The hemibiotrophic fungus Zymoseptoria tritici causes Septoria tritici blotch disease of wheat (Triticum aestivum). Pathogen reproduction on wheat occurs without cell penetration, suggesting that dynamic and intimate intercellular communication occurs between fungus and plant throughout the disease cycle. We used deep RNA sequencing and metabolomics to investigate the physiology of plant and pathogen throughout an asexual reproductive cycle of Z. tritici on wheat leaves. Over 3,000 pathogen genes, more than 7,000 wheat genes, and more than 300 metabolites were differentially regulated. Intriguingly, individual fungal chromosomes contributed unequally to the overall gene expression changes. Early transcriptional down-regulation of putative host defense genes was detected in inoculated leaves. There was little evidence for fungal nutrient acquisition from the plant throughout symptomless colonization by Z. tritici, which may instead be utilizing lipid and fatty acid stores for growth. However, the fungus then subsequently manipulated specific plant carbohydrates, including fructan metabolites, during the switch to necrotrophic growth and reproduction. This switch coincided with increased expression of jasmonic acid biosynthesis genes and large-scale activation of other plant defense responses. Fungal genes encoding putative secondary metabolite clusters and secreted effector proteins were identified with distinct infection phase-specific expression patterns, although functional analysis suggested that many have overlapping/redundant functions in virulence. The pathogenic lifestyle of Z. tritici on wheat revealed through this study, involving initial defense suppression by a slow-growing extracellular and nutritionally limited pathogen followed by defense (hyper) activation during reproduction, reveals a subtle modification of the conceptual definition of hemibiotrophic plant infection.

Cadmium (Cd) tolerance seems to be a constitutive species-level trait in Arabidopsis halleri sp. halleri. Therefore, an interspecific cross was made between A. halleri and its closest nontolerant interfertile relative, Arabidopsis lyrata sp. petraea, and a firstgeneration backcross population (BC1) was used to map quantitative trait loci (QTL) for Cd tolerance. Three QTL were identified, which explained 43%, 24%, and 16% of the phenotypic variation in the mapping population. Heavy metal transporting ATPases4 (HMA4), encoding a predicted heavy metal ATPase, colocalized with the peak of the major QTL Cdtol-1 and was consequently further studied. HMA4 transcripts levels were higher in the roots and the shoots of A. halleri than in A. lyrata sp. petraea. Pollutants such as heavy metals can occur at very high concentrations in the soil as a consequence of either ancient natural processes or recent human activities such as mining, industrial, and agricultural practices. Although heavy metals are very toxic at high concentrations, some metal-tolerant plant species have developed the ability to grow and reproduce on soils highly polluted by heavy metals (Macnair, 1987). Among those metal-tolerant plants, some possess a remarkable physiological trait called hyperaccumulation that enables them to tolerate and accumulate high heavy metal concentrations in their shoots (about 100 times those occurring in nonaccumulator plants growing in the same substrates). Metal hyperaccumulators are consequently a particularly valuable resource to study the genetic basis of metal homeostasis.Arabidopsis halleri is an emerging model species for the molecular elucidation of metal tolerance and hyperaccumulation (Becher et al., 2004;Weber et al., 2004;Craciun et al., 2006;Filatov et al., 2006). This species belongs to the family Brassicaceae and shares about 94% DNA sequence identity within coding regions with the non-metal-tolerant plant species Arabidopsis (Arabidopsis thaliana; Becher et al., 2004). A. halleri constitutively tolerates zinc (Zn;Pauwels et al., 2006) and so far all the identified accessions are able to tolerate cadmium (Cd) and to hyperaccumulate Zn (Macnair et al., 1999). Hyperaccumulation of Cd was

