The effects of pathogen infection on nitrogen remobilization in Arabidopsis thaliana

by 1979- Boercker, Michelle Ann

The natural enemies of plants are ubiquitous and can reduce plant fitness. Plants have evolved two defense strategies to ameliorate the fitness cost associated with natural enemy attack. The first strategy, resistance, reduces the frequency and/or severity of natural enemy damage. The second strategy, tolerance, attenuates the fitness cost of natural enemy damage. Very little is known about the traits through which tolerance is manifested, particularly with respect to plant-pathogen systems (pathosystems). Diseased and naturally senescing leaves are often similar in their visible symptoms and molecular activities, suggesting that they may involve similar processes. One process that may be shared by the two phenomena is the efficient remobilization of nitrogen, a limiting nutrient that is heavily remobilized during natural leaf senescence. Nitrogen metabolism during foliar infections is largely unexplored, although plants are known to remobilize nitrogen from diseased leaves. Efficient remobilization of nitrogen from diseased leaves may ameliorate the fitness cost of infection, thereby manifesting tolerance to infection. Using the model pathosystem Arabidopsis thaliana – Pseudomonas syringae we asked the following questions: 1) Does infection by P. syringae pathovar tomato strain DC3000 (Pst DC3000) affect the amount of nitrogen remobilized from leaves? 2) Is there a relationship between the amount of nitrogen remobilized from infected leaves and plant tolerance to infection? To our knowledge, our study is the first to explore the effect of infection on leaf nitrogen remobilization in the context of tolerance. Results show that infected A. thaliana leaves remobilized nitrogen, however infection substantially reduced the amount of nitrogen remobilized. Plant fitness was inversely correlated with the amount of nitrogen retained by infected, senesced leaves, suggesting that the infection-caused impairment of nitrogen remobilization imposed a fitness cost. We detected little genetic variation in the effect of infection on the amount of nitrogen remobilized from infected leaves among 10 A. thaliana accessions. Similarly, we detected little genetic variation in A. thaliana symptom severity and tolerance to infection by Pst DC3000. The latter results contradict recent studies of this pathosystem, iii indicating that estimates of the broad-sense heritability of resistance and tolerance in this system are highly conditional. The challenge involved with understanding tolerance in an evolutionary context is discussed. We explored the effects of infection on additional A. thaliana traits and found that infected A. thaliana plants produce shorter main stems. The inverse correlation between the nitrogen content of senesced, infected leaves and fitness supports efficient nitrogen remobilization as a promising candidate tolerance trait. iv