Cellular functions are fundamentally regulated by intracellular temperature, which influences biochemical reactions inside a cell. Despite the important contributions to biological and medical applications that it would offer, intracellular temperature mapping has not been achieved. Here we demonstrate the first intracellular temperature mapping based on a fluorescent polymeric thermometer and fluorescence lifetime imaging microscopy. The spatial and temperature resolutions of our thermometry were at the diffraction limited level (200 nm) and 0.18–0.58 °C. The intracellular temperature distribution we observed indicated that the nucleus and centrosome of a COS7 cell, both showed a significantly higher temperature than the cytoplasm and that the temperature gap between the nucleus and the cytoplasm differed depending on the cell cycle. The heat production from mitochondria was also observed as a proximal local temperature increase. These results showed that our new intracellular thermometry could determine an intrinsic relationship between the temperature and organelle function.

Salicylic acid (SA) is a phytohormone best known for its role in plant defense. It is synthesized in response to diverse pathogens and responsible for the large scale transcriptional induction of defense-related genes and the establishment of systemic acquired resistance. Surprisingly, given its importance in plant defense, an understanding of the underlying enzymology is lacking. In Arabidopsis thaliana, the pathogen-induced accumulation of SA requires isochorismate synthase (AtICS1). Here, we show that AtICS1 is a plastid-localized, stromal protein using chloroplast import assays and immunolocalization. AtICS1 acts as a monofunctional isochorismate synthase (ICS), catalyzing the conversion of chorismate to isochorismate (IC) in a reaction that operates near equilibrium (K eq ‫؍‬ 0.89). It does not convert chorismate directly to SA (via an IC intermediate) as does Yersinia enterocolitica Irp9. Using an irreversible coupled spectrophotometric assay, we found that AtICS1 exhibits an apparent K m of 41.5 M and k cat ‫؍‬ 38.7 min ؊1 for chorismate. This affinity for chorismate would allow it to successfully compete with other pathogen-induced, chorismate-utilizing enzymes. Furthermore, the biochemical properties of AtICS1 indicate its activity is not regulated by light-dependent changes in stromal pH, Mg 2؉ , or redox and that it is remarkably active at 4°C consistent with a role for SA in cold-tolerant growth. Finally, our analyses support plastidic synthesis of stress-induced SA with the requirement for one or more additional enzymes responsible for the conversion of IC to SA, because non-enzymatic conversion of IC to SA under physiological conditions was negligible.

We have isolated 5 cDNA clones (din2, din6, din9, din10 and din11) corresponding to genes, the transcripts of which accumulated in leaves of Arabidopsis thaliana kept in the dark. These cDNA clones encode proteins similar to beta-glucosidase (EC 3.2.1.21, din2), asparagine synthetase (EC 6.3.5.4, din6), phosphomannose isomerase (EC 5.3.1.8, din9), seed imbibition protein (din10) and 2-oxoacid-dependent dioxygenases (din11). Accumulation of the transcripts from din6 and din10 occurred within 3 h after plants were transferred to darkness. The transcripts from din2, din9 and din11 were only detected after 24 h of dark treatment. We also observed the accumulation of the din transcripts in senescing leaves. Application of a photosynthesis inhibitor, 3-(3,4-dichlorophenyl)-1-1-dimethyl-urea, induced the expression of the din genes under illumination. Application of sucrose to detached leaves suppressed the accumulation of the din transcripts in the dark. These results indicate that expression of these genes partly depends on cellular sugar level. The sugar-modulated expression of the din genes suggests that dark-induced expression of these genes might be related to sugar starvation occurring in leaf cells in the dark, when the photosynthesis is hindered.

