New insights into the role of constitutive bacterial rhizobiome and phenolic compounds in two Pinus spp. with contrasting susceptibility to pine pitch canker

The rhizobiome is being increasingly acknowledged as a key player in plant health and breeding strategies. The pine pitch canker (PPC), caused by the fungus Fusarium circinatum, affects pine species with varying susceptibility degrees. Our aims were to explore the bacterial rhizobiome of a susceptible (Pinus radiata) and a resistant (Pinus pinea) species together with other physiological traits, and to analyze shifts upon F. circinatum inoculation. Pinus seedlings were stem inoculated with F. circinatum spores and needle gas exchange and antioxidant-related parameters were analyzed in non-inoculated and inoculated plants. Rhizobiome structure was evaluated through 16S rRNA gene massive parallel sequencing. Species (non-inoculated plants) harbored distinct rhizobiomes (<40% similarity), where P. pinea displayed a rhizobiome with increased abundance of taxa described in suppressive soils, displaying plant-growth-promoting (PGP) traits and/or antifungal activity. Plants of this species also displayed higher levels of phenolic compounds. Fusarium circinatum induced slight changes in the rhizobiome of both species and a negative impact in photosynthetic-related parameters in P. radiata. We concluded that the rhizobiome of each pine species is distinct and higher abundance of bacterial taxa associated to disease protection was registered for the PPC-resistant species. Furthermore, differences in the rhizobiome are paralleled by a distinct content in phenolic compounds, which are also linked to plants’ resistance against PPC. This study unveils a species-specific rhizobiome and provides insights to exploit the rhizobiome for plant selection in nurseries and for rhizobiome-based plant-growth-promoting strategies, boosting environmentally friendly disease control strategies.

Genome-Wide Identification and Characterization of Fusarium circinatum-Responsive lncRNAs in Pinus radiata

One of the most promising strategies of Pine Pitch Canker (PPC) management is the use of reproductive plant material resistant to the disease. Understanding the complexity of plant transcriptome that underlies the defence to the causal agent Fusarium circinatum, would greatly facilitate the development of an accurate breeding program. Long non-coding RNAs (lncRNAs) are emerging as important transcriptional regulators under biotic stresses in plants. However, to date, characterization of lncRNAs in conifer trees has not been reported. In this study, transcriptomic identification of lncRNAs was carried out using strand-specific paired-end RNA sequencing, from Pinus radiata samples inoculated with F. circinatum at an early stage of infection. Overall, 13,312 lncRNAs were predicted through a bioinformatics approach, including long intergenic non-coding RNAs (92.3%), antisense lncRNAs (3.3%) and intronic lncRNAs (2.9%). Compared with protein-coding RNAs, pine lncRNAs are shorter, have lower expression, lower GC content and harbour fewer and shorter exons. A total of 164 differentially expressed (DE) lncRNAs were identified in response to F. circinatum infection in the inoculated versus mock-inoculated P. radiata seedlings. The predicted cis-regulated target genes of these pathogen-responsive lncRNAs were related to defence mechanisms such as kinase activity, phytohormone regulation, and cell wall reinforcement. Co-expression network analysis of DE lncRNAs, DE protein-coding RNAs and lncRNA target genes also indicated a potential network regulating pectinesterase activity and cell wall remodelling. This study presents the first analysis of conifer lncRNAs involved in the regulation of defence network and provides the basis for future functional characterizations of lncRNAs in relation to pine defence responses against F. circinatum.

Tolerance of 2-benzoxazolinone and interactions with grass and pine hosts in a population of Fusarium circinatum

Fusarium circinatum, the causal agent of pitch canker in pines and a cryptic endophyte of grasses, was examined for heritable variation in tolerance of the grass defense compound 2-benzoxazolinone (BOA). A diverse population of F. circinatum progeny was assayed for growth rate on potato dextrose agar amended with BOA. Matings were conducted to allow for selection of progeny with lower and higher tolerance of BOA. The results confirmed heritable variation in BOA tolerance in F. circinatum. A subset of differentially tolerant progeny was used for inoculations of growth chamber grown Zea mays and greenhouse grown Pinus radiata. No differences were detected in the rate of infection or extent of colonization of Z. mays inoculated with F. circinatum progeny differing in tolerance of BOA. Pitch canker symptoms in inoculated P. radiata trees showed that high BOA tolerating isolates induced significantly longer lesion lengths than those induced by low BOA tolerating isolates. Results from this study were consistent with the proposition that F. circinatum evolved from grass colonizing ancestors, and that pathogenicity to pine is a relatively recent evolutionary innovation.

