High-level secretion of a virally encoded anti-fungal toxin in transgenic tobacco plants

Subtilases - versatile tools for protein turnover, plant development, and interactions with the environment

Subtilases (SBTs) constitute a large family of serine peptidases. They are commonly found in Archaea, Bacteria and Eukarya, with many more SBTs in plants as compared to other organisms. The expansion of the SBT family in plants was accompanied by functional diversification, and novel, plant-specific physiological roles were acquired in the course of evolution. In addition to their contribution to general protein turnover, plant SBTs are involved in the development of seeds and fruits, the manipulation of the cell wall, the processing of peptide growth factors, epidermal development and pattern formation, plant responses to their biotic and abiotic environment, and in programmed cell death. Plant SBTs share many properties with their bacterial and mammalian homologs, but the adoption of specific roles in plant physiology is also reflected in the acquisition of unique biochemical and structural features that distinguish SBTs in plants from those in other organisms. In this article we provide an overview of the earlier literature on the discovery of the first SBTs in plants, and highlight recent findings with respect to their physiological relevance, structure and function.

A novel antifungal peptide from foxtail millet seeds

BACKGROUND

Antifungal proteins (AFP) help plants to combat phytopathogenic fungi and thus protect plants from the devastating damage caused by fungal infections and prevent massive economic losses. To date, several proteins with antibacterial and/or antifungal properties have been isolated and characterized from different plant species and tissues; however, there are no reports concerning the antifungal peptide from foxtail millet seeds.

RESULTS

An antifungal peptide with a molecular mass of 26.9 kDa was isolated from dry seeds of the foxtail millet (Setaria italica (L.) Beauv.), using a procedure that involved four chromatographic steps. The antifungal peptide was adsorbed on CM-Sepharose, Affi-gel blue gel and Superdex 75. It was further purified by C(18) reverse-phase high-performance liquid chromatography and submitted for analysis of peptide mass fingerprint. The Mascot peptide mass fingerprint of the isolated protein hit no existing protein (score >60), and it was proved to be a novel antifungal peptide. It inhibited mycelial growth in Alternaria alternate with an IC(50) of 1.3 µmol L(-1) , and it also exhibited antifungal activity against Trichoderma viride, Botrytis cinerea and Fusarium oxysporum. Transmission electron microscopy of mold forms of Alternaria alternate after incubation with 20 µg mL(-1) of the antifungal protein for 48 h revealed marked ultrastructural changes in the fungus.

CONCLUSION

A novel antifungal peptide with high potency was isolated from foxtail millet seeds.

Fungicidal and binding properties of three plant peptides

The fungicidal properties of plant seed peptides from Heuchera sanginea (Hs-AFP1), Raphanus sativus (EA-AFP2), and Impatiens balsamina (Ib-AMP3) were determined for the non-germinated and germinated conidia of Aspergillus flavus and Fusarium moniliforme. These peptides were weakly lethal for germinated but not for non-germinated conidia of A. flavus. Both non-germinated and germinated conidia of F. moniliforme were susceptible to these peptides. Overall, F. moniliforme was more susceptible than A. flavus to the peptides. The peptides bound strongly to chitin, mannan, galactocerebrosides, and sphingomyelin. Binding results varied for ergosterol, cholesterol, and beta1,3-glucan.

Rewiring mitogen-activated protein kinase cascade by positive feedback confers potato blight resistance

Late blight, caused by the notorious pathogen Phytophthora infestans, is a devastating disease of potato (Solanum tuberosum) and tomato (Solanum lycopersicum), and during the 1840s caused the Irish potato famine and over one million fatalities. Currently, grown potato cultivars lack adequate blight tolerance. Earlier cultivars bred for resistance used disease resistance genes that confer immunity only to some strains of the pathogen harboring corresponding avirulence gene. Specific resistance gene-mediated immunity and chemical controls are rapidly overcome in the field when new pathogen races arise through mutation, recombination, or migration from elsewhere. A mitogen-activated protein kinase (MAPK) cascade plays a pivotal role in plant innate immunity. Here we show that the transgenic potato plants that carry a constitutively active form of MAPK kinase driven by a pathogen-inducible promoter of potato showed high resistance to early blight pathogen Alternaria solani as well as P. infestans. The pathogen attack provoked defense-related MAPK activation followed by induction of NADPH oxidase gene expression, which is implicated in reactive oxygen species production, and resulted in hypersensitive response-like phenotype. We propose that enhancing disease resistance through altered regulation of plant defense mechanisms should be more durable and publicly acceptable than engineering overexpression of antimicrobial proteins.

