Biosecurity capacity building for the Australian avocado industry: Laurel Wilt

Ambrosia beetles share one special feature with humans, the ability to farm. These weevil-sized beetles colonize wood but rather than consuming this rather nutrient-poor material, they inoculate the tree with a particular type of fungus, which then becomes the food for the beetle larvae. Many tree species have evolved specialized chemical defences against this type of insect infestation and normally the fungal symbiont is only weakly pathogenic. However, there are some notable exceptions to this rule where perfectly healthy trees have succumbed to ambrosia beetle infestation and their fungal symbionts have acted as aggressive pathogens. One classic example occurs in the south-eastern states of the USA, where cultivated avocado and its wild relatives are afflicted by a lethal disease called laurel wilt. The culprit is the beetle Xyleborus glabratus, which transmits the fungus Raffaelea lauricola. X. glabratus is native to Asia and entered the USA as a stowaway in wooden crates. X. glabratus is considered one of the most serious biosecurity threats to the Australian avocado industry. In a pre-emptive move to prevent X. glabratus entering Australia, scientists from Brisbane have visited their counterparts in the USA to learn about laurel wilt disease. As a result of this collaboration, a diagnostic manual for laurel wilt has been produced, which will become the national standard for Australia. During preparation of this manual, new molecular diagnostic assays for both X. glabratus and R. lauricola were developed. Armed with these new assays, ambrosia beetle surveys were done in subtropical and tropical avocado production areas of the east coast of Australia and thankfully no X. glabratus found, confirming Australia’s pest-free status. However, there is no room for complacency, as two more ambrosia beetle species were discovered (Euwallacea sp. aff. fornicatus and Microperus sp.) were found on the Sunshine Coast and Atherton Tablelands in Queensland. Infestations of these beetles were associated with avocado canopy thinning and most likely yield loss. The fungal symbionts of these ambrosia beetles were isolated and shown to be new species in the genera Fusarium and Bionectria. When pure cultures of these fungi were injected into the stems of young avocado plants, large brown lesions were produced in the sapwood, confirming that these fungi were causing disease

Function of FXYD Proteins, Regulators of Na, K-ATPase

: In this short review, we summarize our work on the role of members of the FXYD protein family as tissue-specific modulators of Na, K-ATPase. FXYD1 or phospholemman, mainly expressed in heart and skeletal muscle increases the apparent affinity for intracellular Na+ of Na, K-ATPase and may thus be important for appropriate muscle contractility. FXYD2 or γ subunit and FXYD4 or CHIF modulate the apparent affinity for Na+ of Na, K-ATPase in an opposite way, adapted to the physiological needs of Na+ reabsorption in different segments of the renal tubule. FXYD3 expressed in stomach, colon, and numerous tumors also modulates the transport properties of Na, K-ATPase but it has a lower specificity of association than other FXYD proteins and an unusual membrane topology. Finally, FXYD7 is exclusively expressed in the brain and decreases the apparent affinity for extracellular K+, which may be essential for proper neuronal excitabilit

Badnavirus sequences identified from Passiflora spp. in Guadeloupe and Australia [Abstract]

Badnaviruses (family Caulimoviridae, genus Badnavirus) have non-enveloped bacilliform particles containing a single copy of a circular dsDNA genome of c. 7.4R7.6 kbp arranged in at least three conserved open reading frames. Badnaviruses are transmitted in a semi-persistent mode by mealybugs and, for some species, also by aphids, scale insects or lacebugs. Several badnaviruses infect economically important tropical crops such as banana, cocoa, pineapple, sugarcane, taro and yam, and have a serious impact on yields. Over the past 10 years, endogenous viral sequences have been discovered and characterized in the genome of numerous crops, including badnavirus sequences in the genome of banana, yam and pineapple. It is expected that in silico analysis of plant genomic sequence data will reveal more endogenous viral sequences. Passiflora spp. from Guadeloupe and Australia were screened for badnavirus sequences by PCR across the conserved reverse transcriptase/RNaseH region of the ORF 3 polyprotein. PCR was performed on purified total DNA, using two sets of degenerate primers. Badnavirus sequences were amplified from Passiflora edulis f. flavicarpa, a yellow mutant introduced to Australia and Guadeloupe in the beginning of the XXth century. Badnavirus sequences were also amplified from species P. edulis, P. foetida, P. laurifolia, P. maliformis, P. quadrangularis and P. serrato-digitata, which were introduced in Guadeloupe from South and Central America at the turn of the XXth century. On the contrary, no badnavirus sequences could be amplified from Australian native P. aurantia and P. herbertiana. Phylogenetic analyses show that there is no host speciation of badnavirus sequences in the Passiflora spp. used in this work. They also show that P. edulis samples from Guadeloupe and Australia share highly similar sequences. The nature of these sequences (episomal vs integrated) is currently being explored. Although no badnavirus has been reported yet in Passiflora spp., bacilliform particles could be observed in an Australian P. edulis f. flavicarpa plant showing severe fruit puckering and leaf shoestring symptoms. Nevertheless, all other plants used in this study were asymptomatic. (Texte intégral

