| Summary: | <i>Verticillium dahliae</i> is one of the most destructive soilborne plant pathogens since it has a broad host range and there is no chemical disease management. Therefore, there is a need to unravel the molecular interaction between the pathogen and the host plant. For this purpose, we examined the role of 1-aminocyclopropane-1-carboxylic acid synthases (ACSs) of <i>Arabidopsis thaliana</i> upon <i>V. dahliae</i> infection. We observed that the <i>acs2</i>, <i>acs6</i>, and <i>acs2/6</i> plants are partially resistant to <i>V. dahliae</i>, since the disease severity of the <i>acs</i> mutants was lower than the wild type (wt) Col-0 plants. Quantitative polymerase chain reaction analysis revealed that <i>acs2</i>, <i>acs6</i>, and <i>acs2/6</i> plants had lower endophytic levels of <i>V. dahliae</i> than the wt. Therefore, the observed reduction of the disease severity in the <i>acs</i> mutants is rather associated with resistance than tolerance. It was also shown that <i>ACS2</i> and <i>ACS6</i> were upregulated upon <i>V. dahliae</i> infection in the root and the above ground tissues of the wt plants. Furthermore, the addition of 1-aminocyclopropane-1-carboxylic acid (ACC) and aminooxyacetic acid (AOA), the competitive inhibitor of ACS, in wt <i>A. thaliana</i>, before or after <i>V. dahliae</i> inoculation, revealed that both substances decreased <i>Verticillium</i> wilt symptoms compared to controls irrespectively of the application time. Therefore, our results suggest that the mechanism underpinning the partial resistance of <i>acs2</i> and <i>acs6</i> seem to be ethylene depended rather than ACC related, since the application of ACC in the wt led to decreased disease severity compared to control.
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