Root-Associated Microbiomes, Growth and Health of Ornamental Geophytes Treated with Commercial Plant Growth-Promoting Products.

The microbial community inhabiting a plant's root zone plays a crucial role in plant health and protection. To assess the ability of commercial plant growth-promoting products to enhance the positive effects of this environment, two products containing beneficial soil bacteria and a product containing plant extracts were tested on Zantedeschia aethiopica and Ornithogalum dubium. The products were tested in two different growing media: a soil and a soilless medium. The effects of these products on Pectobacterium brasiliense, the causal agent of soft rot disease, were also evaluated in vitro, and on naturally occurring infections in the greenhouse. The growing medium was found to have the strongest effect on the microbial diversity of the root-associated microbiome, with the next-strongest effect due to plant type. These results demonstrate that either a single bacterial strain or a product will scarcely reach the level that is required to influence soil microbial communities. In addition, the microbes cultured from these products, could not directly inhibit Pectobacterium growth in vitro. We suggest density-based and functional analyses in the future, to study the specific interactions between plants, soil type, soil microbiota and relevant pathogens. This should increase the effectiveness of bio-supplements and soil disinfestation with natural products, leading to more sustainable, environmentally friendly solutions for the control of bacterial plant diseases.

Ecological adaptations influence the susceptibility of plants in the genus Zantedeschia to soft rot Pectobacterium spp.

Soft rot disease caused by Pectobacterium spp. is responsible for severe agricultural losses in potato, vegetables, and ornamentals. The genus Zantedeschia includes two botanical groups of tuberous ornamental flowers that are highly susceptible to the disease. Previous studies revealed that Z. aethiopica, a member of the section Zantedeschia, is significantly more resistant to Pectobacterium spp. than members of the same genus that belong to the section Aestivae. During early infection, we found different patterns of bacterial colonization on leaves of hosts belonging to the different sections. Similar patterns of bacterial colonization were observed on polydimethylsiloxane (PDMS) artificial inert replicas of leaf surfaces. The replicas confirmed the physical effect of leaf texture, in addition to a biochemical plant-bacterium interaction. The differential patterns may be associated with the greater roughness of the abaxial leaf surfaces of Aestivae group that have evolutionarily adapted to mountainous environments, as compared to Zantedeschia group species that have adapted to warm, marshy environments. Transverse leaf sections also revealed compact aerenchyma and reduced the total volume of leaf tissue air spaces in Aestivae members. Finally, an analysis of defense marker genes revealed differential expression patterns in response to infection, with significantly higher levels of lipoxygenase 2 (lox2) and phenylalanine ammonia lyase (pal) observed in the more resistant Z. aethiopica, suggesting greater activation of induced systemic resistance (ISR) mechanisms in this group. The use of Zantedeschia as a model plant sheds light on how natural ecological adaptations may underlay resistance to bacterial soft rot in cultivated agricultural environments.

Host Specificity and Differential Pathogenicity of Pectobacterium Strains from Dicot and Monocot Hosts.

Recent phylogenetic studies have transferred certain isolates from monocot plants previously included in the heterogeneous group of Pectobacteriumcarotovorum (Pc) to a species level termed Pectobacterium aroidearum. The specificity of Pectobacterium associated infections had received less attention, and may be of high scientific and economic importance. Here, we have characterized differential responses of Pectobacterium isolates from potato (WPP14) and calla lily (PC16) on two typical hosts: Brassica oleracea var. capitata (cabbage) a dicot host; and Zantedeschia aethiopica (calla lily) a monocot host. The results revealed clear host specific responses following infection with the two bacterial strains. This was demonstrated by differential production of volatile organic compounds (VOCs) and the expression of plant defense-related genes (pal, PR-1, lox2, ast). A related pattern was observed in bacterial responses to each of the host's extract, with differential expression of virulence-related determinants and genes associated with quorum-sensing and plant cell wall-degrading enzymes. The differences were associated with each strain's competence on its respective host.

Transcriptome Profiling of Ornithogalum dubium Leaves and Flowers to Identify Key Carotenoid Genes for CRISPR Gene Editing.

Ornithogalum dubium is a popular ornamental monocot native to South Africa with flower colors ranging from pure white to deep orange. Gene editing based on the CRISPR/Cas9 system has recently been shown to hold potential for color improvement in ornamental flower crops. To apply this approach to Ornithogalum color manipulation, genomic or transcriptomic data must first be collected. Here, cDNA libraries of O. dubium leaves and flowers were constructed and sequenced using the Illumina HiSeq 2500. Over 155 million 100-bp paired-end reads were assembled into a transcriptome database of 360,689 contigs, of which 18,660 contigs were differentially expressed between leaves and flowers. Carotenoids are the main pigment imparting spectrum of orange hues to O. dubium flowers. By querying our database, we identified a total of 16 unique transcripts (unigenes) predicted to be involved in the carotenoid biosynthesis pathway of Ornithogalum. Combining carotenoid profiles, we further inferred several key unigenes responsible for floral coloration and accumulation in O. dubium, of which the gene LCYB/comp146645_c0 was found as a suitable target to generate potentially red flower varieties of O. dubium. Our research thus provides a framework for the application of CRISPR/Cas9 technology to improve this ornamental crop.

SELENOPROTEIN O is a chloroplast protein involved in ROS scavenging and its absence increases dehydration tolerance in Arabidopsis thaliana.

The evolutionary conserved family of Selenoproteins performs redox-regulatory functions in bacteria, archaea and eukaryotes. Among them, members of the SELENOPROTEIN O (SELO) subfamily are located in mammalian and yeast mitochondria, but their functions are thus far enigmatic. Screening of T-DNA knockout mutants for resistance to the proline analogue thioproline (T4C), identified mutant alleles of the plant SELO homologue in Arabidopsis thaliana. Absence of SELO resulted in a stress-induced transcriptional activation instead of silencing of mitochondrial proline dehydrogenase, and also high elevation of Δ(1)-pyrroline-5-carboxylate dehydrogenase involved in degradation of proline, thereby alleviating T4C inhibition and lessening drought-induced proline accumulation. Unlike its animal homologues, SELO was localized to chloroplasts of plants ectopically expressing SELO-GFP. The protein was co-fractionated with thylakoid membrane complexes, and co-immunoprecipitated with FNR, PGRL1 and STN7, all involved in regulating PSI and downstream electron flow. The selo mutants displayed extended survival under dehydration, accompanied by longer photosynthetic activity, compared with wild-type plants. Enhanced expression of genes encoding ROS scavenging enzymes in the unstressed selo mutant correlated with higher oxidant scavenging capacity and reduced methyl viologen damage. The study elucidates SELO as a PSI-related component involved in regulating ROS levels and stress responses.