Leaf trichomes in Metrosideros polymorpha can contribute to avoiding extra water stress by impeding gall formation

Rapid Loss of Nutritional Symbionts in an Endemic Hawaiian Herbivore Radiation Is Associated with Plant Galling Habit

Insect herbivores frequently cospeciate with symbionts that enable them to survive on nutritionally unbalanced diets. While ancient symbiont gain and loss events have been pivotal for insect diversification and feeding niche specialization, evidence of recent events is scarce. We examine the recent loss of nutritional symbionts (in as little as 1 MY) in sap-feeding Pariaconus, an endemic Hawaiian insect genus that has undergone adaptive radiation, evolving various galling and free-living ecologies on a single host-plant species, Metrosideros polymorpha within the last ∼5 MY. Using 16S rRNA sequencing, we investigated the bacterial microbiomes of 19 Pariaconus species and identified distinct symbiont profiles associated with specific host-plant ecologies. Phylogenetic analyses and metagenomic reconstructions revealed significant differences in microbial diversity and functions among psyllids with different host-plant ecologies. Within a few millions of years, Pariaconus species convergently evolved the closed-gall habit twice. This shift to enclosed galls coincided with the loss of the Morganella-like symbiont that provides the essential amino acid arginine to free-living and open-gall sister species. After the Pariaconus lineage left Kauai and colonized younger islands, both open- and closed-gall species lost the Dickeya-like symbiont. This symbiont is crucial for synthesizing essential amino acids (phenylalanine, tyrosine, and lysine) as well as B vitamins in free-living species. The recurrent loss of these symbionts in galling species reinforces evidence that galls are nutrient sinks and, combined with the rapidity of the evolutionary timeline, highlights the dynamic role of insect-symbiont relationships during the diversification of feeding ecologies. We propose new Candidatus names for the novel Morganella-like and Dickeya-like symbionts.

How does the life cycle of Clinodiplosis profusa (Cecidomyiidae) adjust to phenological variations of the host plant Eugenia uniflora (Myrtaceae) in sun and shade?

Galls are plant neoformations induced by specialized parasites. Since gall inducers rely on reactive plant sites for gall development, variations in abiotic factors that affect plant phenology are expected to impact the life cycle of gall inducers. To test the hypothesis that different light conditions affect both host plant and gall inducer life cycles, we studied the system Eugenia uniflora (Myrtaceae) - Clinodiplosis profusa (Cecidomyiidae), comparing plants occurring in sunny and shaded environments. We mapped phenological differences among individuals of E. uniflora occurring in the two environments and related them to the influence of luminosity on the life cycle of the gall inducer. Shade plants showed lower intensity of leaf sprouting throughout the year compared to sun-exposed plants, especially during the rainy season. Young and mature galls are synchronized with the peak of leaf sprouting at the beginning of the rainy season, lasting longer in sun-exposed plants - approximately two months longer compared to shade plants. The greater light intensity positively impacts the formation and growth of leaves and galls, with an extended period available for their induction and growth. Thus, light is an important factor for the development of gallers, considering that variations in luminosity influenced not only the phenology of the host plant, but also determined the life cycle of gall inducers. Furthermore, changes in plant-environment interactions are expected to affect the life cycle and richness of other host plant-gall inducer systems.

Physiological and transcriptional analyses reveal the resistance mechanisms of kiwifruit (Actinidia chinensis) mutant with enhanced heat tolerance

High temperature is an environmental stressor that severely threatens plant growth, development, and yield. In this study, we obtained a kiwifruit mutant (MT) of 'Hongyang' (WT) through 60Co-γ irradiation. The MT possessed different leaf morphology and displayed prominently elevated heat tolerance compared to the WT genotype. When exposure to heat stress, the MT plants exhibited stabler photosynthetic capacity and accumulated less reactive oxygen species, along with enhanced antioxidant capacity and higher expression levels of related genes in comparison with the WT plants. Moreover, global transcriptome profiling indicated that an induction in genes related to stress-responsive, phytohormone signaling, and transcriptional regulatory pathways, which might contribute to the upgrade of thermotolerance in the MT genotype. Collectively, the significantly enhanced thermotolerance of MT might be mainly attributed to profitable leaf structure variations, improved photosynthetic and antioxidant capacities, as well as extensive transcriptome reprogram. These findings would be insightful in elucidating the sophisticated mechanisms of kiwifruit response to heat stress, and suggest the MT holds great potential for future kiwifruit improvement with enhanced heat tolerance.

