The Study Of Plant Pathogen Interactions

Published Date: 02 Nov 2017

The study of plant-pathogen interactions requires the use of the whole plant in order to get a precise idea of the whole-plant response against the pathogen (Ruz et al., 2008). This is unlike the use and inoculation of detached plant organs which have nevertheless been widely use to test the pathogenicity of Erwinia amylovora and host resistance (Cabrefiga & Montesinos, 2005).

Numerous methods of inoculation have been used in evaluating plant response against pathogens. For instance, Ralstonia solanacearum can be inoculated by cutting lateral roots and then pouring the inoculum into the cut (Velupillai & Stall, 1984) or by pouring the inoculum onto the soil without wounding the roots (Milling et al., 2011). Plants can also be inoculated into the stem by puncturing the stem at the axils of the second or third fully expanded leaves from the apex with a needle dipped in the inoculum (Lemessa & Zeller, 2007), by forcing the needle into the stem through a drop of inoculum placed at the axil of leaf (Vellupillai & Stall, 1984) or by using a syringe to directly deliver the inoculum into the stem (Paret et al., 2008). In addition, R. solanacearum can also be inoculated by the leaf infiltration method in which the bacteria is inoculated by gentle pressure infiltration through the stomata of the abaxial surface of the leaf using a syringe with its needle removed (James et al., 2003; Poiatti et al., 2009). This method can be carefully controlled to produce uniform water soaked areas about 4 mm in diameter by varying the pressure on the leaf blade between the flat syringe and a sterile gloved finger (Bertoni & Mills, 1987).

In this study, the leaf infiltration method was employed. Leaf infiltration is known to result in early development of infection. The pathogen is introduced directly into the leaf mesophyll and thus symptoms appear quickly in the infiltrated leaf area. In addition, the pathogen is more efficiently exposed to the plant cell through this method. However, disease progression is slow and the final severity levels are low (Ruz et al., 2008). Several studies have shown that inoculating tobacco plants with R. solanacearum by the leaf infiltration method resulted in the same phenotype as when inoculated by the root inoculation method (Kanda et al., 2003; Shinohara et al., 2005). Furthermore this method is widely used to study the molecular events in many plant-bacteria pathosystems, including quantitative and qualitative analysis of molecular events during plant-R. solanacearum interaction (Kiba et al., 2007). Reproducibility of plant responses is confirmed in many plant-bacteria pathosystems (Kiba et al., 2003).

In this study, symptoms were visible as from day 3 p.i. and the plants were dead 8 to more than 30 days p.i. (Tables ….). This timeframe is in line with other studies. For instance, in one study involving the ginger- R. solanacearum pathosystem, bacterial wilt symptoms were observed 5 to 21 days p.i. (dpi) and by 21 dpi all affected plants were severely wilted or dead (Paret et al., 2008). In the work of Milling et al. (2011), tomato plants were inoculated by naturalistic soil soak inoculation method. It was found that plants of the susceptible cultivar Bonny Best were dead by 8 dpi. However, among the quite resistant tomato breeding line H7996 plants only 12% were dead by 14 dpi.

The current study showed that there was variation in the virulence of the different isolates. Differences in aggressiveness (virulence) among strains can be attributed to the wide range of origin of the strains, dose of pathogen, plant material type, time of incubation and accurate control of environmental conditions during the assays (Cabrefiga & Montesinos, 2005). However, in context of this study, only the first factor seems to be relevant since all the other factors were kept constant as far as possible. The inoculation results were also dependent on the plant variety. Indeed several studies support this observation. For instance, variability was seen, in susceptibility to fire blight among hosts, cultivars and plant material type (Thomas & Jones, 1992; Paulin et al., 1993; Bogs et al., 2004) and in virulence among strains of E. amylovora (Norelli et al., 1988).

All tomato plants were inoculated when they were 4-week old. Plant age was considered important in this study since it has been reported that time of inoculation affects disease development on different hosts depending on the type of host and race (Lemessa & Zeller, 2007). It was found that R. solanacearum Race 1 caused reduced disease development in pepper and tobacco at the 4 to 5 true leaves stage as compared to earlier stages. However, plant age during inoculation was found to have no effect on the expression of symptoms in potato, tomato and eggplant. This study provides an interesting idea which can be put into practice to reduce the risk of infection of tobacco and pepper plants by R. solanacearum. This involve employing mechanisms that delay contact between the host and the bacteria, for example, delaying transplantation of pepper where seedlings can grow on R. solanacearum-free nursery soil and transplanted to R. solanacearum infected fields. In addition, Arabidopsis has been found to become more resistant to virulent Pseudomonas syringae (pv. tomato or maculicola) as they grow. In a study involving the potato-Phytophthora infestans pathosystem Mutty & Hossenkhan (2008) reported that out of 6 varieties, those exhibiting high to moderate levels of resistance to late blight became consistently more resistant as they aged, while the effect was less pronounced on the 2 remaining susceptible varieties. However, cases of plants becoming more susceptible to some pathogens as they mature have also been reported.

Resistance in old plants can be attributed to the toxic antimicrobial compounds during the life cycle of the plant or to an increase in endogenous SA as demonstrated in mature tobacco plants (Yalpani et al., 1993) and Arabidopsis (Kus et al., 2002). In their study, Mutty & Hossenkhan (2008) deduced that blight resistance in potato plants might be a developmentally regulated response and as the potato plant undergoes transition from vegetative to reproductive growth, the rewiring of certain developmental pathways induces the expression, or results in the accumulation of, certain genes which might also play an important role in defense responses.