NLR- and mlo-Based Resistance Mechanisms against Powdery Mildew in Cannabis sativa

Abstract

Powdery mildew (PM) is one of the most common Cannabis sativa diseases. In spite of this, very few documented studies have characterized the resistance genes involved in PM defense mechanisms, or sources of natural genetic resistance in cannabis. The focus of the present work is on the two primary mechanisms for qualitative resistance against PM. The first is based on resistance (R) genes characterized by conserved nucleotide-binding site and/or leucine-rich repeat domains (NLRs). The second one involves susceptibility (S) genes, and particularly mildew resistance locus o (MLO) genes, whose loss-of-function mutations seem to be a reliable way to protect plants from PM infection. Cannabis defenses against PM are thus discussed, mainly detailing the strategies based on these two mechanisms. Emerging studies about this research topic are also reported and, based on the most significant results, a potential PM resistance model in cannabis plant-pathogen interactions is proposed. Finally, innovative approaches, based on the pyramiding of multiple R genes, as well as on genetic engineering and genome editing methods knocking out S genes, are discussed, to obtain durable PM-resistant cannabis cultivars with a broad-spectrum resistance range.

Keywords: Cannabis sativa; broad-spectrum resistance; disease resistance genes; mildew resistance locus o; nucleotide-binding and leucine-rich repeat receptors; powdery mildew.

Figure 1

A model of the main mechanisms of PM resistance in cannabis. PAMPs are perceived by membrane-associated PRRs, which activate defense signaling. NLRs recognize pathogen-secreted proteins. These recognitions, in turn, activate immune signaling cascades, resulting in the synthesis of numerous pathogenesis-related proteins to confer PM resistance. Proteins encoded by PM1 gene, represented by a single dominant locus and associated with a region containing NLRs, are shown. Proteins encoded by MLO genes (MLO1 and MLO4), which can lead to PM cannabis resistance, are also included. Abbreviations: Mildew resistance locus o (MLO) gene; NLR, nucleotide-binding and leucine-rich repeat receptor; PM, powdery Mildew; PRR, pattern recognition receptor.