Anish Shah

The variability of the rhizosphere microbial community has not been well-studied in avocado plantations at the field-scale. This research aimed to determine if the bacterial and fungal communities in the rhizosphere and/or the soil physicochemical properties from two commercial avocado orchards varied with different sampling designs (grid-based, longitudinal transect, and zigzag transect), and in turn if this changed the soil physicochemical properties driving the composition of the microbial communities. There were no differences in alpha diversity of bacteria or fungi based on sampling design in either orchard, and bacterial and fungal alpha diversity showed no evidence of spatial autocorrelation. Bacterial community composition in Orchard 1 varied with sampling design, whereas no differences were observed for bacterial community composition in Orchard 2 or for fungal community composition in either orchard. In each orchard, at least 50 % of the most abundant bacterial taxa were common between the sampling designs, however, less than 40 % the most abundant fungal taxa were common between the sampling designs. Canonical correspondence analysis indicated that the edaphic drivers of bacterial and fungal communities in Orchard 1 differed based on the sampling design. These results highlight the importance of field-scale sampling design for accurately characterising avocado rhizosphere bacterial and fungal communities particularly when such data will inform orchard management decisions. Soil sampling using a random, grid-based design is recommended as a simple and reliable method for monoculture fruit tree orchards.

Roots of tomato plants (Solanum lycopersicum) are vulnerable to soil-borne pathogenic fungi, bacteria and nematodes. Current control methods for these biotic stressors have limitations, necessitating the need for new eco-friendly alternatives. The rhizosphere microbiome is an effective natural barrier to invasion by soil-borne pathogens, and there is scope to harness this inherent capacity to improve the management of pathogens. This review examined molecular analyses of the taxonomic composition, abundance and function of the rhizosphere microbiome in healthy and diseased field-grown tomato plants for evidence of the role of the microbiome in disease suppression. The role of biological products in manipulating the rhizosphere microbiome to suppress soil-borne pathogens in field-grown tomato crops was also analysed. We discuss likely mechanisms underpinning microbiome-mediated tolerance to biotic stress in tomato crops and highlight research gaps to be considered in future investigations. Identifying functionally beneficial rhizosphere microbiota in healthy tomato crops may provide new insights into understanding plant–pathogen interactions and allow new strategies for exploring disease control.

Introduction: Through the combined use of two nitrification inhibitors, Dicyandiamide (DCD) and chlorate with nitrogen amendment, this study aimed to investigate the contribution of comammox Nitrospira clade B, ammonia oxidizing bacteria (AOB) and archaea (AOA) to nitrification in a high fertility grassland soil, in a 90-day incubation study.

Methods: The soil was treated with nitrogen (N) at three levels: 0 mg-N kg-1 soil, 50 mg-N kg-1 soil, and 700 mg-N kg-1 soil, with or without the two nitrification inhibitors. The abundance of comammox Nitrospira, AOA, AOB, and nitrite oxidising bacteria (NOB) was measured using qPCR. The comammox Nitrospira community structure was assessed using Illumina sequencing.

Results and Discussion: The results showed that the application of chlorate inhibited the oxidation of both NH4+ and NO2- in all three nitrogen treatments. The application of chlorate significantly reduced the abundance of comammox Nitrospira amoA and nxrB genes across the 90-day experimental period. Chlorate also had a significant effect on the beta diversity (Bray-Curtis dissimilarity) of the comammox Nitrospira clade B community. Whilst AOB grew in response to the N substrate additions and were inhibited by both inhibitors, AOA showed little or no response to either the N substrate or inhibitor treatments. In contrast, comammox Nitrospira clade B were inhibited by the high ammonium concentrations released from the urine substrates. These results demonstrate the differential and niche responses of the three ammonia oxidising communities to N substrate additions and nitrification inhibitor treatments. Further research is needed to investigate the specificity of the two inhibitors on the different ammonia oxidising communities.
(Display omitted)

