Main scientific achievements and significance

This BARD project resulted in the publication ofseveral refereed papers and symposia proceedings that provide an increased understanding of potyviral determinants influencing systemic infection, symptom development, host range, and aphid transmission. The main findings ofeach subproject and associated publications are listed below.  

1. Analvsis ofthe role ofthe amino terminus (NT) ofthe potyviral coat protein rep) in systemic infection, symptom development and host range.

Publications: Ullah Z, Gal-On A, Raccah B, Grumet R. Testing the role ofthe amino terminus ofthe coat protein in systemic infection and host range determination ofpotyviruses. In preparation.

Ullah Z, Grumet R Localization of Zucchini yellow mosaic virus to the veinal regions and role of viral coat protein in veinal chlorosis conditioned by the zym potyvirus resistance locus in cucumber. Submitted.

Grumet, R., Ullah, Z., Gal-On, A., and Raccah, B. 2000. Cucurbit-potyvirus interactions influencing virus movement and accumulation. Pages 327-334 in Eucarpia 2000. N. Katzir and H. Parris, eds .. Invited symposium paper.

Production ofhybrid ZYMV infectious clones with substitutions in the variable CP-NT indicated that when the CP-NT came from a non-cucurbit adapted virus (tobacco etch virus, TEV), the plant was able to mount a successful recovery leading to reduced symptoms, virus titer, and virus specific protection against secondary infection. When the CP-NT came from another cucurbit potyvirus (watermelon mosaic virus, WMV), recovery did not occur. These results suggest host recognition ofthe CP-NT from a non-adapted virus and provide direct evidence for a role ofthe variable CP-NT in host range adaptation.

The observed recovery phenotype closely resembles phenotypes associated with gene silencing (Ratcliff et al. 1997, 1999; van den Boogaart et al. 1998) and so raises interesting questions. HC-Pro is known to act as a suppressor ofgene silencing (Brignetti et al., 1998; Anandalaskshmi et al. 1998; Kasschau and Carrington, 1998) interactions between the CP-NT and HC-Pro already are well-established for aphid transmission and in-vitro (including work from this BARD project, Peng et al. 1998) and both are involved in long distance virus movement (Dolja et al. 1995; Atreya et al. 1995; Kleinet al. 1994; Huet et al. 1994; Cronin et al. 1995; Peng et al. 1998). Future investigations will examine whether the TNT recovery phenomenon is related to gene silencing, and ifso, what role the CP-NT plays.

The chimeric viruses also were tested for ability to systemically infect an array ofZYMV local lesion or non-hosts, including those normally infected by TEV or WMV. We did not observe any modification in host range. The failure ofthe heterologous CP-NTs to extend host range also has implications for risk assessment involving deployment oftransgenic virus resistant plants expressing potyvirus CPs.

The CP-NT was also found to playa role in naturally occurring resistance conferred by the zym locus in the cucumber line 'Dina-I'. Analysis ofresistance indicated that the zym allele confers a block in long distance movement; the ability to do so varied among ZYMV strains and was influenced by the specific ZYMV CP-NT.

Collectively, these results indicate that the variable potyvirus CP-NT plays a role in host adaptation and can be either a trigger (in the case ofa non-adapted CP) or target (in the case ofa resistant genotype) for host defenses.

2. Analvsis ofthe role ofthe helper component protease (He-Pro) in systemic infection. symptom development andaphid transmission.

Publications

Peng YH, Kadoury D, Gal-On A, Huet H, Wang Y, Raccah B. 1998. Mutation in the He-Pro

gene ofthe zucchini yellow mosaic potyvirus: Effects on aphid transmission and on

binding to purified virions. 1. Gen. Virol. 79:897-904. Kadouri D, Peny YH, Wang Y, Singer S, Huet H, Raccah B, Gal-On A. 1998. Affinity

purification ofHC-Pro ofpotyviruses with Ni²+-NTA resin. 1. Virol. Meth. 76:19-29. Raccah B. 1999. Insect transmission ofplant viruses. In: Encyclopedia ofLife Sciences. McMillan

Publication. An on-line publication. http://www.els.netlelsonline Raccah B, Blanc S, Huet H. 2000. Potyviruses. In: Current Perspectives on Virus-Vector-Plant Interactions. K.F. Harris, ed. Academic Press, In Press.

Finding domains in the HC-Pro that are involved in milder symptoms has economical importance for preparing mild virus isolates that may serve for cross protection (see Gal-On, 2000), and for genetic tailoring of infectious clones with mild symptoms that can serve for transient expression in plants of foreign proteins of economical value. The potential for application of mild viruses is immediate and the technology is available. In Israel alone, ca. 1000 ha of watermelons are currently cross protected with naturally found mild ZYMV. Assuming 10% of severe epidemics with total loss in bad virus years, this means a yield loss of 100 ha (ca. 7000 tons with an approximate value of$1 million. Additional work is needed to verify ifthe domains found in ZYMV HC-Pro are also acting in other viruses.

Finding domains in the HC-Pro that are involved in aphid transmissibility may be useful in the future for expression in plants of deficient HC molecules that will trap virions and render them non-transmissible. Much work, however, is needed to reach this goal.