To elucidate host processes and components required for the sustained growth and reproduction of the obligate biotrophic fungus Golovinomyces orontii on Arabidopsis thaliana , laser microdissection was used to isolate cells at the site of infection at 5 days postinfection for downstream global Arabidopsis expression profiling. Site-specific profiling increased sensitivity dramatically, allowing us to identify specific host processes, process components, and their putative regulators hidden in previous whole-leaf global expression analyses. For example, 67 transcription factors exhibited altered expression at the powdery mildew (PM) infection site, with subsets of these playing known or inferred roles in photosynthesis, cold/dehydration responses, defense, auxin signaling, and the cell cycle. Using integrated informatics analyses, we constructed putative regulatory networks for a subset of these processes and provided strong support for host cell cycle modulation at the PM infection site. Further experimentation revealed induced host endoreduplication occurred exclusively at the infection site and led us to identify MYB3R4 as a transcriptional regulator of this process. Induced endoreduplication was abrogated in myb3r4 mutants, and G. orontii growth and reproduction were reduced. This suggests that, by increasing gene copy number, localized endoreduplication serves as a mechanism to meet the enhanced metabolic demands imposed by the fungus, which acquires all its nutrients from the plant host.

Changes in intracellular temperatures reflect the activity of the cell. Thus, the tool to measure intracellular temperatures could provide valuable information about cellular status. We previously reported a method to analyze the intracellular temperature distribution using a fluorescent polymeric thermometer (FPT) in combination with fluorescence lifetime imaging microscopy (FLIM). Intracellular delivery of the FPT used in the previous study required microinjection. We now report a novel FPT that is cell permeable and highly photostable, and we describe the application of this FPT to the imaging of intracellular temperature distributions in various types of mammalian cell lines. This cell-permeable FPT displayed a temperature resolution of 0.05°C to 0.54°C within the range from 28°C to 38°C in HeLa cell extracts. Using our optimized protocol, this cell-permeable FPT spontaneously diffused into HeLa cells within 10 min of incubation and exhibited minimal toxicity over several hours of observation. FLIM analysis confirmed a temperature difference between the nucleus and the cytoplasm and heat production near the mitochondria, which were also detected previously using the microinjected FPT. We also showed that this cell-permeable FPT protocol can be applied to other mammalian cell lines, COS7 and NIH/3T3 cells. Thus, this cell-permeable FPT represents a promising tool to study cellular states and functions with respect to temperature.

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Acquisition of cell identity requires communication among neighboring cells. To dissect the genetic pathways regulating cell signaling in later leaf development, a screen was performed to identify mutants with chloroplast pigmentation sectors that violate cell lineage boundaries in maize (Zea mays) leaves. We have characterized a recessive mutant, tie-dyed1 (tdy1), which develops stable, nonclonal variegated yellow and green leaf sectors. Sector formation requires high light, occurs during a limited developmental time, and is restricted to leaf blade tissue. Yellow tdy1 sectors accumulate excessive soluble sugars and starch, whereas green sectors appear unaffected. Significantly, starch accumulation precedes chlorosis in cells that will become a yellow sector. Retention of carbohydrates in tdy1 leaves is associated with a delay in reproductive maturity, decreased stature, and reduced yield. To explain the tdy1 sectoring pattern, we propose a threshold model that incorporates the light requirement and the hyperaccumulation of photoassimilates. A possible function consistent with this model is that TDY1 acts as a sugar sensor to regulate an inducible sugar export pathway as leaves develop under high light conditions.

In 2003, we successfully created the first fluorescent polymeric thermometer by combining a thermo-responsive polymer and an environment-sensitive (polarity and hydrogen bonding-sensitive) fluorophore. Its high sensitivity to temperature variation and high hydrophilicity, even under conditions of high ionic strength, enabled intracellular temperature measurements. Along with the progress of our research projects, the development of new luminescent molecular thermometers and the establishment of novel methods for measuring intracellular temperature have matured in this field. In this Feature Article, we summarize the background and history of intracellular temperature measurements using fluorescent polymeric thermometers based on studies performed in our laboratory and the relationship between our methods and those of other eminent research groups. Future research directions regarding intracellular temperature measurements are also discussed.