Wounding- and pathogen-induced defense responses in plants

Abstract

Microbial pathogens and herbivores that cause disease or inflict damage to plants are ubiquitous in nature. To withstand and counteract invasions by these, plants have evolved several overlapping layers of defense. A preformed barrier consisting of physical impediments and toxic secondary metabolites limits the progress of most attackers. If these are overcome, a second line of inducible defense responses can be activated in plants through the recognition of non-self structures. Enormous progress has been made in the field of plant pathology over the last decades. Many of the mechanism by which plants perceive pathogens and pests, and the downstream signaling events that ultimately lead to immune responses have been characterized on a molecular level. Yet a comprehensive understanding for how plants can fend off invaders and achieve immunity with such finesse remains to be attained. Two aspects of plant immunity are addressed in this thesis: I) the cell-to-cell communication that governs local defense and II) the genetic machinery and the biochemical processes that underlie wounding and pathogen-induced accumulation of complex lipids. One of the most effective plant defense strategies against parasites is termed hypersensitive response (HR) and involves programmed cell death in infected and neighboring cells. In here, evidence is presented that the glucosinolate breakdown product sulforaphane is released from Arabidopsis thaliana cells undergoing HR induced by the bacterial effector AvrRpm1, and that sulforaphane can cause cell death when infiltrated into naïve tissue (Paper II). Hence, sulforaphane is identified as a novel regulator of plants’ local defense. Plants unable to synthesize sulforaphane displayed impaired HR response and enhanced pathogen susceptibility. A proposed mode of action for sulforaphane is that it binds glutathione and thereby affects the cellular redox status. Galactolipids containing the phytohormone 12-oxo-phytodienoic (OPDA), also called arabidopsides, are formed quickly and to high concentrations following mechanical wounding and pathogen elicitation in Arabidopsis. Data presented show that lipid-bound OPDA is formed while the fatty acid remains attached to the glycerol backbone (Paper III), and that all steps in this synthesis are enzyme catalyzed (Paper IV). Paper VI reports on the development of a LC-MS based method for the profiling of plant glycerolipids. This method was subsequently used to investigate natural variation in arabidopside accumulation (Paper IV), delimit the occurrence of OPDA-containing and acylated galactolipids in the plant kingdom (Paper V), and for the phospholipid profiling of the HR in Arabidopsis (Paper VI). Some of the findings from these studies include support that the gene Hydroperoxide lyase (HPL) is involved in arabidopside formation, that acylated MGDG species are omnipresent in the plant kingdom, and that a not previously described class of acylated OPDAcontaining phosphatidylglycerols is induced during effector-triggered HR in Arabidopsis. Taken together, these results show that the plant membrane lipid composition is highly dynamic and that distinct lipids profiles are generated during different types of defense responses. Crop losses due to diseases-causing pathogen and pests are estimated at around 30% globally. Understanding the mechanisms that determine resistance in plants and how plant diseases can be controlled is therefore of great value. The work presented in this thesis is my contribution to a deepened understanding of the plant innate immune system.