N. van den Berg

Unusual coloration in scarabaeid beetles

In this paper we investigate the reflection of circularly polarized light from the exocuticle of the scarabaeid beetle Gymnopleurus virens. Reflection spectra are deeply modulated, exhibiting a number of relatively narrow well-defined peaks, which differ from previously studied specimens. By comparing model calculations and electron microscopy work with the recorded spectra, we can propose the presence of specific structural defects responsible for the unusual spectra.

Biological, physiological and pathogenic variation in a genetically homogenous population of Fusarium oxysporum f.sp. cubense

Fusarium oxysporum f.sp. cubense (Foc) is the causal agent of Fusarium wilt of banana and is divided into three races and 21 vegetative compatibility groups (VCGs). Within a VCG, Foc proved to be genetically homogenous. Previous studies on phenotypic characteristics were performed with isolates that represented different races and VCGs from different banana-producing countries. The aim of this study was to determine the phenotypic variation in a genetically similar South African population of Foc, where all isolates belong to VCG 0120. Twenty-six isolates of Foc ‘subtropical’ race 4 (VCG 0120) were selected for this study. Differences in growth rate, spore production and the number of spores were determined. Virulence of Foc isolates was determined by inoculating susceptible, tissue-cultured, banana plantlets. Differences in cultural characteristics and virulence among isolates were found. The South African isolates can be divided into sporodochial, cottony and slimy pionnotal types. All isolates produced microconidia in abundance, but production of macroconidia and sporodochia varied. Disease severity varied from no vascular discoloration to complete discoloration of vascular tissue. This study showed that, although the South African population of Foc is homogenous in terms of its genetic make-up, it varies considerably in terms of phenotypic characteristics.

Infection studies of Alternaria cassiae on cowpea

Alternaria cassiae is the causal pathogen of a new, destructive foliar disease of cowpea (Vigna unguiculata). Cowpea leaves were inoculated with conidia of A. cassiae. Conidium germination, appressorium formation, penetration and colonisation of the plant surface were studied using light and scanning electron microscopy. Multiple germ-tubes developed randomly from each conidium and grew in any direction across the leaf surface. Terminal (71%) or intercalary (29%) appressoria were formed above epidermal cells or over stomata. Penetration of the plant surface, whether directly through the epidermis or via stomata, occurred with or without the formation of appressoria. Following penetration through a stoma, bulbous primary hyphae developed within the sub-stomatal cavity. Secondary hyphae developed from the primary hyphae and grew within the intercellular spaces, penetrating epidermal and mesophyll cells. Conidiophores emerged directly through the epidermis or through stomata.

Transcriptome responses of an ungrafted Phytophthora root rot tolerant avocado (Persea americana) rootstock to flooding and Phytophthora cinnamomi.

BackgroundAvocado (Persea americana Mill.) is a commercially important fruit crop worldwide. A major limitation to production is the oomycete Phytophthora cinnamomi, which causes root rot leading to branch-dieback and tree death. The decline of orchards infected with P. cinnamomi occurs much faster when exposed to flooding, even if flooding is only transient. Flooding is a multifactorial stress compromised of several individual stresses, making breeding and selection for tolerant varieties challenging. With more plantations occurring in marginal areas, with imperfect irrigation and drainage, understanding the response of avocado to these stresses will be important for the industry.ResultsMaintenance of energy production was found to be central in the response to flooding, as seen by up-regulation of transcripts related to glycolysis and induction of transcripts related to ethanolic fermentation. Energy-intensive processes were generally down-regulated, as evidenced by repression of transcripts related to processes such as secondary cell-wall biosynthesis as well as defence-related transcripts. Aquaporins were found to be down-regulated in avocado roots exposed to flooding, indicating reduced water-uptake under these conditions.ConclusionsThe transcriptomic response of avocado to flooding and P. cinnamomi was investigated utilizing microarray analysis. Differences in the transcriptome caused by the presence of the pathogen were minor compared to transcriptomic perturbations caused by flooding. The transcriptomic response of avocado to flooding reveals a response to flooding that is conserved in several species. This data could provide key information that could be used to improve selection of stress tolerant rootstocks in the avocado industry.

