Sarah Refi Hind

Plants exposed to volatile signals from herbivore-infested neighbors can activate faster and stronger defenses against subsequent herbivore attack, a phenomenon called defense priming. However, the specific volatile components responsible for activating defense priming remain unclear. Here, we examined the role of green leaf volatiles (GLV) by silencing their biosynthesis using virus-induced gene silencing technique. Exposure to full blend of herbivore-induced plant volatiles (HIPV) primed receiver plants for enhanced production of all 5 groups of HIPV (GLV, monoterpenes, sesquiterpenes, aldoximes, and indole). When GLV production was silenced in emitter plants, receiver plants were no longer primed for terpene production. However, exposure to ( Z )-3-hexenol (Z3HOL) alone primed receiver plants for terpene production. These results suggest that GLV are necessary, and Z3HOL alone is sufficient, to prime terpene production in receiver plants. Consistent with enhanced resistance, Manduca sexta larvae feeding on Z3HOL-or HIPV-primed plants consumed less leaf tissue and exhibited reduced growth compared with controls. Importantly, priming did not impose fitness costs, as Z3HOL-exposed plants showed normal growth but produced more seed capsules and seeds than control plants. Together, these findings suggest that Z3HOL alone is sufficient to prime plants for better defense without compromising their ability to grow and reproduce.

Bacterial spot disease of cucurbits, caused by Xanthomonas cucurbitae , is a major problem in cucurbit‐growing areas worldwide. In the Midwestern region of the United States, pumpkin and squash fields can have greater than 90% infected fruits, leading to high yield losses. While reference genomes are available for this bacterial species, the genetic diversity between different strains and populations is unknown. After performing restriction‐site associated DNA sequencing (RAD‐seq) analysis of X . cucurbitae isolates collected from the Midwestern region, we selected five representative isolates for further characterization, which included whole‐genome sequencing and in vitro enzyme and in planta virulence assays. Our results suggest that minimal genetic diversity exists between these isolates, and that the isolates have differential virulence on different cucurbit host plants. This study contributes to our understanding of X . cucurbitae population dynamics in the Midwestern region and may assist with developing additional management strategies for controlling bacterial spot disease of cucurbits.

The pattern‐triggered immunity (PTI) response is triggered at the plant cell surface by the recognition of microbe‐derived molecules known as microbe‐ or pathogen‐associated molecular patterns or molecules derived from compromised host cells called damage‐associated molecular patterns. Membrane‐localized receptor proteins, known as pattern recognition receptors, are responsible for this recognition. Although much of the machinery of PTI is conserved, natural variation for the PTI response exists within and across species with respect to the components responsible for pattern recognition, activation of the response, and the strength of the response induced. This review describes what is known about this variation. We discuss how variation in the PTI response can be measured and how this knowledge might be utilized in the control of plant disease and in developing plant varieties with enhanced disease resistance.

SUMMARY: Wild relatives of tomato are a valuable source of natural variation in tomato breeding, as many can be hybridized to the cultivated species (Solanum lycopersicum). Several, including Solanum lycopersicoides, have been crossed to S. lycopersicum for the development of ordered introgression lines (ILs), facilitating breeding for desirable traits. Despite the utility of these wild relatives and their associated ILs, few finished genome sequences have been produced to aid genetic and genomic studies. Here we report a chromosome‐scale genome assembly for S. lycopersicoides LA2951, which contains 37 938 predicted protein‐coding genes. With the aid of this genome assembly, we have precisely delimited the boundaries of the S. lycopersicoides introgressions in a set of S. lycopersicum cv. VF36 × LA2951 ILs. We demonstrate the usefulness of the LA2951 genome by identifying several quantitative trait loci for phenolics and carotenoids, including underlying candidate genes, and by investigating the genome organization and immunity‐associated function of the clustered Pto gene family. In addition, syntenic analysis of R2R3MYB genes sheds light on the identity of the Aubergine locus underlying anthocyanin production. The genome sequence and IL map provide valuable resources for studying fruit nutrient/quality traits, pathogen resistance, and environmental stress tolerance. We present a new genome resource for the wild species S. lycopersicoides, which we use to shed light on the Aubergine locus responsible for anthocyanin production. We also provide IL boundary mappings, which facilitated identifying novel carotenoid quantitative trait loci of which one was likely driven by an uncharacterized lycopene β‐cyclase whose function we demonstrate.

Increasingly, new evidence has demonstrated variability in the epitope regions of bacterial flagellin, including in regions harboring the microbe-associated molecular patterns flg22 and flgII-28 that are recognized by the pattern recognition receptors FLS2 and FLS3, respectively. Additionally, because bacterial motility is known to contribute to pathogen virulence and chemotaxis, reductions in or loss of motility can significantly reduce bacterial fitness. In this study, we determined that variations in flg22 and flgII-28 epitopes allow some but not all Xanthomonas spp. to evade both FLS2- and FLS3-mediated oxidative burst responses. We observed variation in the motility for many isolates, regardless of their flagellin sequence. Instead, we determined that past growth conditions may have a significant impact on the motility status of isolates, because we could minimize this variability by inducing motility using chemoattractant assays. Additionally, motility could be significantly suppressed under nutrient-limited conditions, and bacteria could “remember” its prior motility status after storage at ultracold temperatures. Finally, we observed larger bacterial populations of strains with flagellin variants predicted not to be recognized by either FLS2 or FLS3, suggesting that these bacteria can evade flagellin recognition in tomato plants. Although some flagellin variants may impart altered motility and differential recognition by the host immune system, external growth parameters and gene expression regulation appear to have more significant impacts on the motility phenotypes for these Xanthomonas spp. [ ] Copyright © 2022 The Author(s). This is an open access article distributed under the CC BY 4.0 International license .

