The ecophysiology of a neotropical mistletoe depends on the leaf phenology of its tree hosts

Protein Profiling of Psittacanthus calyculatus during Mesquite Infection

Psittacanthus calyculatus is a hemiparasite mistletoe that represents an ecological problem due to the impacts caused to various tree species of ecological and commercial interest. Although the life cycle for the Psittacanthus genus is well established in the literature, the development stages and molecular mechanism implicated in P. calyculatus host infection are poorly understood. In this study, we used a manageable infestation of P. laevigata with P. calyculatus to clearly trace the infection, which allowed us to describe five phenological infective stages of mistletoe on host tree branches: mature seed (T1), holdfast formation (T2), haustorium activation (T3), haustorium penetration (T4), and haustorium connection (T5) with the host tree. Proteomic analyses revealed proteins with a different accumulation and cellular processes in infective stages. Activities of the cell wall-degrading enzymes cellulase and β-1,4-glucosidase were primarily active in haustorium development (T3), while xylanase, endo-glucanase, and peptidase were highly active in the haustorium penetration (T4) and xylem connection (T5). Patterns of auxins and cytokinin showed spatial concentrations in infective stages and moreover were involved in haustorium development. These results are the first evidence of proteins, cell wall-degrading enzymes, and phytohormones that are involved in early infection for the Psittacanthus genus, and thus represent a general infection mechanism for other mistletoe species. These results could help to understand the molecular dialogue in the establishment of P. calyculatus parasitism.

Transcriptional Basis for Haustorium Formation and Host Establishment in Hemiparasitic Psittacanthus schiedeanus Mistletoes

The mistletoe Psittacanthus schiedeanus, a keystone species in interaction networks between plants, pollinators, and seed dispersers, infects a wide range of native and non-native tree species of commercial interest. Here, using RNA-seq methodology we assembled the whole circularized quadripartite structure of P. schiedeanus chloroplast genome and described changes in the gene expression of the nuclear genomes across time of experimentally inoculated seeds. Of the 140,467 assembled and annotated uniGenes, 2,000 were identified as differentially expressed (DEGs) and were classified in six distinct clusters according to their expression profiles. DEGs were also classified in enriched functional categories related to synthesis, signaling, homoeostasis, and response to auxin and jasmonic acid. Since many orthologs are involved in lateral or adventitious root formation in other plant species, we propose that in P. schiedeanus (and perhaps in other rootless mistletoe species), these genes participate in haustorium formation by complex regulatory networks here described. Lastly, and according to the structural similarities of P. schiedeanus enzymes with those that are involved in host cell wall degradation in fungi, we suggest that a similar enzymatic arsenal is secreted extracellularly and used by mistletoes species to easily parasitize and break through tissues of the host.

Tapping into the physiological responses to mistletoe infection during heat and drought stress

Mistletoes are important co-contributors to tree mortality globally, particularly during droughts. In Australia, mistletoe distributions are expanding in temperate woodlands, while their hosts have experienced unprecedented heat and drought stress in recent years. We investigated whether the excessive water use of mistletoes increased the probability of xylem emboli in a mature woodland during the recent record drought that was compounded by multiple heatwaves. We continuously recorded transpiration ($T_{SLA}$) of infected and uninfected branches from two eucalypt species over two summers, monitored stem and leaf water potentials ($\Psi $) and used hydraulic vulnerability curves to estimate percent loss in conductivity (PLC) for each species. Variations in weather (vapor pressure deficit, photosynthetically active radiation, soil water content), host species and % mistletoe foliage explained 78% of hourly $T_{SLA}$. While mistletoe acted as an uncontrollable sink for water in the host even during typical summer days, daily $T_{SLA}$ increased up to 4-fold in infected branches on hot days, highlighting the previously overlooked importance of temperature stress in amplifying water loss in mistletoes. The increased water use of mistletoes resulted in significantly decreased host $\Psi _{\rm{leaf}}$ and $\Psi _{\rm{trunk}}$. It further translated to an estimated increase of up to 11% PLC for infected hosts, confirming greater hydraulic dysfunction of infected trees that place them at higher risk of hydraulic failure. However, uninfected branches of Eucalyptus fibrosa F.Muell. had much tighter controls on water loss than uninfected branches of Eucalyptus moluccana Roxb., which shifted the risk of hydraulic failure towards an increased risk of carbon starvation for E. fibrosa. The contrasting mechanistic responses to heat and drought stress between both co-occurring species demonstrates the complexity of host-parasite interactions and highlights the challenge in predicting species-specific responses to biotic agents in a warmer and drier climate.

Evaluating the impact of roads on the diversity pattern and density of trees to improve the conservation of species

Roadside trees alter biotic and abiotic factors of plants diversity in an ecosystem. Rows of plants grow along the roadside due to the interplay between the arrival of propagule and seedling establishment, which depends on the road's specifications, land pattern, and road administration and protection practices. A field study was conducted to measure the roadside tree diversity in the city of Karachi (Pakistan). A total of 180 plots, divided into three primary road groups, were surveyed. The highest quantity of tree biomass per unit area was found on wide roads, followed by medium roads. On narrow roads, the least biomass was detected. A single species or a limited number of species dominated the tree community. Conocarpus erectus was the most dominant non-native species on all types of sidewalks or roadsides, followed by Guaiacum officinale. A total of 76 species (32 non-natives and 44 natives) that were selectively spread along the roadsides of the city were studied. There was a significant difference in phylogenetic diversity (PD), phylogenetic mean pairwise distance (MPD), and phylogenetic mean nearest taxon distance (MNTD) among wide, medium, and narrow roads. Management practices have a significant positive correlation with diversity indices. Our study identified patterns of diversity in roadside trees in Karachi. It provides the basis for future planning for plant protection, such as the protection of plant species, the maintenance of plant habitats, and the coordination of plant management in Karachi.

Differential reproductive responses to contrasting host species and localities in Psittacanthus calyculatus (Loranthaceae) mistletoes

Host trees are immediate environments for multi-host mistletoes, so parasitizing different hosts implies exposure to varying environmental conditions between mistletoe generations. Therefore, to maximize reproductive fitness in heterogeneous environments (host species) mistletoes should adjust its reproductive responses in relation to circumstances. Here, we ask how reproductive traits of Psittacanthus calyculatus mistletoes vary on two host tree species (Crataegus mexicana, Prunus serotina) at three different localities. We followed individual mistletoes on each host species and at three localities through the flowering season to quantify traits related to reproductive effort and success, e.g. total number of buds, flowers and fruits produced. In buds about to open, we measured two anthers and quantified the number of pollen grains and their viability. Individual flowers were marked to obtain flower longevity data and were followed until fruit formation. At which time we measured and weighed the fruits and the seeds. Mistletoes from one locality produced more buds, flowers and fruits, and the flowers lasted longer, had larger anthers and produced more pollen as compared to the other two localities. However, mistletoes on Prunus serotina produced fewer floral buds, but their fruits were heaviest, longest and widest and ripened fastest across localities. The probability of fruit formation, percentage of fruits formed, and pollen viability were similar among the mistletoes, regardless of host species or locality. We propose that the observed differences in reproductive effort and success associated with host species or locality are plastic or adaptive in this mistletoe in response to varying conditions.