Transfer of NHR mechanisms across species may lead to development of broad-spectrum and durable resistance in economically important crop species. Identification of NHO1 and PEN genes established the molecular basis of NHR. It also suggested the feasibility of transferring single gene-encoded NHR across plant species for creating durable and broad-spectrum resistance
Here we have described the Arabidopsis PSS1 locus that carries one of the nonhost resistance genes conferring immunity of Arabidopsis against two important soybean pathogens, P. sojae and F. virguliforme. Considering the disease phenotypes observed in detached leaves of pss1 as opposed to that in detached leaves of the pen1-1 mutant following P. sojae inoculation (Figures
2), the NHR mechanism governed by PSS1 is most likely important not only to provide penetration resistance, but also to confer necessary protection against further spread of the pathogen. pss1 supports secondary hyphal growth and sporulation of P. sojae (Figure
2). These observations suggest that PSS1 encodes a NHR defense mechanism that regulates both penetration and post-penetration resistance. It has been shown that the NHR mechanism at the post-haustorial stage is most important in sow thistle for providing resistance against a poorly adapted powdery mildew fungus, Golovinomyces cichoracearum UMSG1
. Similar mechanism could also be important for NHR of Arabidopsis against the non-adapted oomycete pathogen, P. sojae.
Segregation data from a cross between pss1 and Nd-0 revealed 1:2:1 genotypic segregation ratio for the alleles at the PSS1 locus (Table
2); and therefore, it is a single gene. Alleles at the PEN1 locus segregated independently of the alleles at the PSS1 locus (Figure
3). The P. sojae susceptible phenotype of the pss1 allele is manifested even in the presence of PEN1. Thus, PSS1 controls a novel defense mechanism for penetration resistance against the oomycete pathogen, P. sojae and the fungal pathogen, F. virguliforme. PEN genes have been shown to regulate two distinct NHR mechanisms that are involved in penetration resistance. Monogenic inheritance of PSS1 with no epistatic effect from PEN1 suggests that an additional Arabidopsis NHR mechanism is operative against penetration by oomycete and Fusarium pathogens. PSS1 is located in an approximately 2.75 Mb region flanked by two sequence-based polymorphic markers, SBP_20.71 and the telomere-specific SBP_23.46 (Figure
5C). This region does not contain any characterized plant defense or disease resistance genes. Thus, most likely we have identified a novel nonhost resistance mechanism in Arabidopsis.
The important hallmarks of a successful adapted pathogen are its ability to establish feeding structures, derive nutrition from the host and finally to complete its lifecycle in the host plant
. Aniline blue staining has previously been used to show oomycete feeding structures such as runner hyphae
. We observed secondary hyphae even after 6 hpi suggesting that P. sojae is able to form feeding structures in pss1 leaves at a very early stage following inoculation (Figure
2A). Sporangia are specialized asexual reproductive structures of oomycetes which can either germinate into hyphae or release about 10–30 zoospores to complete the asexual life-cycle. The male and female reproductive structures, antheridia and oogonia, are fused to develop oospores and complete the sexual life
. P. sojae developed both sporangia and oogonia in infected pss1 leaves; and thus, completed its life cycle in this mutant (Figure
2B). In contrast, in pen1-1 leaves the pathogen was able to penetrate single cells, which die following penetration; while in the wild type Col-0 leaves, germinated P. sojae zoospores failed to penetrate host cells (Figure
Lack of epistasis of PEN1 on PSS1 (Figure
3), growth of secondary hyphae and rapid induction of effector genes in the pss1 mutant, and most importantly completion of the P. sojae’s life cycle in infected pss1 mutant leaves suggest that PSS1 encodes a novel NHR mechanism that regulates both pre- and post-invasive resistance of Arabidopsis against the nonhost pathogen. Transfer of this to soybean could play an important role in creating broad-spectrum disease resistant not only against P. sojae, but also F. virguliforme. It is also possible that PSS1 encoded resistance may be applicable to fighting diseases caused by oomycete pathogens in other crop species; such as potatoes and tomatoes.
It has been shown that lack of either of a functional pathway, the PEN1/SNAP33/VAMP721/722 or the indole- glucosinolates/metabolites pathway, involving the PEN2/PEN3 activity is sufficient to allow a non-adapted fungal pathogen to enter Arabidopsis mutant plants at a rate similar to that in an adapted host
. However, a complete loss of the subsequent post-invasion resistance mechanism encoded by plant defense genes PAD4 and SAG101 is necessary for a nonhost plant species to become a host for such non-adapted fungal pathogens
. In light of the critical role of the post-invasion genes as determinants of the nonhost status of Arabidopsis against non-adapted fungal pathogens, PSS1’s role at both pre- and post-haustorial levels in conferring NHR of Arabidopsis against P. sojae is novel.
In vivo trans-specific gene silencing in Fusarium verticillioides from transgenic tobacco provides molecular evidence suggesting a possible short biotrophic phase in Fusarium species. F. virguliforme has been considered to be semi-biotrophic fungus with its ability to feed on live host soybean cells
. Thus, most likely PSS1 may regulate the immunity against both hemibiotrophs, P. sojae and F. virguliforme, by using the same mechanism. The differing lifestyles of the two pathogens, P. sojae and F. virguliforme and the importance of PSS1 in providing nonhost resistance against both of these pathogens hints at a crucial role of this gene in broader nonhost resistance of the model plant, Arabidopsis.