It is possible that senescence-associated modifications of the leaf tissue enabled the penetration of the mycelium inside the host cells and the saprotrophic development of these strains. It should be noted that some mycelium development could be detected by real-time RT-PCR prior to any visible necrotic
symptom, as early as 1 dpi in case of E139, E70 and CCP. We suspect that these isolates may have a phase of epiphytic development before the mycelium penetrates through the cells upon toxin action (necrotrophy) or Selleck SB431542 senescence-induced alteration of the tissues (saprotrophy). In the case of the isolate E78, which remained avirulent even at 9 dpi, learn more we cannot rule out all saprobic activity but the very low amount of mycelium detected at 5 and 9 dpi demonstrated that it is clearly less competitive than the other isolates in senescing tissue. Discovery of new cassiicolin gene homologues New cassiicolin gene homologues potentially encoding two new cassiicolin precursor protein isoforms (Cas3 and Cas4) were found in the endophytic C. cassiicola isolates. Their predicted amino acid sequence is similar to that of the Cas1 reference isoform. In particular, the
Go6983 mature cassiicolin domain is highly conserved, with only one amino acid substitution (S instead of T) at position 2. This amino acid is especially important because it carries the sugar moiety (0-methyl-mannose) of the active cassiicolin (Barthe et al. 2007; de Lamotte et al. 2007). of Although the role played by this sugar in toxicity is still unknown, it should be noted that Serine (S), like Threonine (T), can be 0-glycosylated. Therefore, the glycosylation of the toxin is not jeopardized by the T to S substitution. The cassiicolin gene may be under purifying selection pressure, as indicated by the low (<1) d N /d S ratios. This suggests that this gene is playing and important functional role in C. cassiicola. However, this will have to be confirmed when a higher number of Cas gene sequences reflecting C. cassiicola
evolution history will be available. Although the genes encoding Cas3 and Cas4 appear structurally functional, no Cas3 and Cas4 transcripts could be detected post-inoculation. Therefore, if Cas3 and Cas4 genes are functional, it seems that their transcription is negatively controlled under the conditions used in this experiment. We have previously shown (Déon et al. 2012) that Cas1 is transiently expressed, with a sharp peak of expression at 1 or 2 dpi depending on the cultivar. This was confirmed in this work for RRIM 600 inoculated with CCP. In the cultivar FDR 5788 inoculated with CCP, Cas1 was expressed, but no peak of expression was observed. We suggest that the peak may have occurred at a different time-point not tested in this experiment. Whether Cas3 and 4 can be switched on and under which conditions is unknown.