In the past two decades, tremendous progress has been made in our understanding of the molecular basis of plant immunity against adapted pathogens. However, why plants can ward off numerous “would be” pathogens–a phenomenon called non-host resistance (NHR)–remains largely elusive. One typical example of NHR is illustrated by rice’s complete immunity to powdery mildew and rust fungi, infectious to many other cereal crops such as barley, wheat, and rye. Many plant pathologists have dreamed of understanding the genetic mechanisms of rice’s NHR and utilizing them for creating cereal crops resistant to powdery mildew and rust fungi.
In our review, published in MPP (Wu et al., 2022), we first defined NHR as “effective resistance or immunity, exhibited by an entire plant species, to all genetic variants of a phytopathogen species”. We also noted that NHR “reflects the inability of a non-adapted but potentially pathogenic microbe to complete its asexual or sexual life cycle on a particular plant species”. A central idea surrounding NHR is illustrated in a cartoon (Title figure). That is: all plants possess multiple spatio-temporally connected layers of defence and each layer alone, or several layers acting together, can prevent an invading microbe from becoming a successful pathogen.
Due to the multi-layered complexity of NHR, as typified by rice’s immunity to powdery mildew and rust fungi, conventional (single-gene-based) “forward genetics” is rather ineffective in breaking down NHR. To help the research community study NHR, we summarized the approaches that have been deployed by researchers to identify new genes contributing to NHR under different plant-microbe interaction scenarios. Researchers have employed the following methods to break NHR: (1) chemical inhibition of specific genes or pathways, (2) whole-genome random mutagenesis (conforming to “forward genetics”) of a near-host using chemical mutagens or fast neutron radiation, (3) targeted mutagenesis (by VIGS or CRISPR) of genes and gene families (conforming to “reverse genetics”).
The identification of many key immunity genes known to contribute to resistance against adapted pathogens and the development of multiplexed CRISPR mutagenesis tools have now enabled new strategies to identify novel genes involved in NHR. For example, a “bottom-up” forward genetics approach can be deployed to identify genes involved in penetration resistance (i.e., layer #1 and #2 in figure 1) against non-adapted filamentous pathogens. A “nullify-knowns” forward genetics strategy can be used to identify new genes contributing to deeper layers of NHR (i.e., layer #5 in Figure 1) against non-adapted pathogens that are more phylogenetically distant to similar pathogens adapted to the plant.
“Bottom-up forward genetics” is a strategy that involves knocking out genes known to be required for post-penetration resistance (layer #3 – #5 in title Figure) to enable a more sensitive forward genetic screens for novel genes involved in penetration resistance (layer #1 and #2).
“Nullify-knowns forward genetics” is a strategy that involves knocking out key known immune genes by CRISPR in the same individual and then using the immune-compromised plant for identification of additional genes that prevent infection of a non-adapted pathogen (layer #5 in figure 1). There may be no strict demarcation of host and non-host.
A “near-host” is a plant species that is evolutionarily close to a host plant species with its NHR subverted by the pathogen. In some cases, a near-host’s NHR to the pathogen may largely rely on one defence layer and thus is breakable by whole-genome random mutagenesis.
“Virus-induced gene silencing (VIGS)” is an approach to suppress endogenous gene expression by infecting plants with a recombinant virus vector (VIGS vector) carrying host-derived sequence. VIGS offers a rapid means to knock down expression of a given gene and reveal its role in a given biological pathway in plants.
“Clustered regularly interspaced short palindromic repeat (CRISPR)” is a new gene-editing technology to create targeted genetic disruption in one gene or multiple genes (by multiplexed CRISPR).
Ying Wu, William Sexton, Bing Yang and Shunyuan Xiao published this review in Molecular Plant Pathology:
Genetic approaches to dissect plant nonhost resistance mechanisms.
TITLE IMAGE: For any filamentous pathogens to achieve reproductive success in a particular plant species, they must overcome several (five depicted) spatiotemporally distinct or connected defence barriers of the plant. Like an onion, multi-layered NHR against a non-adapted filamentous pathogen is dissectible using appropriate strategies and tools. Red stars indicate the defence layers where the microbial invasion is halted; grey lines in between layers imply possible mechanistic connections. All images used with permission of the author.
