224 (Auon et al. 2009). Phytoalexins (antimicrobial substances synthesized de novo in plants) have been implicated in DED defense, particularly mansonone (Duchesne 1993), yet a subsequent study noted that mansonone elicitation in elm callus culture does not require the DED patho- gen (Meier and Remphrey 1997), suggesting its synthesis may be a more generalized stress reac- tion. In one review, the extensive list (see Table 1 in Büchel et al. 2015) of chemical defense com- pounds isolated from elms demonstrated that a multitude of biochemicals (e.g., mansonones, cadalene derivatives, lignin, scopoletin, flavo- noids) are synthesized by elms after infection and perhaps work in concert to thwart the pathogen. Little is known about the genetic factors regu- lating DED resistance. It has been demonstrated that in interspecific elm hybrids, DED resistance is heritable and is linked to the amount of highly resistant U. pumila DNA in the hybrid. This high degree of additive resistance indicates that back- crossing strategies might be successful to move DED resistance into other backgrounds (Solla et al. 2014). Using Ulmus minor, an extensive genetic sequencing of the transcriptome (the entire collection of RNA sequences that allows one to determine when and where each gene is turned on or off) was performed (Perdiguero et al. 2015). By using elm genotypes with dif- ferent levels of DED tolerance and exposing the trees to biotic and abiotic stress, analysis of dif- ferential gene expression between tolerant and susceptible genotypes was accomplished. By studying the upregulation of genes after inocu- lation of elm callus cultures with an aggressive strain of the DED fungus and performing dif- ferential screening, 53 sequences were consid- ered upregulated, demonstrating that reaction to the pathogen causes numerous changes in gene expression in elm (Auon et al. 2010). Many genes coded for some branch in the pathway to phen- ylpropanoids, a broad class of biochemicals that function to elicit inducible physical or chemi- cal barriers against infection or act as signal molecules involved in local systemic signaling for defense gene induction (Dixon et al. 2002). It was hypothesized that the delay in the response of Ulmus americana cells to react to the pathogen could reflect a suboptimal coordination ©2017 International Society of Arboriculture Marcotrigiano: Elms Revisited of defense strategies that might, in the end, fail to produce adequate resistance (Aoun et al. 2010). It had been previously reported that increases in lignin in inoculated xylem tissue occurred earlier in resistant elms than in susceptible ones (Martin et al. 2007). When tolerant and susceptible Amer- ican elms were mechanically inoculated with an aggressive strain of Ophiostoma novo-ulmi, defense, as indicated by gene expression, occurred within 144 hours (Sherif et al. 2016). Defense molecules, such as jasmonic acid (JA) and sali- cylic acid, appeared to act as defense response elicitors. The tolerant elms expressed JA induc- tion more quickly. In this, and many other studies, it appears that temporal factors (host reaction time to infection, developmental age of vessels at the time of inoculation, age of tree, etc.) are a key to determining the level of DED tolerance. Gene expression studies could be informative in establishing which genes should be transferred or upregulated (e.g., with genetic engineering) to com- bat DED. A greater understanding of gene action may help researchers understand which genes play the most significant role in DED resistance. Recently, a Spanish team demonstrated that DED-resistant and DED-susceptible clones of Ulmus minor reacted differently to pathogen inoculation, and the reaction to the pathogen was correlated to biochemical profiles that were ana- lyzed after infection (Li et al. 2016). Again, the timing of the response was a significant indicator for susceptibility. After infection with the patho- gen, leaf water potential and net photosynthetic rate declined, and the loss of hydraulic conductiv- ity increased in susceptible trees. Resistant clones showed elevated levels of phenolic compounds, saturated hydrocarbons, cellulose, and hemicel- lulose when compared to susceptible clones. It was hypothesized that susceptible clones had a weak activation of their defense mechanisms and quickly began to display physiological parameters that could be correlated to susceptibility to DED. Quicker depletion of carbohydrate reserves were implicated in the weakened defense. Defense against DED is multifaceted. It is polygenic and temporal and includes a complex dynamic between host, vector, and pathogen. Yet, with new genetic tools, researchers are getting closer to understanding what makes an elm combat DED.
November 2017
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