Variability in phosphite sensitivity observed within and between seven Phytophthora species

Phosphite is used to control and manage many phytophthora diseases in horticultural systems worldwide and natural ecosystems in Australia, Africa, New Zealand and parts of Northern America and Europe. Phosphite does not kill Phytophthora species, but inhibits growth while also stimulating host defence responses. Phytophthora species differ in their underlying tolerance to phosphite and isolates have been shown to acquire tolerance after prolonged exposure. Intra- and inter-specific variability in phosphite sensitivity is of interest to determine the efficacy and sustainability of phosphite for the treatment of phytophthora diseases, which continue to spread globally. Seven Phytophthora species were tested for their sensitivity to phosphite in vitro in a mycelial growth experiment. Phytophthora agathidicida was the species most sensitive to phosphite, being inhibited by 98.7% on average at the lowest phosphite treatment (15 µg/mL phosphite), followed by P. aleatoria, P. cinnamomi, P. pluvialis, P. multivora, P. kernoviae and P. citricola. Huge intraspecific variability was observed with P. kernoviae, which raises the question of whether diseases caused by P. kernoviae such as phytophthora needle blight of Pinus radiata could be managed effectively with phosphite. Further work is required to determine the phosphite sensitivity of different introduced and native Phytophthora species growing in key hosts and whether tolerance observed in vitro is also expressed in vivo.

Defence Responses Associated with Elicitor-Induced, Cultivar-Associated Resistance to Latania Scale in Kiwifruit

Latania scale insect is a pest of global significance affecting kiwifruit. The sessile insect (life stage: settled crawler—mature adult) is covered with a waxy cap that protects it from topical pesticides, so increasingly, a selection of resistant cultivars and application of elicitors are being used in pest control. Thus far, the application of a salicylic acid (SA) phytohormone pathway elicitor, acibenzolar-S-methyl (ASM), has been shown to reduce insect development (as indicated by cap size) on one kiwifruit cultivar (‘Hayward’). To investigate how cultivar-associated resistance is affected by the ability to respond to different elicitors, we measured phytohormones (by LCMS) and gene expression (by qPCR and NanoString) on latania scale-tolerant ‘Hort16A’ and susceptible ‘Hayward’ kiwifruit over two seasons. Potted plants in the presence/absence of settled latania scales were treated with ASM (0.2 g/L) or methyl jasmonate (MeJA, 0.05% v/v), representing elicitors of the SA and JA signalling pathways, respectively. ‘Hort16A’ cultivar resistance to latania scale was associated with elevated expression of SA and SA-related defence genes (PR1 and two PR2 family genes) in the ASM treatment. MeJA treatments did not significantly affect insect development in ‘Hayward’ (latania scale did not survive on ‘Hort16A’) and did not correlate with phytohormone and gene expression measurements in either cultivar. ‘Hayward’ had greater concentrations than ‘Hort16A’ of inert storage forms of both SA and JA across all treatments. This information contributes to the selection of tolerant cultivars and the effective use of elicitors for control of latania scale in kiwifruit.

Dissecting the membrane-microtubule sensor in grapevine defence

Specific populations of plant microtubules cooperate with the plasma membrane to sense and process abiotic stress signals, such as cold stress. The current study derived from the question, to what extent this perception system is active in biotic stress signalling. The experimental system consisted of grapevine cell lines, where microtubules or actin filaments are visualised by GFP, such that their response became visible in vivo. We used the bacterial elicitors harpin (inducing cell-death related defence), or flg22 (inducing basal immunity) in combination with modulators of membrane fluidity, or microtubules. We show that DMSO, a membrane rigidifier, can cause microtubule bundling and trigger defence responses, including activation of phytoalexin transcripts. However, DMSO inhibited the gene expression in response to harpin, while promoting the gene expression in response to flg22. Treatment with DMSO also rendered microtubules more persistent to harpin. Paradoxically, Benzylalcohol (BA), a membrane fluidiser, acted in the same way as DMSO. Neither GdCl3, nor diphenylene iodonium were able to block the inhibitory effect of membrane rigidification on harpin-induced gene expression. Treatment with taxol stabilised microtubule against harpin but amplified the response of PAL transcripts. Therefore, the data support implications of a model that deploys specific responses to pathogen-derived signals.

