Rust diseases are caused by obligate biotrophic fungal parasites that require living host tissue to complete their lifecycles (Staples 2000), in contrast to necrotrophic pathogens that kill host tissue and metabolise dead plant cells. Rust pathogens use specialised sub-cellular structures called haustoria to extract nutrients from living cells (Mendgen and Hahn 2002). These structures penetrate plant cell walls and invaginate the cell plasma membrane.
There are about 160 genera of rust pathogen with approximately 5,000 species affecting a wide range of crops including cereals, legumes and trees (Helfer 2014; Sillero et al. 2006). Among these pathogens, Puccinia is the largest rust genera, mainly parasitizing cereal or grass species, followed by the Uromyces genus which infect grains and forage legumes. DNA analysis has shown that both these genera have polyphyletic origins (Maier et al. 2003). These species have closely co-evolved with their respective hosts (Gilbert 2002) and cross-species analysis of bean rust (Uromyces viciae-fabae) on wheat and wheat rust (Puccinia striiformis) on bean showed that these pathogens failed to parasitise the non-adapted hosts (Cheng et al. 2012; Zhang et al. 2011).
The evolution has driven a host specificity among adapted and unadapted pathogen species leading host and nonhost resistance (HR and NHR). The plant basal defence response against unadapted pathogens is triggered when conserved Pathogen associated molecular patterns (PAMPs) are detected by plant PAMP receptors, resulting in so called PAMP triggered immunity (NHR) (Zipfel 2008). Adapted pathogens introduce effector molecules in host plant cells that can suppress PAMP triggered basal defence (Hogenhout et al. 2009). Consequently, host plant species have evolved a recognition mechanism (resistance proteins) for specific pathogen effector molecules. In general, a single plant resistance protein recognises a single specific pathogen effector molecule to activate effector triggered immunity (ETI). Effector-triggered resistance is the underlying molecular basis of the gene for gene hypothesis of disease resistance (Flor 1971; Jones and Dangl 2006).
However, effector triggered resistance is not only confined to HR, and it appears to also play a role in NHR to unadapted pathogens (Schulze-Lefert and Panstruga 2011). Although some similarities in HR and NHR exists, the basis of innate plant resistance against NHR is still not well characterised (Fan and Doerner 2012). Current models of NHR suggest that unadapted pathogen faces a variety of impediments ranging from preformed passive physical and chemical barriers to active pathogen recognition followed by defence activation (Heath 2000).
The faba bean rust pathogen (U. viciae-fabae) has a diverse host range including Lathyrus, Pisum, Lens and Vicia species with Vicia faba the neotype host. DNA marker analyses (Barilli et al. 2011; Emeran et al. 2008) and cross-inoculation studies (Rubiales et al. 2013) have identified three specialised pathogen groups amongst the U. viciae-fabae species; U. viciae-fabae ex V. faba infects only faba bean, U. viciae-fabae ex V. sativa infects Vicia sp. apart from V. faba and U. viciae-fabae ex L. culinaris infects L. culinaris only. Lense culinaris, Vicia spp. and Lathyrus spp. were reported as non-hosts for Iranian U. viciae-fabae isolates collected from faba bean (Tohid et al. 2010), indicating Iranian isolates belonged to U. viciae-fabae ex V. faba. Using post-infection epifluorescence microscopy, Ghimire and Mansfield (1996) demonstrated that French bean (Phaseolus vulgaris L.), lima bean (Phaseolus lunatus L.), chickpea (Cicer arietinum L.) and lettuce (Lactuca sativa L.) are non-hosts for U. viciae-fabe ex V. sativa. No significant difference in penetration by the fungus was observed on host and non-host plants but clear differences in post-infection responses were observed. The U. viciae-fabae senu lato species complex, therefore, exhibits diversity in host preference (Sillero et al. 2006).
Faba bean rust pathogen is responsible for causing significant economic losses, worldwide. The pathogen isolates that affect faba bean (Vicia faba) are present almost everywhere (except North America) it is grown and causes moderate to high yield losses (Adhikari et al. 2016; Emeran et al. 2011; Rashid and Bernier 1991). The isolate variants of U. viciae-fabae prevalent in tropical and sub-tropical regions of India and China were reported pathogenic on field pea (Pisum sativum) where the warm climate and high humidity promoted disease progression (Kushwaha et al. 2006). However, pathogen isolates from temperate regions were able to infect field pea only at seedlings and were unable to cause disease outbreaks at the adult plant stage under field conditions (Barilli et al. 2009b). U. viciae-fabae (ex L. culinaris isolates) have been reported to cause complete crop failure of lentil (Lens culinaris) in North-African and South-Asian countries (Bejiga et al. 1998; Erskine et al. 1994). However, under Australian conditions, U. viciae-fabae is a common pathogen of faba bean and vetch (Vicia sativa) because rust infection on field pea and lentil has never been reported (Lindbeck 2009).
Several studies from Europe and Canada have characterised resistance against U. viciae-fabae, where hypersensitive response (complete resistance) was not visible in faba bean (Rashid and Bernier 1986; Sillero et al. 2000). However, in addition to faba bean (Conner and Bernier 1982c; Sillero et al. 2000), smaller pustule size with reduced disease severity (incomplete resistance) was reported in field pea (Barilli et al. 2014; Chand et al. 2006) and lentil (Negussie et al. 2005). In contrast, microscopic analyses demonstrated late hypersensitivity against fungal growth in faba bean (Rashid and Bernier 1984; Sillero et al. 2000). In wheat, hypersensitive cell death (HCD) completely inhibit fungal proliferation that were evident with visible immune infection type (IT: 0;) (McIntosh et al. 1995). Negussie et al. (2012) also observed post-haustorial hypersensitivity completely ceased the fungal growth in rust developing colonies inside the leaf tissue in lentil genotype ‘Gudo’.
Doza#12034 and Ac1655 carry seedling resistance genes Uvf-2 and Uvf-3, respectively. (Chapter 4 and 5). The focus of this study is to determine the host-pathogen interaction microscopically in these genotypes (Doza#12034 and Ac1655). Since there are conflicting reports of faba bean rust infection on other grain legumes, this study will estimate the host range of Australian faba bean rust pathotypes in related grain legumes i.e., field pea, chickpea, lupin, lentil and mung bean (Vigna radiata (L.) R. Wilczek.)