Endogenous caulimovirid sequences are widespread in plant genomes

Endogenous sequences from members of the Caulimoviridae and Geminiviridae families have been identified in the genome of several plant species. They are thought to result from illegitimate recombination events and are generally replication-defective. However, some caulimovirid sequences of tobacco, petunia and banana are capable of causing infection. We performed in silico analyses on nucleotide sequences from plant genome databases and reconstituted 11 full length and potentially infectious viral genomes from endogenous viral sequences embedded in the genomes of monocotyledonous and dicotyledonous Brassicaceae, Euphorbiacae, Fabacae, Myrtaceae, Poaceae, Rutaceae, Saliaceae and Vitaceae plant species. Sequence comparisons show that the corresponding viruses belong to a new genus in the family Caulimoviridae, tentatively named Dionyvirus. Mapping of endogenous Dionyvirus sequences was achieved in two fully sequenced grape genomes, allowing for the first time the study of the distribution pattern of endogenous viral sequences at the host plant genome scale. Dionyvirus-specific primers were designed and used for a PCR-based large scale screening of plant germplasm. It showed that endogenous Dionyvirus sequences are widespread among plants of temperate, tropical and arctic origins, and belong to distinct viral species. Endogenous Dionyvirus sequences belonging to distinct viral species were also identified in single host plants, showing that endogenization of viral sequences is a common phenomenon in plants. Our work shed new lights on plant/virus molecular interactions. The potential contributions of endogenous viral sequences to normal plant functions and to plant and virus evolution will be discussed. (Texte intégral

The Third Sodium Binding Site of Na,K-ATPase Is Functionally Linked to Acidic pH-Activated Inward Current

Sodium- and potassium-activated adenosine triphosphatases (Na,K-ATPase) is the ubiquitous active transport system that maintains the Na+ and K+ gradients across the plasma membrane by exchanging three intracellular Na+ ions against two extracellular K+ ions. In addition to the two cation binding sites homologous to the calcium site of sarcoplasmic and endoplasmic reticulum calcium ATPase and which are alternatively occupied by Na+ and K+ ions, a third Na+-specific site is located close to transmembrane domains 5, 6 and 9, and mutations close to this site induce marked alterations of the voltage-dependent release of Na+ to the extracellular side. In the absence of extracellular Na+ and K+, Na,K-ATPase carries an acidic pH-activated, ouabain-sensitive "leak” current. We investigated the relationship between the third Na+ binding site and the pH-activated current. The decrease (in E961A, T814A and Y778F mutants) or the increase (in G813A mutant) of the voltage-dependent extracellular Na+ affinity was paralleled by a decrease or an increase in the pH-activated current, respectively. Moreover, replacing E961 with oxygen-containing side chain residues such as glutamine or aspartate had little effect on the voltage-dependent affinity for extracellular Na+ and produced only small effects on the pH-activated current. Our results suggest that extracellular protons and Na+ ions share a high field access channel between the extracellular solution and the third Na+ binding sit

Endogenous Dionyvirus sequences are widespread in plant genomes

Endogenous sequences from members of the Caulimoviridae and Geminiviridae families have been identified in the genome of several plant species [1, 2]. They are thought to result from illegitimate recombination events and are generally replication-defective. However, some caulimovirid sequences of tobacco, petunia and banana are capable of causing infection. We performed in silico analyses on nucleotide sequences from plant genome databases and reconstituted 11 full length and potentially infectious viral genomes from endogenous viral sequences embedded in the genomes of monocotyledonous and dicotyledonous plant species. Sequence comparisons show that the corresponding viruses belong to a new genus in the family Caulimoviridae, tentatively named Dionyvirus. Mapping of endogenous Dionyvirus sequences was achieved in the fully sequenced genomes of grape, poplar, peach and rice, allowing for the first time the study of the distribution pattern of endogenous viral sequences at the host plant genome scale. Using the TEannot pipeline from the REPET package [3], we sensitively detected virus fragments and were able to join them to recover fragmented virus sequences. Full length and partial virus sequences were found in all four genomes, evenly distributed along chromosomes; they appear to be repeated throughout the whole genome. Allelic variations of endogenous Dionyvirus sequences were also characterized in grape, using the complete sequences of a near-homozygous line and a highly heterozygous genotype, PN40024 and Pinot Noir clone ENTAV115 respectively. Dionyvirus-specific primers were designed and used for a PCR-based large scale screening of plant germplasm. It showed that endogenous Dionyvirus sequences belonging to distinct viral species are widespread among plants of temperate, tropical and arctic origins, and that endogenization of viral sequences is therefore a common phenomenon in plants. siRNAs homologous to endogenous Dionyvirus species were identified in grape, peach, soybean and orange, providing evidence that the expression of such endogenous sequences is tightly regulated and/or might trigger RNAi-based antiviral defence. (Texte intégral

