Natural allelic variation underlying a major fitness trade-off in Arabidopsis thaliana.

Plants can defend themselves against a wide array of enemies, from microbes to large animals, yet there is great variability in the effectiveness of such defences, both within and between species. Some of this variation can be explained by conflicting pressures from pathogens with different modes of attack. A second explanation comes from an evolutionary 'tug of war', in which pathogens adapt to evade detection, until the plant has evolved new recognition capabilities for pathogen invasion. If selection is, however, sufficiently strong, susceptible hosts should remain rare. That this is not the case is best explained by costs incurred from constitutive defences in a pest-free environment. Using a combination of forward genetics and genome-wide association analyses, we demonstrate that allelic diversity at a single locus, ACCELERATED CELL DEATH 6 (ACD6), underpins marked pleiotropic differences in both vegetative growth and resistance to microbial infection and herbivory among natural Arabidopsis thaliana strains. A hyperactive ACD6 allele, compared to the reference allele, strongly enhances resistance to a broad range of pathogens from different phyla, but at the same time slows the production of new leaves and greatly reduces the biomass of mature leaves. This allele segregates at intermediate frequency both throughout the worldwide range of A. thaliana and within local populations, consistent with this allele providing substantial fitness benefits despite its marked impact on growth.

Live and let die--Arabidopsis nonhost resistance to powdery mildews.

The term "nonhost resistance" (NHR) describes the phenomenon that an entire plant species is resistant to all genetic variants of a non-adapted pathogen species. In nature, NHR represents the most robust form of plant immunity and is therefore of scientific as well as economic importance. Due to its highly complex nature, NHR has previously not been studied in detail. Recently, the establishment of model interaction systems utilizing Arabidopsis and non-adapted powdery mildews allowed the identification of several key components and conceptual conclusions. It is now generally accepted that NHR of Arabidopsis to powdery mildews comprises two distinct layers of defence: pre-invasion entry control at the cell periphery and post-invasion resistance based on cell death execution. The timely production and localised discharge of toxic compounds at sites of fungal attack appear to be pivotal for entry control. This process requires proteins involved in secretion and trans-membrane transport, synthesis and activation of indolic glucosinolates as well as gene regulation and post-translational protein modification. Post-invasion defence relies on lipase-like proteins and salicylic acid signalling. To what extent pathogen-associated molecular pattern- or effector-triggered immunity contribute to NHR remains to be investigated and is likely to depend on the model system studied.

Over-expression of different aldehyde dehydrogenase genes in Arabidopsis thaliana confers tolerance to abiotic stress and protects plants against lipid peroxidation and oxidative stress.

Aldehyde dehydrogenases (ALDHs) play a major role in the detoxification processes of aldehydes generated in plants when exposed to abiotic stress. In previous studies, we have shown that the Arabidopsis thaliana ALDH3I1 gene is transcriptionally activated by abiotic stress, and over-expression of the ALDH3I1 gene confers stress tolerance in transgenic plants. The A. thaliana genome contains 14 ALDH genes expressed in different sub-cellular compartments and are presumably involved in different reactions. The purpose of this study was to compare the potential of a cytoplasmic and a chloroplastic stress-inducible ALDH in conferring stress tolerance under different conditions. We demonstrated that constitutive or stress-inducible expression of both the chloroplastic ALDH3I1 and the cytoplasmic ALDH7B4 confers tolerance to osmotic and oxidative stress. Stress tolerance in transgenic plants is accompanied by a reduction of H2O2 and malondialdehyde (MDA) derived from cellular lipid peroxidation. Involvement of ALDHs in stress tolerance was corroborated by the analysis of ALDH3I1 and ALDH7B4 T-DNA knockout (KO) mutants. Both mutant lines exhibited higher sensitivity to dehydration and salt than wild-type (WT) plants. The results indicate that ALDH3I1 and ALDH7B4 not only function as aldehyde-detoxifying enzymes, but also as efficient reactive oxygen species (ROS) scavengers and lipid peroxidation-inhibiting enzymes. The potential of ALDHs to interfere with H2O2 was also shown for recombinant bacterial proteins.