Untranslated regions from C4 amaranth AhRbcS1 mRNAs confer translational enhancement and preferential bundle sheath cell expression in transgenic C4 Flaveria bidentis

Many aspects of photosynthetic gene expression are posttranscriptionally regulated in C4 plants. To determine if RbcS mRNA untranslated regions (UTRs) in themselves could confer any characteristic C4 expression patterns, 5'- and 3'-UTRs of AhRbcS1 mRNA from the C4 dicot amaranth were linked to a gusA reporter gene. These were constitutively transcribed from a cauliflower mosaic virus promoter and assayed for posttranscriptional expression patterns in transgenic lines of the C4 dicot Flaveria bidentis. Three characteristic C4 expression patterns were conferred by heterologous AhRbcS1 UTRs in transgenic F. bidentis. First, the AhRbcS1 UTRs conferred strong translational enhancement of gusA expression, relative to control constructs lacking these UTRs. Second, while the UTRs did not appear to confer tissue-specific expression when analyzed by beta-glucuronidase activity assays, differences in gusA mRNA accumulation were observed in leaves, stems, and roots. Third, the AhRbcS1 UTRs conferred preferential gusA expression (enzyme activity and gusA mRNA accumulation) in leaf bundle sheath cells. AhRbcS1 UTR-mediated translational enhancement was also observed in transgenic C3 plants (tobacco [Nicotiana tabacum]) and in in vitro translation extracts. These mRNAs appear to be translated with different efficiencies in C4 versus C3 plants, indicating that processes determining overall translational efficiency may vary between these two categories of higher plants. Our findings suggest that the AhRbcS1 5'-UTR functions as a strong translational enhancer in leaves and other tissues, and may work synergistically with the 3'-UTR to modulate overall levels of Rubisco gene expression in different tissues and cell types of C4 plants.

The viral killer system in yeast: from molecular biology to application

Since the initial discovery of the yeast killer system almost 40 years ago, intensive studies have substantially strengthened our knowledge in many areas of biology and provided deeper insights into basic aspects of eukaryotic cell biology as well as into virus-host cell interactions and general yeast virology. Analysis of killer toxin structure, synthesis and secretion has fostered understanding of essential cellular mechanisms such as post-translational prepro-protein processing in the secretory pathway. Furthermore, investigation of the receptor-mediated mode of toxin action proved to be an effective means for dissecting the molecular structure and in vivo assembly of yeast and fungal cell walls, providing important insights relevant to combating infections by human pathogenic yeasts. Besides their general importance in understanding eukaryotic cell biology, killer yeasts, killer toxins and killer viruses are also becoming increasingly interesting with respect to possible applications in biomedicine and gene technology. This review will try to address all these aspects.

The Ustilaginales as plant pests and model systems

The Ustilaginales are a vast and diverse group of fungi, which includes the plant pathogenic smuts that cause significant losses to crops worldwide. Members of the Ustilaginales are also valuable models for the unraveling of fundamental mechanisms controlling important biological processes. Ustilago maydis is an important fungal model system and has been well studied with regard to mating, morphogenesis, pathogenicity, signal transduction, mycoviruses, DNA recombination, and, recently, genomics. In this review we discuss the life cycles of members of the Ustilaginales and provide background on their economic impact as agricultural pests. We then focus on providing a summary of the literature with special attention to topics not well covered in recent reviews such as the use of U. maydis in mycovirus research and as a model for understanding the molecular mechanisms of fungicide resistance and DNA recombination and repair.

Antifungal activity of a virally encoded gene in transgenic wheat

The cDNA encoding the antifungal protein KP4 from Ustilago maydis-infecting virus was inserted behind the ubiquitin promoter of maize and genetically transferred to wheat varieties particularly susceptible to stinking smut (Tilletia tritici) disease. The transgene was integrated and inherited over several generations. Of seven transgenic lines, three showed antifungal activity against U. maydis. The antifungal activity correlated with the presence of the KP4 transgene. KP4-transgenic, soil-grown wheat plants exhibit increased endogenous resistance against stinking smut.

The influence of dsRNA viruses on the biology of plant pathogenic fungi

Double-stranded RNA viruses are ubiquitous in fungi. They are non-infective and, like most prokaryotic plasmids, are only transmitted to compatible strains via cell fusion. Most are cryptic, but some with an established phenotype, such as the hypoviruses of the chestnut-blight fungus, have been studied for their potential as biological control agents of fungi.

The Ustilago maydis virally encoded KP1 killer toxin

Some strains of the plant-pathogenic fungus Ustilago maydis secrete toxins (killer toxins) that are lethal to susceptible strains of the same fungus. There are three well-characterized killer toxins in U. maydis-KP1, KP4, and KP6-which are secreted by the P1, P4, and P6 subtypes, respectively. These killer toxins are small polypeptides encoded by segments of an endogenous, persistent double-stranded RNA (dsRNA) virus in each U. maydis subtype. In P4 and P6, the M2 dsRNA segment encodes the toxin. In this work, the KP1 killer toxin was purified for internal amino acid sequence analysis, and P1M2 was identified as the KP1 toxin-encoding segment by sequence analysis of cDNA clones. The KP1 toxin is a monomer with a predicted molecular weight of 13.4kDa and does not have extensive sequence similarity with other viral anti-fungal toxins. The P1M2 segment is different from the P4 and P6 toxin-encoding dsRNA segments in that the 3' non-coding region of its plus strand has no sequence homology to the 3' ends of the plus strands of P1M1, P4M2, or P6M2.