Identification and characterization of maize ACD6-like gene reveal ZmACD6 as the maize orthologue conferring resistance to Ustilago maydis

Enhancing broad-spectrum resistance is a major goal of crop breeding. However, broad-spectrum resistance has not been thoroughly investigated, and its underlying molecular mechanisms remain elusive. In the model plant Arabidopsis (Arabidopsis thaliana), ACCELERATED CELL DEATH6 (ACD6) is a key component of broad-spectrum resistance that acts in a positive feedback loop with salicylic acid (SA) to regulate multiple pattern recognition receptors. However, the role of ACD6 in disease resistance in crop plants is unclear. Here, we show that the transcript of ANK23, one of the 15 ACD6-like genes in maize (Zea mays), is induced by SA and by infection with the pathogenic fungus Ustilago maydis. Heterologous expression of ANK23 restored disease resistance in the Arabidopsis mutant acd6-2. We show that ANK23 is a maize ortholog of ACD6 and therefore rename ANK23 as ZmACD6. Furthermore, using CRISPR/Cas9, we generated ZmACD6 knockout maize plants, which are more susceptible to U. maydis than wild-type plants. We also identified a maize line (SC-9) with relatively high ZmACD6 expression levels from a diverse natural maize population. SC-9 has increased disease resistance to U. maydis and defense activation, suggesting a practical approach to cultivate elite varieties with enhanced disease resistance.