Nevertheless, this activity is comparable with the activity of the growth-promoting bacteria and efficient native producer of ACCD, P. putida UW4 (Todorovic & Glick, 2008), and is sufficient to induce root elongation in canola seedlings (Table 1). In P. citrinum, it is suggested that ACC derived from ACC synthase activity accumulates in the cells and this induces ACCD activity (Jia et al., 2000). In Trichoderma, the situation

could be similar. ACC synthase sequences are present in all Trichoderma genomes annotated to date (http://genome.jgi-psf.org/Trire2/Trire2.home.html; http://genome.jgi-psf.org/Trive1/Trive1.home.html), and low basal activity of ACCD can be detected in Trichoderma without exogenous induction. We did not see a significant induction of Tas-acdS by plant roots after either 5 or 24 h (data not shown). In bacteria, induction of enzyme activity is a relatively see more slow and complex process (Glick et al., 2007).

It could be that the induction by plant roots will be detectable following an environmental stress. The role of ACCD activity per se in rhizosphere colonization was assessed. Similar survival of wild-type T203 and mutants inside canola roots was assessed after 4–5 days (Fig. 3b) and after two weeks (data not shown). This is in agreement with previous results on the persistence of Pseudomonas brassicacearum Am3 and its ACCD-deficient mutant in the tomato rhizosphere (Belimov et al., 2007), suggesting that changes in ACCD activity do not markedly affect the ability of bacteria or fungi to colonize plant roots at least over this check details time scale. A significant increase in root length can be discerned in seedlings pretreated with T. asperellum WT, suggesting a growth promotion activity that is lost in the ACCD RNAi lines (Fig. 3a). This new observation of ACCD activity in Trichoderma spp. is of potential interest for different types of applications. There is evidence of various Trichoderma spp. contributing to soil contaminants’ degradation (Verma

et al., 2007). The use of ACCD-containing microorganisms in rhizoremediation of organics-contaminated soil has been proposed (Arshad et al., 2007). Prolific root growth could maximize rates of hyperaccumulation of inorganic contaminants or rhizodegradation of of organic pollutants, and thus accelerate phytoremediation. In future work, it will be interesting to evaluate the expression of Tas-acdS in bacterial strains lacking ACCD activity and growth-promoting activity, but possessing other useful biocontrol qualities. We are grateful to Prof. B. Rubin (Plant Sciences, Hebrew University of Jerusalem) for providing canola seeds. This research was partially supported by the USAID-CDR Israel–Uzbekistan–USA, grant no. TA-MOU-03-CA23-036, and by the DFG-Trilateral Cooperation Project between Germany, Israel and the Palestinian Authority grant no.0306458.