Triple Binding Pockets of Atsdh: Reduced Affinity Unerpins Sdhis-Resistance in Alternaria Tenuissima

Abstract

Alternaria tenuissima, a causal agent of violet leaf spots (GVLS) on garlic (Allium sativum L.), causes a massive loss of yields. Boscalid [a Succinate Dehydrogenase Inhibitor (SDHI) fungicide, abbreviated as Bos] has been registered to control the GVLS in China since 2012. Of the 53 isolates separated from Pizhou City of Jiangsu Province in the eastern of China, 6 were resistant to Bos (BosR), and five types of amino acid substitutions were identified in AtSDH among these field isolates. Most BosR isolates exhibited decreased fitness defined as mycelial growth, conidiation, and virulence. Furthermore, the data revealed that gene expressions of AtSDH of the BosR isolates were up-regulated in response to Bos, significantly higher than that of the Bos-sensitivity (BosS) isolates; in the meantime, all of the BosR isolates exhibited comparatively elevated enzymatic activity of AtSDH and increased ATP accumulation compared to the BosS isolates. Positive cross-resistance between Bos and flubeneteram was verified by the use of Spearman rank correlation analysis. Moreover, the resistance mechanism was explored by AlphaFold3 structure prediction that amino acid substitutions at AtSDH were associated to SDHIs-resistance in A.tenuissima. This study revealed novel SDHI-binding sites in Alternaria spp. and highlights the role of amino acid substitutions in SDHIs resistance.

Keywords: Alternaria tenuissima, Boscalid, Amino acid substitutions at SDH, Alphafold3, Molecular docking