The phytohormone ethylene is a principal modulator in many aspects of plant life, including various mechanisms by which plants react to pathogen attack. Induced ethylene biosynthesis and subsequent intracellular signaling through a single conserved pathway have been well characterized. This leads to a cascade of transcription factors consisting of primary EIN3-like regulators and downstream ERF-like transcription factors. The latter control the expression of various effector genes involved in various aspects of systemic induced defense responses. Moreover, at this level significant cross-talk occurs with other defense response pathways controlled by salicylic acid and jasmonate, eventually resulting in a differentiated disease response.

SummaryBasic and applied research involving transgenic plants often requires consistent high-level expression of transgenes. However, high inter-transformant variability of transgene expression caused by various phenomena, including gene silencing, is frequently observed. Here, we show that stable, high-level transgene expression is obtained using Arabidopsis thaliana post-transcriptional gene silencing (PTGS) sgs2 and sgs3 mutants. In populations of first generation (T1) A. thaliana plants transformed with a b-glucuronidase (GUS) gene (uidA) driven by the 35S cauliflower mosaic virus promoter (p35S), the incidence of highly expressing transformants shifted from 20% in wild type background to 100% in sgs2 and sgs3 backgrounds. Likewise, when sgs2 mutants were transformed with a cyclin-dependent kinase inhibitor 6 gene under control of p35S, all transformants showed a clear phenotype typified by serrated leaves, whereas such phenotype was only observed in about one of five wild type transformants. p35S-driven uidA expression remained high and steady in T2 sgs2 and sgs3 transformants, in marked contrast to the variable expression patterns observed in wild type T2 populations. We further show that T-DNA constructs flanked by matrix attachment regions of the chicken lysozyme gene (chiMARs) cause a boost in GUS activity by fivefold in sgs2 and 12-fold in sgs3 plants, reaching up to 10% of the total soluble proteins, whereas no such boost is observed in the wild type background. MAR-based plant transformation vectors used in a PTGS mutant background might be of high value for efficient high-throughput screening of transgene-based phenotypes as well as for obtaining extremely high transgene expression in plants.

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Summary Previously, it was shown that the Arabidopsis thaliana plant defensins AtPDF1.1 (At1g75830) and AtPDF1.2a (At5g44420) exert in vitro antimicrobial properties and that their corresponding genes are expressed in seeds and induced in leaves upon pathogen attack, respectively. In this study, the expression profile of both AtPDF1.1 and AtPDF1.2a is analysed in wild‐type plants upon different stress‐related treatments and the effect of modulation of their expression in transgenic plants is examined in both host and nonhost resistance. AtPDF1.1, which was originally considered to be seed‐specific, is demonstrated to be locally induced in leaves upon fungal attack and exhibits an expression profile distinct from that of AtPDF1.2a, a gene frequently used as marker for the ethylene/jasmonate‐mediated signaling pathway. Transgenic plants with modulated AtPDF1.1 or AtPDF1.2a gene expression show no altered phenotype upon Botrytis cinerea inoculation. However, constitutive overexpression of AtPDF1.1 in A. thaliana leads to a reduction in symptoms caused by the nonhost Cercospora beticola causing non‐spreading spots on A. thaliana leaves. These results indicate that AtPDF1.1 and AtPDF1.2a clearly differ regarding their expression profile and functionality in planta. It emphasizes the additional level of complexity and fine‐tuning within the highly redundant plant defensin genes in A. thaliana.