Plant root PET: visualization of photosynthate translocation to roots in rice plant

Roots are essential to plants for uptake of water and nutrients. For the improvement of crop production, it is necessary to understand the elucidation of the root development and its function under the ground. Especially, photosynthate translocation from plant leaves to roots is an important physiological function that affects the root elongation, adaptation to the soil environment and nutrients uptake. To evaluate the translocation dynamics to roots, positron emission tomography (PET) and 11C tracer have been used. However, the spatial resolution is degraded at roots that develop around the peripheral area of field of view (FOV) due to parallax errors. In this study, to overcome this problem, we developed a small OpenPET prototype applying four-layer depth-of-interaction detectors. We demonstrated the imaging capability of 11C-photosynthate translocation to rice roots that develop throughout the entire PET field. We also tried to obtain structural information of roots by high-throughput X-ray computerized tomography (CT) system using the same test plant. As a result, we succeeded in visualizing the root structure that developed around the peripheral region of FOV and imaging the accumulation of 11C-photosynthate to the roots in those areas without degrading the spatial resolution. From obtained images, we also succeeded in evaluating the translocation dynamics varied by roots. The combined use of the high-throughput CT system and the OpenPET prototype was demonstrated to be appropriate for structural and functional analysis of roots.

Non-invasive 11C-Imaging Revealed the Spatiotemporal Variability in the Translocation of Photosynthates Into Strawberry Fruits in Response to Increasing Daylight Integrals at Leaf Surface

The efficiency of photosynthate translocation from leaves to fruits directly affects dry matter partitioning. Therefore, controlling photosynthate translocation dynamics is critical for high-yield and high-quality fruit production. Accordingly, photosynthate translocation changes must be characterized using data obtained at a higher spatiotemporal resolution than those provided by conventional methods. In this study, 11C-photosynthate translocation into strawberry (Fragaria × ananassa Duch.) fruits in individual plants was visualized non-invasively and repeatedly using a positron emission tracer imaging system (PETIS) to assess the spatiotemporal variability in the translocation dynamics in response to increasing daylight integrals (i.e., 0.5-, 4.5-, and 9-h exposures to 400 μmol m–2 s–1 at the leaf surface). Serial images of photosynthate translocation into strawberry fruits obtained from the PETIS confirmed that 11C-photosynthates were translocated heterogeneously into each fruit on the same inflorescence. The amount of translocated 11C-photosynthates and the translocation rate into each fruit significantly increased as the integrated light intensity at the leaf surface increased. An analysis of the pedicel of each fruit also confirmed that the photosynthate translocation rate increased. The cumulated photosynthesis in leaves increased almost linearly during the light period, suggesting that an increase in the amount of photosynthates in leaves promotes the translocation of photosynthates from leaves, resulting in an increase in the photosynthate translocation rate in pedicels and enhanced photosynthate accumulation in fruits. Additionally, the distribution pattern of photosynthate translocated to fruits did not change during the light period, nor did the order of the sink activity (11C radioactivity/fruit dry weight), which is the driving force for the prioritization of the 11C-partitioning between competing organs, among fruits. Thus, this is the first study to use 11C-radioisotopes to clarify the spatiotemporal variability in photosynthate translocation from source leaves to individual sink fruits in vivo in response to increasing daylight integrals at a high spatiotemporal resolution.

Effect of temperature and feeding on carbon budgets and O2 dynamics in Pocillopora damicornis

Studying carbon dynamics in the coral holobiont provides essential knowledge of nutritional strategies and is thus central to understanding coral ecophysiology. In this study, we assessed the carbon budget in Pocillopora damicornis (using H13CO3) as a function of feeding status and temperature stress. We also compared dissolved oxygen (O2) fluxes measured at the colony scale and at the polyp scale. At both scales, O2 production rates were enhanced for fed vs. unfed corals, and unfed corals exhibited higher bleaching and reduced photosynthetic activity at high temperature. Unfed corals exclusively respired autotrophically acquired carbon, while fed corals mostly respired heterotrophically acquired carbon. As a consequence, fed corals excreted on average >5 times more organic carbon than unfed corals. Photosynthate translocation was higher under thermal stress, but most of the carbon was lost via respiration and/or mucus release (42-46% and 57-75% of the fixed carbon for unfed and fed corals, respectively). Such high loss of translocated carbon, coupled to low assimilation rates in the coral tissue and symbionts, suggests that P. damicornis was nitrogen and/or phosphorus limited. Heterotrophy might thus cover a larger portion of the nutritional demand for P. damicornis than previously assumed. Our results suggest that active feeding plays a fundamental role in metabolic dynamics and bleaching susceptibility of corals.

Productivity and carbon fluxes depend on species and symbiont density in soft coral symbioses

Soft corals often constitute one of the major benthic groups of coral reefs. Although they have been documented to outcompete reef-building corals following environmental disturbances, their physiological performance and thus their functional importance in reefs are still poorly understood. In particular, the acclimatization to depth of soft corals harboring dinoflagellate symbionts and the metabolic interactions between these two partners have received little attention. We performed stable isotope tracer experiments on two soft coral species (Litophyton sp. and Rhytisma fulvum fulvum) from shallow and upper mesophotic Red Sea coral reefs to quantify the acquisition and allocation of autotrophic carbon within the symbiotic association. Carbon acquisition and respiration measurements distinguish Litophyton sp. as mainly autotrophic and Rhytisma fulvum fulvum as rather heterotrophic species. In both species, carbon acquisition was constant at the two investigated depths. This is a major difference from scleractinian corals, whose carbon acquisition decreases with depth. In addition, carbon acquisition and photosynthate translocation to the host decreased with an increase in symbiont density, suggesting that nutrient provision to octocoral symbionts can quickly become a limiting factor of their productivity. These findings improve our understanding of the biology of soft corals at the organism-scale and further highlight the need to investigate how their nutrition will be affected under changing environmental conditions.

Experimentally Induced Bleaching in the Sea Anemone Exaiptasia Supports Glucose as a Main Metabolite Associated with Its Symbiosis

Our current understanding of carbon exchange between partners in the Symbiodinium-cnidarian symbioses is still limited, even though studies employing carbon isotopes have made us aware of the metabolic complexity of this exchange. We examined glycerol and glucose metabolism to better understand how photosynthates are exchanged between host and symbiont. The levels of these metabolites were compared between symbiotic and bleached Exaiptasia pallida anemones, assaying enzymes directly involved in their metabolism. We measured a significant decrease of glucose levels in bleached animals but a significant increase in glycerol and G3P pools, suggesting that bleached animals degrade lipids to compensate for the loss of symbionts and seem to rely on symbiotic glucose. The lower glycerol 3-phosphate dehydrogenase but higher glucose 6-phosphate dehydrogenase specific activities measured in bleached animals agree with a metabolic deficit mainly due to the loss of glucose from the ruptured symbiosis. These results corroborate previous observations on carbon translocation from symbiont to host in the sea anemone Exaiptasia, where glucose was proposed as a main translocated metabolite. To better understand photosynthate translocation and its regulation, additional research with other symbiotic cnidarians is needed, in particular, those with calcium carbonate skeletons.