Hawaiian Treeline Ecotones: Implications for Plant Community Conservation under Climate Change

Species within tropical alpine treeline ecotones are predicted to be especially sensitive to climate variability because this zone represents tree species' altitudinal limits. Hawaiian volcanoes have distinct treeline ecotones driven by trade wind inversions. The local climate is changing, but little is known about how this influences treeline vegetation. To predict future impacts of climate variability on treelines, we must define the range of variation in treeline ecotone characteristics. Previous studies highlighted an abrupt transition between subalpine grasslands and wet forest on windward Haleakalā, but this site does not represent the diversity of treeline ecotones among volcanoes, lava substrates, and local climatic conditions. To capture this diversity, we used data from 225 plots spanning treelines (1500-2500 m) on Haleakalā and Mauna Loa to characterize ecotonal plant communities. Treeline indicator species differ by moisture and temperature, with common native species important for wet forest, subalpine woodland, and subalpine shrubland. The frequency or abundance of community indicator species may be better predictors of shifting local climates than the presence or absence of tree life forms per se. This study further supports the hypothesis that changes in available moisture, rather than temperature, will dictate the future trajectory of Hawaiian treeline ecotone communities.

Roles of lower-side leaf trichomes in diffusion resistance and gas-exchange characteristics across environmental gradients in Metrosideros polymorpha

Leaf trichomes on the lower leaf surface are common in many plant species, especially those grown under dry and/or low-temperature conditions; however, their adaptive significance remains unclear. Lower-side leaf trichomes can directly decrease gas fluxes through increased gas-diffusion resistance but can indirectly increase gas fluxes through increased leaf temperature owing to increased heat-diffusion resistance. We examined whether the combined direct and indirect effects of trichome resistance increase photosynthetic rates and water-use efficiency (WUE) using Metrosideros polymorpha Gaud., which varies widely in the masses of lower-side non-glandular leaf trichomes across various environments on the Hawaiian Islands. We employed both field surveys, including ecophysiological measurements at five elevation sites, and simulation analyses to predict the gas-exchange rates of leaves with various trichome-layer thicknesses across a wide range of environmental conditions. Field surveys showed that the trichome-layer thickness was the largest at the coldest and driest site and the thinnest at the wettest site. Field surveys, experimental manipulations and simulation analyses demonstrated that leaf trichomes significantly increased leaf temperature owing to the increased heat resistance. Simulation analyses showed that the effect of leaf trichomes on heat resistance was much larger than that on gas-flux resistance. Leaf trichomes can increase daily photosynthesis only in cold dry areas by increasing the leaf temperature. However, the increased leaf temperature with leaf trichomes resulted in a consistent decrease in the daily WUE at all elevation sites. The magnitudes of trichome effects on gas-exchange rates were associated with the temperature difference across the elevational gradient, the strong light intensity in Hawaii, the leaf-size variation and the conservative stomatal behavior of M. polymorpha as well as the trichome-layer thickness. In summary, the lower-side leaf trichomes in M. polymorpha can be beneficial for carbon assimilation in low-temperature environments but not for water conservation in most environments in terms of diffusion resistance.

Engineering Themes in Plant Forms and Functions

Living structures constantly interact with the biotic and abiotic environment by sensing and responding via specialized functional parts. In other words, biological bodies embody highly functional machines and actuators. What are the signatures of engineering mechanisms in biology? In this review, we connect the dots in the literature to seek engineering principles in plant structures. We identify three thematic motifs-bilayer actuator, slender-bodied functional surface, and self-similarity-and provide an overview of their structure-function relationships. Unlike human-engineered machines and actuators, biological counterparts may appear suboptimal in design, loosely complying with physical theories or engineering principles. We postulate what factors may influence the evolution of functional morphology and anatomy to dissect and comprehend better the why behind the biological forms.

SlHair2 Regulates the Initiation and Elongation of Type I Trichomes on Tomato Leaves and Stems

Trichomes are hair-like structures that are essential for abiotic and biotic stress responses. Tomato Hair (H), encoding a C2H2 zinc finger protein, was found to regulate the multicellular trichomes on stems. Here, we characterized Solyc10g078990 (hereafter Hair2, H2), its closest homolog, to examine whether it was involved in trichome development. The H2 gene was highly expressed in the leaves, and its protein contained a single C2H2 domain and was localized to the nucleus. The number and length of type I trichomes on the leaves and stems of knock-out h2 plants were reduced when compared to the wild-type, while overexpression increased their number and length. An auto-activation test with various truncated forms of H2 using yeast two-hybrid (Y2H) suggested that H2 acts as a transcriptional regulator or co-activator and that its N-terminal region is important for auto-activation. Y2H and pull-down analyses showed that H2 interacts with Woolly (Wo), which regulates the development of type I trichomes in tomato. Luciferase complementation imaging assays confirmed that they had direct interactions, implying that H2 and Wo function together to regulate the development of trichomes. These results suggest that H2 has a role in the initiation and elongation of type I trichomes in tomato.