The objectives of this study were to investigate the abundance and community composition of comammox Nitrospira under: (i) pasture-based dairy farms from different regions, and (ii) different land uses from the same region and soil type. The results clearly showed that comammox Nitrospira were most abundant (3.0 × 106 copies) under the west coast dairy farm conditions, where they were also significantly more abundant than canonical ammonia oxidisers. This was also true in the Canterbury dairy farm. The six land uses investigated were pine monoculture, a long term no input ecological trial, sheep + beef and Dairy, both irrigated and non-irrigated. It was concluded that comammox Nitrospira was most abundant under the irrigated dairy farm (2.7 × 106 copies). Contrary to the current industry opinion, the relatively high abundance of comammox Nitrospira under fertile irrigated dairy land suggests that comammox Nitrospira found in terrestrial ecosystems may be copiotrophic. it was also determined that comammox Nitrospira was more abundant under irrigated land use than their non-irrigated counterparts, suggesting that soil moisture is a key environmental parameter influencing comammox abundance. Comammox abundance was also positively correlated with annual rainfall, further supporting this theory. Phylogenetic analysis of the comammox Nitrospira detected determined that 17 % of the comammox community belonged to a newly distinguished subclade, clade B.2. The remaining 83 % belonged to clade B.1. No sequences from clade A were found.

The purpose of developing this high throughput assay was to determine whether there was evidence of pH adaptation in strains of rhizobia which nodulate subterranean clover (SC) and white clover (WC), and whether this was related to the pH of the soil of origin. pH is a first-order factor influencing the niche preferences of soil microorganisms and has been convincingly shown to be a key driver of soil bacterial communities. Naturalised strains of Rhizobium spp. that are pH-adapted may have the potential to better compete and/or persist in acidic or alkaline soils compared with introduced commercial strains. Three pilot studies were conducted to design the optimised bioassay. This bioassay tested the effect of pH-amended yeast mannitol broth (seven pH values from pH 4.5–9.0), across three time points, on the in vitro growth of 299 Rhizobium strains isolated from the nodules of SC and WC. The media pH where strains demonstrated fastest growth was related to the pH of the soil that strains were isolated from. However, the correlation between media pH and soil pH was strongly influenced by the growth of strains from alkaline soils (alkaline adaptation), especially in strains isolated from SC nodules.
•Development of a cheap, effective and highly replicable bioassay.•Growth of 299 Rhizobium strains measured in 7 pH broths over 48 h.•Strains tended toward alkaline adaptation, especially those from sub. clover.•Potential to develop some strains into new clover inoculants.

Grapevine trunk diseases (GTDs) are a threat to grape production worldwide, with a diverse collection of fungal species implicated in disease onset. Due to the long-term and complex nature of GTDs, simultaneous detection of multiple microbial species can enhance understanding of disease development. We used DNA metabarcoding of ribosomal internal transcribed spacer 1 (ITS1) sequences, supported by specific PCR and microbial isolation, to establish the presence of trunk pathogens across 11 vineyards (11-26 years old) over three years in Marlborough, the largest wine producing region in New Zealand. Using a reference database of trunk pathogen sequences, species previously associated with GTD, such as Cadophora luteo-olivacea, Diplodia seriata, Diplodia mutila, Neofusicoccum australe, and Seimatosporium vitis, were identified as highly represented across the vineyard region. The well-known pathogens Phaeomoniella chlamydospora and Eutypa lata had especially high relative abundance across the dataset, with P. chlamydospora reads present between 22 and 84% (average 52%) across the vineyards. Screening of sequences against broader, publicly available databases revealed further fungal species within families and orders known to contain pathogens, many of which appeared to be endemic to New Zealand. The presence of several wood-rotting basidiomycetes (mostly Hymenochaetales) was detected for the first time in the Marlborough vineyard region, notably, the native Inonotus nothofagii which was present at 1-2% relative abundance in two vineyards.

