Transcriptomic changes during the establishment of long-term methyl jasmonate-induced resistance in Norway spruce

Tree Longevity: Multifaceted Genetic Strategies and Beyond

Old trees are remarkable for their ability to endure for centuries or even millennia, acting as recordkeepers of historical climate and custodians of genetic diversity. The secret to their longevity has long been a subject of fascination. Despite the challenges associated with studying old trees, such as massive size, slow growth rate, long lifespan and often remote habitat, accumulating studies have investigated the mechanisms underlying tree aging and longevity over the past decade. The recent publication of high-quality genomes of long-lived tree species, coupled with research on stem cell function and secondary metabolites in longevity, has brought us closer to unlocking the secrets of arboreal longevity. This review provides an overview of the global distribution of old trees and examines the environmental and anthropogenic factors that shape their presence. We summarize the contributions of physiological characteristics, stem cell activity, and immune system responses to their extraordinary longevity. We also explore the genetic and epigenetic 'longevity code', which consists of resistance and defense genes, DNA repair genes and patterns of DNA methylation modification. Further, we highlight key areas for future research that could enhance our understanding of the mechanisms underlying tree longevity.

Improved pokeweed genome assembly and early gene expression changes in response to jasmonic acid

BACKGROUND

Jasmonic acid (JA) is a phytohormone involved in regulating responses to biotic and abiotic stress. Although the JA pathway is well characterized in model plants such as Arabidopsis thaliana, less is known about many non-model plants. Phytolacca americana (pokeweed) is native to eastern North Americana and is resilient to environmental stress. The goal of this study was to produce a publicly available pokeweed genome assembly and annotations and use this resource to determine how early response to JA changes gene expression, with particular focus on genes involved in defense.

RESULTS

We assembled the pokeweed genome de novo from approximately 30 Gb of PacBio Hifi long reads and achieved an NG50 of ~ 13.2 Mb and a minimum 93.9% complete BUSCO score for gene annotations. With this reference, we investigated the early changes in pokeweed gene expression following JA treatment. Approximately 5,100 genes were differentially expressed during the 0-6 h time course with almost equal number of genes with increased and decreased transcript levels. Cluster and gene ontology analyses indicated the downregulation of genes associated with photosynthesis and upregulation of genes involved in hormone signaling and defense. We identified orthologues of key transcription factors and constructed the first JA gene response network integrated with our transcriptomic data from orthologues of Arabidopsis genes. We discovered that pokeweed did not use leaf senescence as a means of reallocating resources during stress; rather, most secondary metabolite synthesis genes were constitutively expressed, suggesting that pokeweed directs its resources for survival over the long term. In addition, pokeweed synthesizes several RNA N-glycosylases hypothesized to function in defense, each with unique expression profiles in response to JA.

CONCLUSIONS

Our investigation of the early response of pokeweed to JA illustrates patterns of gene expression involved in defence and stress tolerance. Pokeweed provides insight into the defense mechanisms of plants beyond those observed in research models and crops, and further study may yield novel approaches to improving the resilience of plants to environmental changes. Our assembled pokeweed genome is the first within the taxonomic family Phytolaccaceae to be publicly available for continued research.

Pathophysiology and transcriptomic responses of Pinus armandii defenses to ophiostomatoid fungi

Pinus armandii Franch. is an ecologically and economically important evergreen tree species native to western China. Dendroctonus armandi Tsai and Li and pathogenic ophiostomatoid fungi pose substantial threats to P. armandii. With the interplay between species, the defense mechanisms of P. armandii have evolved to withstand external biotic stressors. However, the interactions between P. armandii and pathogenic ophiostomatoid fungal species/strains remain poorly understood. We aimed to analyze the pathophysiological and molecular changes in P. armandii following artificial inoculation with four ophiostomatoid species (Graphilbum parakesiyea, Leptographium qinlingense, Ophiostoma shennongense and Ophiostoma sp. 1). The study revealed that L. qinlingense produced the longest necrotic lesions, and G. parakesiyea produced the shortest. All strains induced monoterpenoid release, and monoterpene levels of P. armandii were positively correlated with fungal virulence (R2 = 0.93, P < 0.01). Co-inoculation of two dominant highly (L. qinlingense) and weakly virulent (O. shennongense) pathogens reduced the pathogenicity of the highly virulent fungi. Transcriptomic analysis of P. armandii (LQ: L. qinlingense treatments, QS: co-inoculation treatments and OS: O. shennongense treatments) showed that the expression pattern of differentially expressed genes (DEGs) between QS and OS was similar, but different from that of LQ. The DEGs (LQ vs QS) involved in flavonoid biosynthesis and phenylpropanoid biosynthesis were downregulated. Notably, compared with LQ, QS significantly decreased the expression of host defense-related genes. This study provides a valuable theoretical basis for managing infestations of D. armandi and associated ophiostomatoid fungi.