The species Arabidopsis halleri, an emerging model for the study of heavy metal tolerance and accumulation in plants, has evolved a high level of constitutive zinc tolerance. Mapping of quantitative trait loci (QTL) was used to investigate the genetic architecture of zinc tolerance in this species. A firstgeneration backcross progeny of A. halleri ssp. halleri from a highly contaminated industrial site and its nontolerant relative A. lyrata ssp. petraea was produced and used for QTL mapping of zinc tolerance. A genetic map covering most of the A. halleri genome was constructed using 85 markers. Among these markers, 65 were anchored in A. thaliana and revealed high synteny with other Arabidopsis genomes. Three QTL of comparable magnitude on three different linkage groups were identified. At all QTL positions zinc tolerance was enhanced by A. halleri alleles, indicating directional selection for higher zinc tolerance in this species. The two-LOD support intervals associated with these QTL cover 24, 4, and 13 cM. The importance of each of these three regions is emphasized by their colocalization with HMA4, MTP1-A, and MTP1-B, respectively, three genes well known to be involved in metal homeostasis and tolerance in plants.M ETAL tolerance in plants has been considered ''an example of more powerful evolution in action than industrial melanism in moths' ' (Antonovics et al. 1971). Therefore it has been the focus of many evolutionary studies, in which it was argued that metal tolerance could evolve rapidly following exposure to heavy metal stress (Wu et al. 1975;Al-Hiyali et al. 1988). Some heavy metals, like zinc and copper, are oligo-nutrients and thus essential in small quantities for normal plant development. To avoid metal toxicity, all plants have evolved basic tolerance mechanisms. Binding by proteins or nonprotein thiol peptides in the cytoplasm and subsequent sequestration in the vacuole are the major component processes in the cellular heavy metal detoxification (Clemens 2001;Krämer 2005). However, at so-called metalliferous sites, heavy metals can occur at highly elevated concentrations in the soil, either through ancient natural processes, as in nickel-rich serpentine soils, or through recent human activities, as in zinc-and cadmium-rich calamine soils surrounding smelters. The total metal content of contaminated sites, depending on the metal, can be up to 10-to 1000-fold higher than that of uncontaminated sites (Bert et al. 2002). At these extreme concentrations, both essential and nonessential heavy metals become toxic (Clemens 2001;Hall 2002) and only a small number of plant species have evolved tolerance to such concentrations. These species have been classified as either absolute (strict or eu-) or facultative (pseudo-) metallophytes, according to their occurrence either on contaminated sites only or on both metalliferous and nonmetalliferous soils (Lambinon and Auquier 1964).The genetic basis of adaptive quantitative traits is still the matter of strong debates among evolutionists. Current que...

No abstract

Zymoseptoria tritici causes Septoria tritici blotch (STB) on wheat. The disease interaction is characterized by clearly defined temporal phases of infection, ultimately resulting in the death of host tissue. Zymoseptoria tritici is a highly polymorphic species with significant intraspecific variation in virulence profiles. We generated a deep transcriptomic sequencing dataset spanning the entire time course of an infection using a previously uncharacterized, highly virulent Z. tritici strain isolated from a Swiss wheat field. We found that seven clusters of gene transcription profiles explained the progression of the infection. The earliest highly up-regulated genes included chloroperoxidases, which may help the fungus cope with plant defences. The onset of necrotrophy was characterized by a concerted up-regulation of proteases, plant cell wall-degrading enzymes and lipases. Functions related to nutrition and growth characterized late necrotrophy and the transition to saprotrophic growth on dead plant tissue. We found that the peak up-regulation of genes essential for mating coincided with the necrotrophic phase. We performed an intraspecies comparative transcriptomics analysis using a comparable time course infection experiment of the genome reference isolate IPO323. Major components of the fungal infection transcriptome were conserved between the two strains. However, individual small, secreted proteins, proteases and cell wall-degrading enzymes showed strongly differentiated transcriptional profiles between isolates. Our analyses illustrate that successful STB infections involve complex transcriptomic remodelling to up-regulate distinct gene functions. Heterogeneity in transcriptomes among isolates may explain some of the considerable variation in virulence and host specialization found within the species.