Dual RNA-Sequencing Analysis of Resistant (Pinus pinea) and Susceptible (Pinus radiata) Hosts during Fusarium circinatum Challenge

Fusarium circinatum causes one of the most important diseases of conifers worldwide, the pine pitch canker (PPC). However, no effective field intervention measures aiming to control or eradicate PPC are available. Due to the variation in host genetic resistance, the development of resistant varieties is postulated as a viable and promising strategy. By using an integrated approach, this study aimed to identify differences in the molecular responses and physiological traits of the highly susceptible Pinus radiata and the highly resistant Pinus pinea to F. circinatum at an early stage of infection. Dual RNA-Seq analysis also allowed to evaluate pathogen behavior when infecting each pine species. No significant changes in the physiological analysis were found upon pathogen infection, although transcriptional reprogramming was observed mainly in the resistant species. The transcriptome profiling of P. pinea revealed an early perception of the pathogen infection together with a strong and coordinated defense activation through the reinforcement and lignification of the cell wall, the antioxidant activity, the induction of PR genes, and the biosynthesis of defense hormones. On the contrary, P. radiata had a weaker response, possibly due to impaired perception of the fungal infection that led to a reduced downstream defense signaling. Fusarium circinatum showed a different transcriptomic profile depending on the pine species being infected. While in P. pinea, the pathogen focused on the degradation of plant cell walls, active uptake of the plant nutrients was showed in P. radiata. These findings present useful knowledge for the development of breeding programs to manage PPC.

Influence of temperature and moisture duration on pathogenic life-history traits of predominant haplotypes of Fusarium circinatum on Pinus spp. in Spain

Pathogen life-history traits influence epidemic development and pathogen adaptive ability to interact with their hosts in different environments. Reduced traits variation may compromise pathogen evolutionary potential which is particularly important for introduced pathogens. Fusarium circinatum (cause of Pine Pitch Canker) is an invasive fungal pathogen in Europe, with current distribution restricted to forest stands of Pinus radiata and P. pinaster in northern Spain and Portugal. This study aimed to quantify pathogenic traits of Spanish isolates of F. circinatum, with two of the strains representing the two dominant haplotypes in the Spanish population. Disease severity was measured on P. radiata, analyzing the influence of temperature and moisture duration on infection as well as the influence of temperature on spore germination, sporulation and mycelial growth. Results indicated that the isolate representing the most common haplotype caused more severe disease on Pinus radiata at 25 and 30ºC compared to the second most common haplotype, but less severe disease at 15ºC. Spore germination was higher for the most common haplotype, which produced more spores at 20 and 25ºC. The isolate showed hyphal melanization at 5ºC, which has been associated with survival and may be important since no resting structures have been described for F. circinatum. Our study determined that longer moisture periods during infection result in more severe disease from 7 to 24 h, regardless of the isolate virulence. This is the first study on virulence of the most abundant haplotypes of F. circinatum in Spain as affected by temperatures and moisture.

Repeat-Induced Point Mutations (RIP) Drives the Formation of Distinct Sub-Genomic Compartments in Fusarium Circinatum

Repeat-Induced Point mutations (RIP) serves as a genome defence mechanism that impedes the deleterious consequences of repeated motifs such as transposable elements in fungi. Genomic regions with RIP are biased for adenosine and thymine transitions and the cumulative influence of RIP is thought to have a considerable impact on genome composition. We investigated the impact of RIP on localized genomic regions and whole-genome sequences for representatives of the pine pathogen, Fusarium circinatum. We set out to determine the intraspecific variation in acquired RIP and the role of RIP in the development of diverse F. circinatum sub-genomic compartments. The results of the study show that the AT-enriched sub-genomic compartment accounts for ca. 97% of the calculated RIP and was further prominent in both core and accessory genomic regions. However, more extensive RIP was observed in the accessory sub-compartment and more variable regions of the genome. Regions with RIP indicated increased intrinsic curvature of the DNA which may influence DNA-protein interactions and may promote constitutive heterochromatin formation. The results show that RIP is an important source of functional novelty and genome variation. RIP contributes to the evolution of the genetic landscape and differentiation of diverse sub-genomic compartments of this important fungal pathogen.