Epigenetic stress memory in gymnosperms

Gymnosperms are long-lived, cone-bearing seed plants that include some of the most ancient extant plant species. These relict land plants have evolved to survive in habitats marked by chronic or episodic stress. Their ability to thrive in these environments is partly due to their phenotypic flexibility, and epigenetic regulation likely plays a crucial part in this plasticity. We review the current knowledge on abiotic and biotic stress memory in gymnosperms and the possible epigenetic mechanisms underlying long-term phenotypic adaptations. We also discuss recent technological improvements and new experimental possibilities that likely will advance our understanding of epigenetic regulation in these ancient and hard-to-study plants.

The impact of insect egg deposition on Pinus sylvestris transcriptomic and phytohormonal responses to larval herbivory

Plants can improve their resistance to feeding damage by insects if they have perceived insect egg deposition prior to larval feeding. Molecular analyses of these egg-mediated defence mechanisms have until now focused on angiosperm species. It is unknown how the transcriptome of a gymnosperm species responds to insect eggs and subsequent larval feeding. Scots pine (Pinus sylvestris L.) is known to improve its defences against larvae of the herbivorous sawfly Diprion pini L. if it has previously received sawfly eggs. Here, we analysed the transcriptomic and phytohormonal responses of Scots pine needles to D. pini eggs (E-pine), larval feeding (F-pine) and to both eggs and larval feeding (EF-pine). Pine showed strong transcriptomic responses to sawfly eggs and-as expected-to larval feeding. Many egg-responsive genes were also differentially expressed in response to feeding damage, and these genes play an important role in biological processes related to cell wall modification, cell death and jasmonic acid signalling. EF-pine showed fewer transcriptomic changes than F-pine, whereas EF-treated angiosperm species studied so far showed more transcriptional changes to the initial phase of larval feeding than only feeding-damaged F-angiosperms. However, as with responses of EF-angiosperms, EF-pine showed higher salicylic acid concentrations than F-pine. Based on the considerable overlap of the transcriptomes of E- and F-pine, we suggest that the weaker transcriptomic response of EF-pine than F-pine to larval feeding damage is compensated by the strong, egg-induced response, which might result in maintained pine defences against larval feeding.

VvWRKY5 enhances white rot resistance in grape by promoting the jasmonic acid pathway

Grape white rot is a disease caused by Coniella diplodiella (Speg.) Sacc. (Cd) can drastically reduce the production and quality of grape (Vitis vinifera). WRKY transcription factors play a vital role in the regulation of plant resistance to pathogens, but their functions in grape white rot need to be further explored. Here, we found that the expression of the WRKY IIe subfamily member VvWRKY5 was highly induced by Cd infection and jasmonic acid (JA) treatment. Transient injection and stable overexpression (in grape calli and Arabidopsis) demonstrated that VvWRKY5 positively regulated grape resistance to white rot. We also determined that VvWRKY5 regulated the JA response by directly binding to the promoters of VvJAZ2 (a JA signaling suppressor) and VvMYC2 (a JA signaling activator), thereby inhibiting and activating the transcription of VvJAZ2 and VvMYC2, respectively. Furthermore, the interaction between VvJAZ2 and VvWRKY5 enhanced the suppression and promotion of VvJAZ2 and VvMYC2 activities by VvWRKY5, respectively. When VvWRKY5 was overexpressed in grape, JA content was also increased. Overall, our results suggested that VvWRKY5 played a key role in regulating JA biosynthesis and signal transduction as well as enhancing white rot resistance in grape. Our results also provide theoretical guidance for the development of elite grape cultivars with enhanced pathogen resistance.