Copper (Cu) products, including copper oxide nanoparticles (nCuO), are critically important agricultural fungicides and algaecides. Foliar application onto crops and subsequent aerosol drift of these Cu products, especially nCuO, on to soil may alter nutrient cycling and microbial communities in both managed and unmanaged environments. We measured the influence of land use on soil microbial biomass and respiration in response to the addition of nCuO to an alluvial soil. Different land uses included grassland, forest and both organic and conventional managed row crops. Soil samples were amended with 1000 mg Cu per kg soil as CuCl₂, 16 nm CuO (16nCuO), 42 nm CuO (42nCuO), and larger than nanoparticle sized bulk CuO (bCuO). Copper availability immediately increased in all soils following Cu addition in the order of CuCl₂ > 16nCuO > 42nCuO > bCuO. After 70 days Cu availability was diminished across land uses and lowest in soils treated with bCuO. Using X-ray absorption near edge structure (XANES) spectroscopy, we determined that the relatively high availability of Cu after treatment with nanoparticle sized CuO was due to the dissolution of CuO particles and subsequent adsorption by soil materials. Respiration, an indicator of microbial activity, was suppressed by Cu additions, especially CuCl₂. Copper effects on soil microbial biomass were sensitive to land use. In agricultural soils, microbial biomass was unaltered by Cu form, regardless of concentration, whereas in unmanaged soils, it decreased following exposure to CuCl₂ and 42nCuO. Our results suggest that land use history has little impact on Cu chemical fate in soils, but strongly modulates microbial response to Cu exposure. These results are especially important for organic agricultural systems where copper fungicides are widely used but may suppress microbial mineralization of nutrients from soil organic matter.

Bacterial spot of cucurbits, caused by Xanthomonas cucurbitae , is an emerging disease of cucurbits. This study was conducted to identify Cucurbita species that are resistant to X . cucurbitae . We developed a reliable method for inoculating cucurbit plants by spraying plants with X . cucurbitae suspensions containing carborundum. Symptoms of the disease developed within 7 days after inoculation. Subsequently, we evaluated the virulence of six isolates of X . cucurbitae in the greenhouse and observed that X . cucurbitae isolated from cucurbit fields in Illinois, Michigan, Kansas, Ohio, and Wisconsin were more virulent than the reference ATCC 23378 strain. Then, we screened 81 commercial cultivars of gourds, pumpkins, and squashes, as well as 300 Cucurbita spp. accessions, for their resistance to X . cucurbitae under greenhouse and field conditions. In the greenhouse study, all commercial cultivars and some of the accessions developed typical symptoms of bacterial spot disease, while some of the accessions developed fewer lesions. In the field studies, infection of leaves and fruits was caused by both natural inoculum and spray‐inoculation of plants with a suspension of the X . cucurbitae isolate from Illinois. Among 300 accessions tested, 9 and 21 accessions were classified as resistant and less resistant, respectively. Resistant and less resistant accessions belong to the species Cucurbita maxima , C . maxima subsp. maxima , C . maxima subsp. andreana , and C . okeechobeensis subsp. martinezii . This is the first report of potential resistance to bacterial spot of cucurbits.

Bacterial leaf spot disease caused by Xanthomonas cucurbitae has severely affected the pumpkin industries in the Midwestern region of United States, with the bacteria mainly infecting pumpkin leaves and fruits, and leading to significant yield losses. In this study, we utilized genomics and genetics approaches to elucidate X. cucurbitae molecular mechanisms of pathogenesis during interaction with its host. We generated the first reference-quality whole-genome sequence of the X. cucurbitae type isolate and compared with other Xanthomonas species, X. cucurbitae has a smaller genome size with fewer virulence-related genes. RNA-seq analysis of X. cucurbitae under plant-mimicking media conditions showed altered transcriptional responses, with upregulation of virulence genes and downregulation of cellular homeostasis genes. Additionally, characterization of key virulence genes using gene deletion methods revealed that both type II enzymes and type III effectors are necessary for X. cucurbitae to cause infection in the pumpkin host.

This study was conducted to evaluate the effectiveness of crop rotation in managing Xanthomonas cucurbitae, the causal agent of Xanthomonas leaf spot, in pumpkin fields. The study was conducted using ‘Howden’ pumpkin, a susceptible pumpkin cultivar, at four sites with no history of cucurbit production. All sites were planted with pumpkins in the first year of crop rotations. The disease developed at all four sites. In a 3-year rotation with nonhost crops, the severity of Xanthomonas leaf spot in pumpkin plots was significantly (P < 0.05) lower than those of plots in the 2-year rotation with nonhost crops and continuous pumpkin production. Furthermore, X. cucurbitae was recovered from pumpkin plant debris 15 months after harvesting pumpkins in commercial fields, and the pathogen was viable. X. cucurbitae was also isolated from asymptomatic weeds in pumpkin fields. In a greenhouse inoculation of weeds with X. cucurbitae, however, Xanthomonas leaf spot developed only on leaves of burcucumber (Sicyos angulatus) and velvetleaf (Abutilon theophrasti) weeds. In the infested soil with X. cucurbitae, the bacterium was not recovered after 15, 42, and 56 days at 22, 12, and 4°C, respectively. The results showed that crop rotation alone does not provide adequate protection of pumpkin plants against X. cucurbitae.