5 as recurrent parent. CPSMV accumulation was reduced in Cpa/Cpa cowpeas when CPSMV and CPMV were co-inoculated. Co-inoculation of CPMV with tomato ringspot nepovirus (TomRV) to primary leaves of Arlington cowpea reduced the accumulation of TomRV capsid antigen in the inoculated leaves to about 5% of the control value obtained for leaves inoculated with TomRV alone. When TomRV was inoculated 5 min after CPMV, TomRV capsid accumulation (ELISA) was 90% of the control value for TomRV inoculated alone, and for co-inoculated CPMV and TomRV, accumulation was 5% of the control value. For Cpa/Cpa cowpea as host, co-inoculation of CPMV and TomRV resulted in accumulation of TomRV capsid to 10% of the control value. Similar results were obtained by co-inoculating CPMV with Southern bean mosaic sobemovirus and with tomato bushy stunt tombusvirus. Thus CPMV appears to mediate similar protection against another comovirus, CPSMV, and against viruses of three other groups. These results are consistent with a model for Arlington cowpea-derived resistance against CPMV in which the plant recognizes an avirulence factor of CPMV, probably encoded by CPMV RNA1, and activates a mechanism that acts not only against CPMV but also against any of several other viruses.

1. Beier H, Siler DJ, Russell ML, Bruening G, 1977. Phytopathology 76, 917-21. 2. Bruening G, Lee S-L, Beier H, 1979. In Sharp WR et al., eds, Plant Cell and Tissue Culture - Principles and Applications. Ohio State University Press, pp. 421-40.
3. Eastwell KC, Kiefer MC, Bruening G, 1983. In Goldberg RB, ed., Plant Molecular Biology, UCLA Symposia on Molecular and Cell Biology New Series, Vol 12. Alan R. Liss, pp. 201-211.
4. Ponz F, Bruening G, 1986. Annual Review of Phytopathology 24, 355-381.
5. Eastwell KC, Kalmar GB, 1997. Journal of the American Society for Horticultural Science 122, 163-168