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Powdery Mildew resistance: Is there a single gene that confers complete resistance?

Spoiler alert: probably not.

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Plant pathogens infecting marijuana (Cannabis sativa L.) reduce growth and productivity of the crop by affecting all parts of the plant during growth and after harvest. In the recent years, several plant pathogens that impact cannabis and hemp growth and quality, were documented in the literature (7). These pathogens were found to occur on cannabis plants during cultivation in greenhouses and indoor controlled-environment growing facilities throughout the world, as-well-as in outdoor field environments.  Among diseases previously reported to affect cannabis and hemp inflorescences grown under field conditions or indoors, the two most prevalent reported diseases were ‘grey mold’ and ‘powdery mildew’ (PM) (3),(4).

Cannabis powdery mildew was previously associated with at least two parasitic fungi Podosphaera macularis (formerly Sphaerotheca macularis), Leveillula taurica (6) and recently Golovinomyces cichoracearum was demonstrated to cause PM in cannabis (7).

The effect of PM is often devastating, resulting in a high percentage of unmarketable products and heavy yield reduction. PM is of special concern in the industry due to the general susceptibility of clonal material, the pathogenicity of the fungus, large seasonal spore loads, and lack of effective and approved mitigation strategies.

Only little scientific information is available about the genetic diversity of PM pathogens and genetic resistance mechanisms to these pathogens in cannabis. However in other crops, using genetic disease resistance (either naturally occurred or synthetically mutated alleles) to control PM disease incidence, is regarded as the most effective approach and has been successfully applied for a wide range of crops such as wheat, barley, tomato and pepper ((10),(5)). The use of crops that are genetically resistant to pathogens not only decreases the cultivation costs and chemical residues but also reduces the environmental risks caused by chemical control of plant diseases.

 

In other species, resistance to PM is conferred by dominant defense mechanisms that directly target the pathogen and may be encoded by a single major gene. In other cases, PM resistance was found to be determined by one or several dominantly inherited single loci. The most impressive resistance is conferred by the recessive Mildew resistance Locus O (mlo) (8). Naturally occurring or experimentally induced mutations in this locus can lead to the loss of function of the coded transmembrane protein MLO, resulting in strong broad-spectrum immunity to the fungus. The MLO gene family is encoded by a variable number of loci from which only one locus—or, in some cases, multiple loci—needs to be mutated to achieve complete immunity, meaning several MLO deletions are likely to increase resistance. The simplicity of the MLO-associated resistance has attracted multiple experimental mutagenic and gene editing projects that are currently being implemented and may confer efficient, broad-spectrum immunity/resistance to PM in cannabis.

Based on reports by commercial cultivators, inherent genetic resistance to PM exists in the cannabis gene-pool (personal communications), particularly in outdoor-grown ‘hemp’ varieties. Methodic research to test these resistance sources had not been carried out and the genetic basis or genes responsible for resistance, have not yet been elucidated. In the recent publication (2) several candidate gene families, including MLO genes, were associated with partial or quantitative resistance in cannabis, based on presence-absence (PAV) and copy number analyses (CNV), suggesting a multi locus nature for PM resistance in cannabis (2).

While qualitative resistance (cases of major genes conferring durable resistance) are known (8), quantitative resistance is generally regarded as more durable (9), less likely to ”break” (due to pathogen adaptation) and more likely to confer broad- spectrum resistance (resistance to multiple strains of the pathogen).

High throughput sequencing and advanced genomic tools are now available to generate high-resolution genetic linkage maps even in highly heterozygous species, such as cannabis. This approach allows not only to identify single-gene/single-locus effects, but also to identify multiple quantitative loci and generate genetic markers for them. These can be used to efficiently screen large numbers of progeny in a short period of time at a low cost and select for individuals containing multiple desirable alleles (QTL stacking) leading to strong long-term, durable resistance to PM in cannabis.

(A) Early stages of infection on young leaf, showing sparse white mycelium on leaf surface. (B) Advanced stages of infection with profuse sporulation, resulting in a powdery appearance on the leaf surface. (C) Development of powdery mildew on leaves and stem of vegetative cuttings of strain “Pink Kush.” (D) Powdery mildew infection on inflorescences of C. sativa “Pink Kush” showing extensive mycelial development.

References:

  1. Gabor BK, Just BJ, Huang C, Jones CM, Vreugdenhil D, Kniskern JM, Pablo A. Quijada PA, Berke TG, Allersma AP, Xiang W (2017) Methods and compositions for producing capsicum plants with powdery mildew resistance. US Patent NO. US9,689,045 B2
  2. McKernan, Kevin J., et al. “Sequence and annotation of 42 cannabis genomes reveals extensive copy number variation in cannabinoid synthesis and pathogen resistance genes.” bioRxiv (2020).
  3. McPartland JM and Hilig KW (2008) Differentiating powdery mildew from false powdery mildew. J Indus Hemp 13:78–87
  4. Punja, Zamir K., et al. “Pathogens and Molds Affecting Production and Quality of Cannabis sativa L.” Frontiers in plant science 10 (2019).
  5. Ren Y, Hou W, Lan C, Basnet BR, Sing RP, Zhu W, Cheng X, Cui D, Chen F (2017) QTL analysis and nested association mapping for adult plant resistance to powdery mildew in two bread wheat populations. Front Plant Sci. https://doi.org/10.3389/FPLS.2017.01212
  6. Wolfenbarger SN, Massie ST, Ocamb C, Eck EB, Grove GG, Nelson ME, Probst C, Twomey MC, Gent DH. 2016. Distribution and Characterization of Podosphaera macularis Virulent on Hop Cultivars Possessing R6-Based Resistance to Powdery Mildew. Plant disease 100(6): 1212-1221.
  7. Zamir K. Punja (2018) Flower and foliage-infecting pathogens of marijuana (Cannabis sativa L.) plants, Canadian Journal of Plant Pathology, 40:4, 514-527
  8. Acevedo‐Garcia, Johanna, Stefan Kusch, and Ralph Panstruga. “Magical mystery tour: MLO proteins in plant immunity and beyond.” New Phytologist 204.2 (2014): 273-281.
  9. Parlevliet, Jan E. “Durability of resistance against fungal, bacterial and viral pathogens; present situation.” Euphytica 124.2 (2002): 147-156.

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