16. APPROACHES TO AND SUCCESSES IN DEVELOPING TRANSGENICALLY ENHANCED MYCOHERBICIDES Jonathan Gressel, ∗ Sagit Meir, Yoav Herschkovitz, Hani Al-Ahmad, ∗∗ Inbar Greenspoon, † Olubukola Babalola, ‡ and Ziva Amsellem Plant Sciences, Weizmann Institute of Science, Rehovot, 76100, Israel Abstract. Inundative mycoherbicides have not been successful in weed con- trol in row crops, probably due to evolutionary barriers, and adding virulence factors was considered essential. Exogenous addition of the products of vari- ous genes was used to ascertain synergy as a prelude to adding them transgeni- cally. Transgenically over-expressing single “soft” genes (host lytic enzymes such as pectinase, cellulase and expansins, or natural hormones such as IAA), or “hard” genes encoding toxins such as NEP1 and CP1, has enhanced viru- lence, but not enough. Gene stacking to obtain synergies among the various genes is considered a top priority, both to achieve sufficient virulence and to delay the evolution of weed resistance to the fungal pathogens. Keywords: carbohydrases, mycoherbicides, NEP1, phytotoxins, transgenic enhancement 16.1. The Need for Enhancement—Exogenous Synergists versus Endogenous Transgenes Inundative mycoherbicides have rarely been commercialized in row crop agri- culture, where they must compete with conventional herbicides. That is not to say there is no need for them; there are many row crop situations where no conventional herbicide can selectively distinguish between crop and related weed. The barrier is often evolutionary: if the specific pathogen had the ex- treme virulence needed in row crops, it would kill all host plants, and both might become extinct. Thus the need to enhance the potential of mycoherbi- cides with virulence factors from other sources. ∗ To whom correspondence should be addressed, e-mail: Jonathan.Gressel@weizmann.ac.il ∗∗ Present address: Department of Biology & Biotechnology, An-Najah National University, Nablus, Palestinian Authority. † Present address: Faculty of Agriculture, Hebrew University of Jerusalem, Rehovot, Israel. ‡ Present address: Department of Microbiology, Olabisi Onabanjo University, Ago-iwoye, Ogun state, Nigeria. M. Vurro and J. Gressel (eds.), Novel Biotechnologies for Biocontrol Agent Enhancement and Management, 297–305. C  2007 Springer. 297