Hypersensitive-like defense in Live Oak: Discovery and efficiency against a host-specific gall former

Background/Question/Methods Hypersensitive response (HR) is a well-characterized gene-for-gene interaction between plant and pathogen genomes that involves a chemical cascade within plant cells in response to detection of a microbial pathogen which culminates in cell lysis and lesion formation in plant tissue at the site of pathogen infection. HR is established as a host plant defense against pathogens.  Recently, HR has been postulated as a defense mechanism against macroscopic plant invaders (such as gall-forming insects) whose larvae feed within plant tissue.  HR-like responses are just beginning to be characterized for macroscopic plant invaders.  Herein we document the presence, and examine the effects of an HR-like response in the leaves of Live Oak, Quercus fusiformis, to oviposition by the gall-forming wasp, Belonocnema treatae (Hymenoptera:Cynipidae), to determine if the lysis frequently associated with ovipositor insertion scars functions as a plant defense against the gall former.  We investigated (a) the incidence of tissue lysis (i.e., percent of oviposition scars developing lesions) within and among trees and in relation to oviposition intensity at the leaf and tree level, (b) the effect of tissue lysis on the probability of gall initiation and development, and (c) whether among-tree variation in the incidence of tissue lysis correlates with gall density/tree. 

Results/Conclusions In 2008, B. treatae oviposition scars were scored for the presence of lesions and galls from an average of 196 (± 15) leaves from each of 58 Live Oaks at Texas State's Freeman Ranch, San Marcos, Texas.  Across all leaves and trees, 14% of the 142,707 oviposition scars surveyed led to lesion formation.  Lesion incidence varied from 3% to 46% of oviposition scars/tree and was negatively related to oviposition intensity/tree (P < 0.003).  Only 8% of ovipositions associated with lesions led to gall initiation.  Comparison of the rates of gall initiation for oviposition scars associated with lesions versus oviposition scars without lesions showed that lesions significantly reduced the probability of gall establishment (P < 0.001).  Finally, the incidence of lesions was negatively correlated with gall density per tree (r = −0.40; P = 0.001).  This study demonstrates that an HR-like response of Live Oak to B. treatae oviposition functions as a plant defense.  The research will be extended in the 2009 field season by replicating the above descriptive study and by implementing transplant experiments to determine whether lesion formation is an intrinsic tree reaction or results from an insect-plant (gene-for-gene) interaction.