Development of a Nested Quantitative Real-Time PCR for Detecting Phytophthora cinnamomi in Persea americana Rootstocks

Phytophthora cinnamomi causes Phytophthora root rot (PRR) in avocado (Persea americana), an important disease that causes severe economic losses to the avocado industry globally. To date, no PRR-resistant avocado rootstock variety has been discovered, although certain rootstock varieties have been shown to be more tolerant than others. In this study, we developed an accurate, low cost assay for in planta quantification of P. cinnamomi to evaluate disease tolerance. A nested real-time polymerase chain reaction assay was developed to sensitively detect pathogen DNA in plant tissues. Root samples from a highly tolerant (Dusa) and less tolerant (R0.12) rootstock were collected at 0, 3, 7, 14, and 21 days after inoculation with P. cinnamomi and used for pathogen quantification. Nested primers developed in this study were specific and sensitive and could detect P. cinnamomi in root tissues. The amount of P. cinnamomi quantified in roots was significantly higher in the less-tolerant R0.12 plants when compared with the highly tolerant Dusa plants at all time points. This study has confirmed the known status of disease tolerance of Dusa and R0.12 avocado rootstocks in a quantitative manner and provides a reliable molecular tool to assist with industry breeding programs for the selection of PRR-resistant avocado rootstock varieties.

Ruthenium and ruthenium-based contacts to GaAs

This paper deals with the outstanding electrical and structural properties of Ru-based Schottky and ohmic contacts fabricated by electron beam evaporation on n- and p+ -type GaAs, respectively. The effective and flatband barrier heights were evaluated by standard current-voltage (I–V)(T) and capacitance-voltage (C–V)(T) measurements, over the temperature (T) range 100 to 350 K. The modified Richardson constant, A∗∗, varied between 2.2 and 4.9 A cm−2 K-2, depending upon the annealing temperature. AES depth profiles indicated that Ru forms structurally very stable contacts to GaAs, with no evidence from XPS measurements of any compound formation between Ru and GaAs even after annealing up to 500°C. A sharp increase was observed in the formation of Ga2O3 at the interface between the Ru layer and the GaAs after vacuum annealings at temperatures of 450°C and above. Only limited diffusion of arsenic through the Ru layer was observed after annealing at 400°C, but increased rapidly above 450°C. A Au/Ru/p+-GaAs ohmic contact system showed comparable specific contact resistance (5.5×10−6 Ω·cm2) to those systems commonly used, but with superior surface morphology.

Callose and β-1,3-glucanase inhibit Phytophthora cinnamomi in a resistant avocado rootstock

Phytophthora root rot (PRR) of avocado, caused by Phytophthora cinnamomi, is a significant threat to sustainable production wherever the crop is grown. Resistant rootstocks in combination with phosphite applications are the most effective options for managing this disease. Recently, the mechanisms underpinning PRR resistance have been investigated by the avocado community. Here, biochemical assays and confocal and scanning electron microscopy were used to investigate early defence responses in PRR resistant and -susceptible avocado rootstocks. Zoospore germination and subsequent hyphal growth for the pathogen were significantly inhibited on the surface of resistant avocado roots. When penetration occurred in the resistant R0.06 rootstock, callose was deposited in the epidermal cells, parenchyma and cortex of roots. In addition, b-1,3-glucanase was released early (6 h post-inoculation, hpi) in response to the pathogen, followed by a significant increase in catalase by 24 hpi. In contrast, susceptible R0.12 roots responded only with the deposition of lignin and phenolic compounds incapable of impeding pathogen colonization. In this study, PRR resistance was attributed to a timely multilayered response to infection by P. cinnamomi.