Grapevine trunk diseases (GTDs) are a significant problem for New Zealand viticulture. Endophytic actinobacteria are of interest as potential biocontrol agents due to their ability to inhibit plant pathogens and improve plant growth. However, no studies have investigated the diversity of actinobacteria associated with grapevines in New Zealand vineyards and their bioactivity. Actinobacteria diversity in different ‘Sauvignon blanc’ vine tissues from three vineyards (conventional and organic management, and different vine ages) was assessed using different methods and media. Forty-six endophytic actinobacteria were isolated, with more isolates recovered from roots (n = 45) than leaves (n = 1) and shoot internodes (n = 0). More isolates were recovered from the organic (n = 21) than conventional (n = 8) vineyard, mature (25-year old; n = 21) than young (2-year old; n = 2) vines and using a tissue maceration technique (n = 40). Actinomycete Isolation Agar, International Streptomyces Project 2, and Starch Casein media were effective for actinobacteria isolation. Most of the isolates recovered belonged to Streptomyces, with one isolate identified as Mycolicibacterium. Forty isolates were assessed for antifungal activity and plant growth-promoting (PGP) characteristics. Of these, 13 isolates had antifungal activity against test GTD pathogens (Dactylonectria macrodidyma, Eutypa lata, Ilyonectria liriodendri, Neofusicoccum parvum, and N. luteum). Eighteen isolates exhibited more than one PGP trait; 25siderophore production (n = 25), phosphate solubilization (n = 6), and indole acetic acid production (n = 16). Two strains, Streptomyces sp. LUVPK-22 and Streptomyces sp. LUVPK-30, exhibited the best antifungal and PGP properties. This study revealed the diversity of culturable endophytic actinobacteria from grapevines in New Zealand vineyards and their biocontrol potential against GTD pathogens.

Aims
The influence of soil edaphic factors on recruitment and composition of bacteria in the legume nodule is unknown. Typically, low (acidic) pH soils have a negative effect on the plant-rhizobia symbiosis and thereby reduce clover growth. However, the specific relationship between soil pH and the ecology of rhizobia is unknown, in either their free-living or nodule-inhabiting states. We used New Zealand pasture systems with soils of different pH, and white (WC) and subterranean (SC) clovers, to examine the relationship between soil pH and the diversity of bacteria that inhabit the nodules.

Methods and Results
Amplicon sequencing (16S rRNA) assessed the bacterial community in 5299 nodules recovered from both legume species grown in 47 soils of different edaphic (including pH) properties. Fewer nodules were formed on both clovers at low soil pH. As expected, rhizobia comprised ∼92% of the total reads in both clovers, however 28 non-rhizobia genera were also present. Soil pH influenced the community structure of bacteria within the nodule, and this was more evident in non-Rhizobium taxa than Rhizobium. Host strongly influenced the diversity of bacteria in the nodules. The alpha diversity of nodule microbiome in SC nodules was higher than in WC nodules and SC nodules also harbored a higher relative abundance of non-Rhizobium bacteria than WC. Beta diversity of Rhizobium and non-Rhizobium bacteria was influenced more by clover species rather than edaphic factors.

Conclusions
The results indicate that these clover species modified their nodule biomes in response to pH-stress.

Significance and Impact of the Study
The non-Rhizobium bacteria may have some functional significance (such as improved clover persistence in low pH soils) in legume nodules.

For successful molecular breeding it is important to identify targets to the gene family level, and in the specific species of interest, in this case Pisum sativum L. The cytokinins have been identified as a key breeding target due to their influence on plant architecture, and on seed size and sink activity. We focused on the cytokinin biosynthetic gene family (the IPTs) and the gene family key to the destruction of cytokinins (the CKXs), as well as other gene families potentially affected by changing cytokinin levels. These included key meristem genes (WUS and BAM1) and the transporter gene families, sucrose transporters (SUTs) and amino acid permeases (AAPs). We used reverse transcription quantitative PCR (RT-qPCR) to monitor gene expression in the vegetative meristem and in pre- and post-fertilisation young pea fruits. PsWUS expression was specific to the shoot apical meristem while PsBAM1 was highly expressed in the shoot apical meristem (SAM) but was also expressed at a low level in the young fruit. Differential expression was shown between genes and within gene families for IPT, CKX, SUT, and AAP. PsCKX7 showed strong gene family member-specific expression in the SAM, and was also expressed in young pea fruits. We suggest that PsCKX7 is a potential target for downregulation via molecular breeding or gene editing.