Palm leaf catechins improved insulin-related pathways in diabetic rats

Catechin-rich oil-palm leaf extract (OPLE) (Elaeis guineensis) was previously demonstrated to possess benefits for diabetes and cardio metabolic health (vasodilation, antioxidant, cardiovascular, anti-hypertensive, anti-inflammatory, hepatoprotective and nephroprotective properties) in animal models. For insights into OPLE anti-diabetic mode-of-action and possible toxicity, the effects of dietary OPLE on insulin-signaling pathways mRNA expressions in the liver, kidney, pancreas, and spleen of normal and diabetic rats were examined. Type-2-Diabetes Mellitus (T2DM) were induced by chronic high-fat diet and streptozotocin (35 mg/kg) intraperitoneal injection. The OPLE (100 mg/kg body weight) were fed daily to normal and T2DM-induced rats. The OPLE suppressed hyperglycaemia and excessive weight gain in the T2DM rats, and appeared harmless to normal rats. The OPLE supplementation significantly (p<0.05) modulated the mRNA expressions of phosphatidylinositol-3 kinase (PIK3R1); insulin signaling receptor (INSR); insulin-receptor substrates 1 and 2; and ectonucleotide pyrophosphatase-1 (ENPP1) especially in the livers of normal rats and the spleen of diabetic rats. Results suggested the OPLE probably help prevent diabetes in healthy mammals and ameliorate the immune functions of diabetic mammals. The OPLE improved the antioxidant defence responses, insulin-pathways mRNA expressions in the normal and diabetic rats; suppressed hyperglycaemia and excessive weight gain in T2DM rodents without observable liver or kidney toxicity at the dose used.

Plasma-Activated Water Triggers Rapid and Sustained Cytosolic Ca2+ Elevations in Arabidopsis thaliana

Increasing evidence indicates that water activated by plasma discharge, termed as plasma-activated water (PAW), can promote plant growth and enhance plant defence responses. Nevertheless, the signalling pathways activated in plants in response to PAW are still largely unknown. In this work, we analysed the potential involvement of calcium as an intracellular messenger in the transduction of PAW by plants. To this aim, Arabidopsis thaliana (Arabidopsis) seedlings stably expressing the bioluminescent Ca2+ reporter aequorin in the cytosol were challenged with PAW generated by a plasma torch. Ca2+ measurement assays demonstrated the induction by PAW of rapid and sustained cytosolic Ca2+ elevations in Arabidopsis seedlings. The dynamics of the recorded Ca2+ signals were found to depend upon different parameters, such as the operational conditions of the torch, PAW storage, and dilution. The separate administration of nitrate, nitrite, and hydrogen peroxide at the same doses as those measured in the PAW did not trigger any detectable Ca2+ changes, suggesting that the unique mixture of different reactive chemical species contained in the PAW is responsible for the specific Ca2+ signatures. Unveiling the signalling mechanisms underlying plant perception of PAW may allow to finely tune its generation for applications in agriculture, with potential advantages in the perspective of a more sustainable agriculture.

Effects of Phytophthora Inoculations on Photosynthetic Behaviour and Induced Defence Responses of Plant Volatiles in Field-Grown Hybrid Poplar Tolerant to Bark Canker Disease

Bark cankers accompanied by symptoms of decline and dieback are the result of a destructive disease caused by Phytophthora infections in woody plants. Pathogenicity, gas exchange, chlorophyll a fluorescence, and volatile responses to P. cactorum and P. plurivora inoculations were studied in field-grown 10-year-old hybrid poplar plants. The most stressful effects of P. cactorum on photosynthetic behaviour were found at days 30 and 38 post-inoculation (p.-i.), whereas major disturbances induced by P. plurivora were identified at day 30 p.-i. and also belatedly at day 52 p.-i. The spectrum of volatile organic compounds emitted at day 98 p.-i. was richer than that at day 9 p.-i, and the emissions of both sesquiterpenes α-cubebene and germacrene D were induced solely by the Phytophthora inoculations. Significant positive relationships were found between both the axial and the tangential development of bark cankers and the emissions of α-cubebene and β-caryophyllene, respectively. These results show that both α-cubebene and germacrene D are signal molecules for the suppression of Phytophthora hyphae spread from necrotic sites of the bark to healthy living tissues. Four years following inoculations, for the majority of the inoculated plants, the callus tissue had already closed over the bark cankers.

A glutamate synthase mutant of Bradyrhizobium sp. strain ORS285 is unable to induce nodules on Nod factor-independent Aeschynomene species

The Bradyrhizobium sp. strain ORS285 is able to establish a nitrogen-fixing symbiosis with both Nod factor (NF) dependent and NF-independent Aeschynomene species. Here, we have studied the growth characteristics and symbiotic interaction of a glutamate synthase (GOGAT; gltD::Tn5) mutant of Bradyrhizobium ORS285. We show that the ORS285 gltD::Tn5 mutant is unable to use ammonium, nitrate and many amino acids as nitrogen source for growth and is unable to fix nitrogen under free-living conditions. Moreover, on several nitrogen sources, the growth rate of the gltB::Tn5 mutant was faster and/or the production of the carotenoid spirilloxanthin was much higher as compared to the wild-type strain. The absence of GOGAT activity has a drastic impact on the symbiotic interaction with NF-independent Aeschynomene species. With these species, inoculation with the ORS285 gltD::Tn5 mutant does not result in the formation of nodules. In contrast, the ORS285 gltD::Tn5 mutant is capable to induce nodules on NF-dependent Aeschynomene species, but these nodules were ineffective for nitrogen fixation. Interestingly, in NF-dependent and NF-independent Aeschynomene species inoculation with the ORS285 gltD::Tn5 mutant results in browning of the plant tissue at the site of the infection suggesting that the mutant bacteria induce plant defence responses.