A novel family of transmembrane proteins interacting with β subunits of the Na,K-ATPase

We characterized a family consisting of four mammalian proteins of unknown function (NKAIN1, 2, 3 and 4) and a single Drosophila ortholog dNKAIN. Aside from highly conserved transmembrane domains, NKAIN proteins contain no characterized functional domains. Striking amino acid conservation in the first two transmembrane domains suggests that these proteins are likely to function within the membrane bilayer. NKAIN family members are neuronally expressed in multiple regions of the mouse brain, although their expression is not ubiquitous. We demonstrate that mouse NKAIN1 interacts with the β1 subunit of the Na,K-ATPase, whereas Drosophila ortholog dNKAIN interacts with Nrv2.2, a Drosophila homolog of the Na,K-ATPase β subunits. We also show that NKAIN1 can form a complex with another β subunit-binding protein, MONaKA, when binding to the β1 subunit of the Na,K-ATPase. Our results suggest that a complex between mammalian NKAIN1 and MONaKA is required for NKAIN function, which is carried out by a single protein, dNKAIN, in Drosophila. This hypothesis is supported by the fact that dNKAIN, but not NKAIN1, induces voltage-independent amiloride-insensitive Na+-specific conductance that can be blocked by lanthanum. Drosophila mutants with decreased dNKAIN expression due to a P-element insertion in the dNKAIN gene exhibit temperature-sensitive paralysis, a phenotype also caused by mutations in the Na,K-ATPase α subunit and several ion channels. The neuronal expression of NKAIN proteins, their membrane localization and the temperature-sensitive paralysis of NKAIN Drosophila mutants strongly suggest that this novel protein family may be critical for neuronal functio

Les protéines FXYD : nouveaux régulateurs de la Na,K-ATPase

Les protéines FXYD appartiennent à une famille de petites protéines membranaires. Des études récentes suggèrent que six des sept membres de cette famille, FXYD1 (phospholemman), FXYD2 (sous-unité γ de la Na,K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), FXYD5 (Ric) et FXYD7, sont des sous-unités auxiliaires de la Na, K-ATPase régulant son activité de manière tissu et isoforme spécifique. Ces résultats soulignent la complexité de la régulation des ions Na+ et K+ par la Na,K-ATPase qui est nécessaire pour assurer les fonctions propres de différents tissus comme la réabsorption du Na+ par le rein, la contraction musculaire et l’excitabilité neuronale. De plus, une mutation dans FXYD2 a été liée à certains cas d’hypomagnésémie, suggérant que des perturbations de la régulation de la Na,K-ATPase par les protéines FXYD seraient impliquées dans des états physiopathologiques. Une meilleure compréhension de ce nouveau mécanisme de régulation de la Na,K-ATPase pourrait nous aider à mieux comprendre son rôle dans les états physiopathologiques. Dans cet article, nous discutons les données les plus récentes sur le rôle des protéines FXYD dans la modulation de la Na, K-ATPase.Members of the FXYD protein family are small membrane proteins which are characterized by an FXYD motif, two conserved glycines and a serine residue. FXYD proteins show a tissue-specific distribution. Recent evidence suggests that 6 out of 7 FXYD proteins, FXYD1 (phospholemman), FXYD2 (γ subunit of Na,K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), FXYD5 (Ric) and FXYD7 associate with Na,K-ATPase and modulate its transport properties e.g. its Na+ and/or its K+ affinity in a distinct way. These results highlight the complex regulation of Na+ and K+ transport which is necessary to ensure proper tissue functions such as renal Na+-reabsorption, muscle contractility and neuronal excitability. Moreover, mutation of a conserved glycine residue into an arginine residue in FXYD2 has been linked to cases of human hypomagnesemia indicating that dysregulation of Na,K-ATPase by FXYD proteins may be implicated in pathophysiological states. A better characterization of this novel regulatory mechanism of Na,K-ATPase may help to better understand its role in physiological and pathophysiological conditions