EVALUATION OF DOWNY MILDEW (HYALOPERONOSPORA BRASSICAE) INFECTION SEVERITY ON DIFFERENT CRUCIFEROUS OILSEED CROPS

Diseases constitute an important economic problem in oilseed rape (Brassica napus) cultivation. Although downy mildew has been counted so far as a minor disease, under intensive cultivation system and short rotation interval, the impact of diseases could increase in the future, especially under predicted more humid northern climatic conditions. This research study is the first report about the severity of downy mildew infection on cruciferous crops in Estonia. During two years (2010 − 2011), downy mildew infection severity was assessed in six different cruciferous crops (B. napus, B. juncea, B. nigra, Sinapis alba, Raphanus sativus, Eruca sativa) in field trials located in Eerika, Tartu County, Estonia. On both study years, four disease assessments were done. Downy mildew infection started a week earlier in 2010, but the overall disease pressure was lower compared to 2011. Based on our field trial results, S. alba, E. sativa and R. sativus plants were significantly more resistant to downy mildew compared to other tested crops. Furthermore, in 2010 downy mildew symptoms were not recorded on two cruciferous species E. sativa and R. sativus. Brassica nigra and B. napus plants were the most susceptible to downy mildew, with especially severe infection in 2011. Based on our two years data, downy mildew can damage foliage on spring oilseed rape (B. napus) and black mustard (B. nigra) in a notable extent. We conclude that downy mildew is an important pathogen, which should be monitored on different oilseed cruciferous crops in Estonia

A genetic framework for H2O2 induced cell death in Arabidopsis thaliana

Background: To survive in a changing environment plants constantly monitor their surroundings. In response to several stresses and during photorespiration plants use reactive oxygen species as signaling molecules. The Arabidopsis thaliana catalase2 (cat2) mutant lacks a peroxisomal catalase and under photorespiratory conditions accumulates H2O2, which leads to activation of cell death. Methods: A cat2 double mutant collection was generated through crossing and scored for cell death in different assays. Selected double mutants were further analyzed for photosynthetic performance and H2O2 accumulation. Results: We used a targeted mutant analysis with more than 50 cat2 double mutants to investigate the role of stress hormones and other defense regulators in H2O2 -mediated cell death. Several transcription factors (AS1, MYB30, MYC2, WRKY70), cell death regulators (RCD1, DND1) and hormone regulators (AXR1, ERA1, SID2, EDS1, SGT1b) were essential for execution of cell death in cat2. Genetic loci required for cell death in cat2 was compared with regulators of cell death in spontaneous lesion mimic mutants and led to the identification of a core set of plant cell death regulators. Analysis of gene expression data from cat2 and plants undergoing cell death revealed similar gene expression profiles, further supporting the existence of a common program for regulation of plant cell death. Conclusions: Our results provide a genetic framework for further study on the role of H2O2 in regulation of cell death. The hormones salicylic acid, jasmonic acid and auxin, as well as their interaction, are crucial determinants of cell death regulation.Peer reviewe

Summary of <i>cat2</i> double mutant cell death phenotypes.

<p>The <i>cat2</i> single mutant develops extensive cell death when grown on soil in 12/12h light dark conditions [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0170532#pone.0170532.ref024" target="_blank">24</a>]. The double mutants were used for gene expression analysis to determine potential signaling pathways involved in gene regulation during PCD.</p

PCD marker gene expression related to hormone, ROS and pathogen response.

<p>Expression of <i>FMO1</i>, <i>PLA2A</i>, <i>WRKY75</i>, <i>WRKY40</i> and <i>GLTP</i> as visualized by investigating selected hormone, pathogen and ROS treatments in Genevestigator using the Perturbations tool. Green indicates decreased expression and red increased expression. Plant age is indicated in parenthesis.</p

Marker gene expression in <i>cat2</i> and in double mutants defective in hormone signaling pathways or TFs.

<p>Letters indicate differences between leaf age classes (p<0.05; n = 3) and asteriks show significant differences relative to <i>cat2</i> at the corresponding leaf age. The values represent the mean (box) and standard error (bar).</p

Marker gene expression in Col-0 and positive regulators of PCD in three leaf age classes.

<p>Mutants were divided into functional classes based on their primary function in defense signaling. Letters indicate differences between leaf age classes (p<0.05; n = 3) and asterisks differences relative to Col-0 at the corresponding leaf age. Leaves divided into age classes are: young (white boxes), mature (grey) and old (green). The values represent the mean (box) and standard error (bar).</p

The expression of SA marker genes in Col-0, <i>cat2</i>, <i>axr1</i> and <i>cat2 axr1</i>.

<p>Letters indicate differences between leaf age classes (p<0.05; n = 3) and asterisks significant differences relative to <i>cat2</i> at the corresponding leaf age. The values represent the mean (box) and standard error (bar).</p