Performing Department
Plant Pathology
Non Technical Summary
Rust is considered to be a major threat to soybean production in the United States. Serious yield losses can occur if P. pachyrhizi, the causal agent of Asian Soybean Rust (ARS) becomes well established in the Continental United States, which is likely since there are no soybean cultivars with broad spectrum durable resistance. The major objective of most breeding programs is to identify genetic resistance to plant diseases. This is especially true of soybean. Significant efforts are being made to determine the location of the gene, or genes, controlling resistance that can assist in the rapid and efficient development of new resistant varieties. The durability and stability of plant disease resistance is also a fundamental issue, as it is an important asset in new cultivars. Considering that durable resistance is a reasonable goal, the specific strategies used to achieve this goal may be improved by a thorough characterization and analysis of the inheritance of
resistance. To characterize the inheritance pattern of soybean resistance to P. pachyrhizi, this project proposes to identify genes involved in defense, determine their potential function and better understand the molecular and genetic processes of soybean and P. pachyrhizi interactions that lead to disease resistance (specific and slow-rusting) or susceptibility, with the intent of rationally developing durable broad spectrum disease resistance strategies. To our knowledge, the proposed project will provide the first look at the relationship of the molecular, genetic and morphological events involved in soybean race specific and slow-rusting resistance to P. pachyrhizi. An important part of this work is the elucidation of the genetic regulation involved in specific and slow-rusting resistance to ASR. This lends the opportunity to manipulate the resistance response by possibly up-regulating identified genes that are necessary for resistance in response to rust infection and
down-regulating genes that are unnecessary or possibly have negative effects. Probably the most intriguing aspect of this work will be the comparison of inheritance patterns of soybean slow-rusting resistance with those of wheat and maize, where partial resistance has been successful in controlling rust fungal diseases. Questions regarding plant disease resistance that have left breeders, farmers and scientists perplexed include; Why has slow-rusting resistance proven to be more durable than specific resistance and how can this type of resistance be used to develop new durable resistant varieties? To address these questions, it will be necessary to gain a thorough understanding of the molecular basis of resistance that is unstable (specific) and one that remains effective (slow-rusting), providing incite as to the requirements for durable resistance and the mechanism controlling this resistance, whether it be gene-for-gene or other mechanisms. The work outlined below will contribute
to a framework for understand the molecular and genetic basis of slow-rusting resistance and designing novel strategies to develop soybean cultivars whose resistance to ASR lasts over time.
Animal Health Component
(N/A)
Research Effort Categories
Basic
(N/A)
Applied
(N/A)
Developmental
(N/A)
Goals / Objectives
The proposed experiments address our long-term goals of gaining a better understanding of the mechanism/genetics of soybean plant resistance and responses to P. pachyrhizi, the causal agent of Asian Soybean Rust (ARS) and developing more durable resistance strategies. Resistance to ASR is controlled by single-gene (race-specific) resistance that has proven to lack durability, losing resistance within the first few years of cultivar deployment. However, the aggressiveness of rust pathogens withstanding, there are a few soybean cultivars with slow-rusting (Partial) resistance that are potentially more durable than cultivars with complete resistance (race-specific), possibly remaining effective in the field for an extended period of time. This suggests that cultivars with partial resistance may somehow be unaffected by shifts in the pathogen population. It is clear that durability and stability of plant disease resistance to ASR is required for effective and efficient
cultivar development. Therefore, developing or improving strategies for durable and stable resistance to ASR through characterization and analysis of the inheritance of resistance is a necessity. The work outlined in this project will contribute to a framework for understanding the molecular and genetic basis of slow-rusting resistance. Thus, provide incite as to why some sources of resistance are more durable than others and facilitate the design of novel strategies to develop soybean cultivars whose resistance to ASR lasts over time. To work towards our long-term goals, this proposal will address three specific objectives: Objective 1: Compare the expression pattern of defense related genes in resistant (complete and slow-rusting) and susceptible isogenic soybean lines in response to pathogen (Phakopsora pachyrhizi) infection to identify genes that are differentially regulated, select candidate genes involved in defense and determine their potential function. Objective 2: Elucidate
the P. pachyrhizi infection process and parallel plant responses in resistant (complete and slow-rusting) and susceptible interactions by analyzing the morphology of rust fungal infection structures and the timing of rust infection in resistant and susceptible soybean cultivars. Objective 3: Compare the results from objectives one and two to analyze the relationship of defense related genes expressed in response to rust infection with the process and timing of rust infection and plant responses in resistant (complete and slow-rusting) and susceptible interactions to better understand the molecular and genetic basis of partial resistance durability.
Project Methods
The expression pattern of defense related genes in L87-0842 (complete resistance), Williams 82 (susceptible) and SRE-C-56E (slow-rusting resistance) in response to race Taiwan 80-2 infection will be determined with Affymetrix GeneChip Soybean Genome Array technology. The three isolines (L87-0482, SRE-C-56E and Williams 82) will constitute one experiment with two factors (1. Resistant (complete)/resistant (slow-rusting)/susceptible 2. inoculated/mock inoculated) and 3 biological replications (eighteen experimental units). The effects of rust inoculation on gene expression in the three soybean cultivars will be determined by comparing the expression profile data from resistant (complete) vs. susceptible, resistant (slow-rusting) vs. susceptible and resistant (complete) vs. resistant (slow-rusting) or the effect of inoculated vs. mock-inoculated. This will facilitate the identification of up-regulated PR (pathogenesis related) proteins and defense-related proteins after
inoculation. The expression patterns of up- and down-regulated genes identified for the three accessions will be confirmed by real time PCR. By comparing the changes in gene expression in L87-0842 and SRE-C-56E induced by rust inoculation, PR and defense-related proteins associated with either specific resistance or slow-rusting resistance can be identified. Therefore, in addition to providing a baseline of transcript profiles for soybean isolines with specific and slow-rusting resistance to P. pachyrhizi, this dataset will be useful for generating and evaluating hypotheses regarding the mechanisms of specific resistance that has not been durable and slow-rusting resistance that has proven to remain effective. The three soybean cultivars selected for this work will allow for comparisons of P. pachyrhizi infection of soybean isolines with complete and slow rusting resistance and to identify the differences and similarities in the infection process of these two resistances. The three
isolines will constitute one experiment with 3 factors (same two factors described for expression analysis plus a time factor) and 2 biological replications. To follow P. pachyrhizi development and infection within the plant cells of leaf sections collected from the three isolines, light microscope scans of the three cultivars for six points in a time course (12, 24, 48, 72, 96 and 144 hpi) will be analyzed by comparing the number of haustoria properly formed. The corresponding plant responses will be determined by monitoring the progression or hindrance of rust development. In our case, characterization of both the changes in gene expression and plant cell responses induced by inoculation will not only identify the type of plant-pathogen interaction mediating complete (specific) and slow-rusting resistance to P. pachyrhizi, but also provide a beginning sketch of the molecular events and genetic regulation involved in resistance to this pathogen, thus deciphering the mechanism(s) that
cause slow-rusting resistance to be more difficult for the pathogen to overcome. This new knowledge base will direct discussion and initial steps to enhance natural resistance mechanisms in soybean and similar crops.