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

ADP-glucose pyrophosphorylase genes are differentially regulated in sugar-dependent or -independent manners in tomato (Solanum lycopersicum L.) fruit

In early developing tomato (Solanum lycopersicum L.) fruit, starch accumulates at high levels and is used by various primary metabolites in ripening fruits. ADP-glucose pyrophosphorylase is responsible for the first key step of starch biosynthesis. Although it has been reported that AgpL1 and AgpS1 isoforms are mainly expressed in early developing fruit, their regulatory mechanism has not been elucidated. The present study investigated the transcriptional response of AgpL1 and AgpS1 to various metabolizable sugars, nonmetabolizable sugar analogues, hexokinase inhibitors and proline by an experimental system using half-cut fruits. AgpL1 was upregulated in response to sucrose and constituted hexoses such glucose, whereas the AgpS1 gene almost did not exhibit a prominent sugar response. Further analyses revealed that other disaccharides such maltose and trehalose did not show a remarkable effect on both AgpL1 and AgpS1 expressions. These results indicate that there are two distinct regulatory mechanisms, namely, sugar metabolism-dependent and -independent, for the regulation of AGPase gene expression. Interestingly, the ADP treatment, a hexokinase inhibitors, cancelled the sugar response of AgpL1, indicating that hexokinase-mediated sugar signaling should be involved in the sugar response of AgpL1. These results suggest that sugar-dependent (AgpL1) and sugar-independent (AgpS1) pathways coordinatively regulate starch biosynthesis in immature tomato fruit.

Visualization of the initial radiocesium dynamics after penetration in living apple trees with bark removal using a positron-emitting 127Cs tracer

Following the Fukushima nuclear accident in March 2011, radiocesium (rCs) contamination in deciduous trees remains over 10 years later even though the trees were leafless at the time of the accident. This phenomenon is considered to be the result of repeated retranslocation of rCs that initially penetrated the bark into the internal tissues. To implement effective measures after a possible accident in the future, it is necessary to clarify how rCs is translocated in the tree after penetration. In this study, rCs translocation was dynamically visualized using a positron-emitting tracer imaging system (PETIS) and autoradiography after the bark of apple branches was removed. The PETIS results showed the translocation of ¹²⁷Cs from the branch to young shoots and the main stem in apple trees under controlled spring growing conditions. The transport velocity of rCs in the branch was faster than that in the main stem. The transport of rCs, which was either acropetal or basipetal, in the main stem through the branch junction favored basipetal movement. Autoradiography of transverse sections of the main stem indicated that basipetal translocation was due to transport in the phloem. This study demonstrated the initial translocation responses of rCs similar to previous field research, which indicates that rCs transport to the young shoots tends to be higher under controlled conditions. Our laboratory-based experimental system may be useful to gain an improved understanding of rCs dynamics in deciduous trees.

Rice immediately adapts the dynamics of photosynthates translocation to roots in response to changes in soil water environment

Rice is susceptible to abiotic stresses such as drought stress. To enhance drought resistance, elucidating the mechanisms by which rice plants adapt to intermittent drought stress that may occur in the field is an important requirement. Roots are directly exposed to changes in the soil water condition, and their responses to these environmental changes are driven by photosynthates. To visualize the distribution of photosynthates in the root system of rice plants under drought stress and recovery from drought stress, we combined X-ray computed tomography (CT) with open type positron emission tomography (OpenPET) and positron-emitting tracer imaging system (PETIS) with ¹¹C tracer. The short half-life of ¹¹C (20.39 min) allowed us to perform multiple experiments using the same plant, and thus photosynthate translocation was visualized as the same plant was subjected to drought stress and then re-irrigation for recovery. The results revealed that when soil is drier, ¹¹C-photosynthates mainly translocated to the seminal roots, likely to promote elongation of the root with the aim of accessing water stored in the lower soil layers. The photosynthates translocation to seminal roots immediately stopped after rewatering then increased significantly in crown roots. We suggest that when rice plant experiencing drought is re-irrigated from the bottom of pot, the destination of ¹¹C-photosynthates translocation immediately switches from seminal root to crown roots. We reveal that rice roots are responsive to changes in soil water conditions and that rice plants differentially adapts the dynamics of photosynthates translocation to crown roots and seminal roots depending on soil conditions.

Design Study of a Novel Positron Emission Tomography System for Plant Imaging

Positron Emission Tomography is a non-disruptive and high-sensitive digital imaging technique which allows to measure in-vivo and non invasively the changes of metabolic and transport mechanisms in plants. When it comes to the early assessment of stress-induced alterations of plant functions, plant PET has the potential of a major breakthrough. The development of dedicated plant PET systems faces a series of technological and experimental difficulties, which make conventional clinical and preclinical PET systems not fully suitable to agronomy. First, the functional and metabolic mechanisms of plants depend on environmental conditions, which can be controlled during the experiment if the scanner is transported into the growing chamber. Second, plants need to be imaged vertically, thus requiring a proper Field Of View. Third, the transverse Field of View needs to adapt to the different plant shapes, according to the species and the experimental protocols. In this paper, we perform a simulation study, proposing a novel design of dedicated plant PET scanners specifically conceived to address these agronomic issues. We estimate their expected sensitivity, count rate performance and spatial resolution, and we identify these specific features, which need to be investigated when realizing a plant PET scanner. Finally, we propose a novel approach to the measurement and verification of the performance of plant PET systems, including the design of dedicated plant phantoms, in order to provide a standard evaluation procedure for this emerging digital imaging agronomic technology.