More detail background information of my research can be found in this page.
- Bremia lactucae
- Gene-for-Gene Theory
- Microsattellite marker
- International Bremia Evaluation Board (IBEB) lettuce cultivars differencial test set
Scientific classification and life cycle
Downy mildew of lettuce causes foliar blights and leaf distortion. Cool temperatures and wetted leaves are necessary for germination of sporangia of B.lactucae. Oospores, conidia and sporangia as well as the endophytic mycelium are important elements of the downy mildew infection cycle. The germinative spore penetrates into the leaf epidermal cell wall or stomata and commences infection. Figure 1: Disease cycle of downy mildew caused by B. lactucae (Source from http://ucce.ucdavis.edu/files/filelibrary/2028/23067.pdf)
Gene-for-Gene Theory which was raised by Harold H. Flor in 1940s, Avirulence of pathogens (Avr gene) corresponds to resistance gene (R gene) in the hosts and compatible interactions drive successful infection. R gene encodes a specific domain that can recognize the Avr protein to confer resistance. The absence of an R gene will make plants become susceptible. The R gene of lettuce corresponding to B. lactucae Avr gene is the Dm gene. The model below constructed by PPI HS lab explains this theory very clearly:
"R is a plant resistance gene and 'r' indicates that the plant lacks this gene. A particular pathogen may express a certain effector gene or it may lack that gene. The effector gene is designated as 'A' and the lack of the effector gene is designated as 'a'. The gene-for-gene model states that when a plant expresses R and the pathogen expresses A then recognition occurs, ETI is activated, and no disease is formed. However, if the plant doesn¡'t have R, or the pathogen doesn¡'t have A, then recognition does not occur and the pathogen is able to escape detection and cause disease. " (by PPI HS lab)
Model is constracted by PPI HS lab
Microsatellites or simple sequence repeats (SSRs), are tandem repeats of 2-8 base pairs (bp) that can vary extensively in the number of repeats. They are valuable as genetic markers because they are co-dominant, detect high levels of allelic diversity, and are easily and economically assayed by PCR.
The differential test set of lettuce cultivars is based on that used by the International Bremia Evaluation Board (IBEB) (Arend et al., 2006). The results of the test give a unique value per isolate/race. Figure 3 is the denomination and identification of B.lactuace isolates commonly used in breeding which was also developed by IBEB, a joint effort of European lettuce breeders and authorities since 1998. IBEB has identified and named new races of B. lactucae (Bl:) in the last 8 years. (van der Arend et al., 2006).
Figure 3 illustrates those Reactions: - resistant, (-) resistant with necrosis may interfere with result, (m) moderate/mixed, (+) susceptible with necrosis, + susceptible. Bl: 1 to Bl: 16 are new names for NL1 -NL16. Bl: 3, 11 and 19 are not available and found anymore and therefore removed from the list (van der Arend et al., 2006).
B.lactuace is a highly variable pathogen (Petrželova and Lebeda, 2004). The variation was first categorized in terms of physiological races (Crute and Dixon, 1981), and later virulence factors (v factors) virulence phenotypes according to the interpretation of the host-pathogen interaction in a gene-for-gene relationship (Crute, 1987). It provided a basis for the population studies in this pathogen (Lebeda, 1982; Petrželova and Lebeda, 2004). Population genetics have been wildly used in studies of virulence variation of pathogen populations occurring on lettuce (Petrželova and Lebeda, 2004). A total of 132 isolates of B.lactuace collected from infected and naturally growing plants of L. serriola during 1998-2000 were used for v-factor analysis (Lebeda and Petrželova, 2003). 37 isolates were used for determination of v-factors on the basic differential set of Lactuca spp. Genotype (van Ettekoven and van der Arend, 1999) and 19 v-factors (v1-v18, v36-v38) were established (Petrželova and Lebeda, 2004).
Figure 2. Denomination of B.lactucae isolates commonly used in breeding (from http://www.plantum.nl/pdf/IBEB_identification_and_nomination_2006.pdf )