Methyl jasmonate, salicylic acid, and oxalic acid affects growth, inducible defenses, and pine weevil resistance in Norway spruce

The large pine weevil (Hylobius abietis) is a major regeneration pest in commercial forestry. Pesticide application has historically been the preferred control method, but pesticides are now being phased out in several countries for environmental reasons. There is, thus, a need for alternative plant protection strategies. We applied methyl jasmonate (MeJA), salicylic acid (SA) or oxalic acid (OxA) on the stem of 2-year-old Norway spruce (Picea abies) plants to determine effects on inducible defenses and plant growth. Anatomical examination of stem cross-sections 9 weeks after application of 100 mM MeJA revealed massive formation of traumatic resin ducts and greatly reduced sapwood growth. Application of high concentrations of SA or OxA (500 and 200 mM, respectively) induced much weaker physiological responses than 100 mM MeJA. All three treatments reduced plant height growth significantly, but the reduction was larger for MeJA (~55%) than for SA and OxA (34-35%). Lower MeJA concentrations (5-50 mM) induced comparable traumatic resin duct formation as the high MeJA concentration but caused moderate (and non-significant) reductions in plant growth. Two-year-old spruce plants treated with 100 mM MeJA showed reduced mortality after exposure to pine weevils in the field, and this enhanced resistance-effect was statistically significant for three years after treatment.

Transcriptional Analysis of Tea Plants (Camellia sinensis) in Response to Salicylic Acid Treatment

Salicylic acid (SA) is an important plant hormone and signal required for establishing resistance to diverse pathogens and plant diseases. The abundant polyphenols in tea plants also defend plants from biotic and abiotic stresses. However, whether exogenous SA would increase the resistance of tea plants to adversity and the relationship between SA and polyphenols are still poorly understood. Here, we carried out SA treatment on tea seedlings and performed transcriptome sequencing. SA treatment inhibited the phenylpropanoid and flavonoid metabolic pathways but promoted the lignin metabolic pathways. The increased accumulation of lignin in tea leaves after treating with SA indicated that lignin might coordinate SA, enhance, and improve plant defense and disease resistance. Simultaneously, an SA-inducible flavonoid glucosyltransferase (CsUGT0554) specifically involved in 7-OH site glycosylation was characterized in vitro. These results provided valuable information about the effects of SA on tea seedlings and the molecular basis for SA-mediated immune responses.

Genotype-Dependent Jasmonic Acid Effect on Pinus sylvestris L. Growth and Induced Systemic Resistance Indicators

Due to temperature changes, forests are expected to encounter more stress than before, both in terms of biotic factors, such as increased insect attacks, and abiotic factors, such as more frequent droughts. Priming trees to respond to these changes faster and more effectively would be beneficial. Induced systemic resistance (ISR) is a mechanism that is turned on when plants encounter unfavorable conditions. Certain elicitors, such as jasmonic acid (JA) are known to induce plants' metabolic response. However, even though studies on ISR in herbaceous species are common and varied ISR elicitors can be used in agriculture, the same cannot be said about trees and forestry enterprises. We aimed to investigate whether JA used in different concentrations could induce metabolic changes (total phenol content, total flavonoid content, photosynthesis pigment content, antioxidant enzyme activity) in Pinus sylvestris seedlings and how this varies between different pine half-sib families (genotypes). After six weeks with a single application of JA, pine seedlings in several pine genetic families exhibited increased antioxidant enzyme activity, total phenol content and carotenoid content that correlated positively with JA concentrations used. Results from other genetic families were varied, but in many cases, there was a significant response to JA, with a noticeable increase as compared to the unaffected group. The impact on chlorophyll content and flavonoids was less noticeable overall. A positive effect on seedling growth parameters was not observed in any of the test cases. We conclude that JA can induce systemic resistance after a single application exogenously in P. sylvestris seedlings and recommend that the use of JA needs to be optimized by selecting appropriate concentrations.