Coronatine (1), a small-molecular virulence factor produced by plant-pathogenic bacteria, promotes bacterial infection by inducing the opening of stomatal pores, the major route of bacterial entry into the plant, via the jasmonate-mediated COI1-JAZ signaling pathway. However, this pathway is also important for multiple plant functions, including defense against wounding by herbivorous insects. Thus, suppression of the COI1-JAZ signaling pathway to block bacterial infection would concomitantly impair plant defense against herbivorous wounding. Here, we report additional, COI1-JAZ-independent, action of 1 in Arabidopsis thaliana guard cells. First, we found that a stereoisomer of 1 regulates the movement of Arabidopsis guard cells without affecting COI1-JAZ signaling. Second, we found using alkyne-tagged Raman imaging (ATRI) that 1 is localized to the endoplasmic reticulum (ER) of living guard cells of Arabidopsis. The use of arc6 mutant lacking chloroplast formation was pivotal to circumvent the issue of autofluorescence during ATRI. These findings indicate that 1 has an ER-related action on Arabidopsis stomata that bypasses the COI1-JAZ signaling module. It may be possible to suppress the action of 1 on stomata without impairing plant defense responses against herbivores.

Calcium ion (Ca2+) is one of the most important second messengers. However, conventional Ca2+ indicators cannot be applied in plant cells. In previous paper, we developed the first-generation protein-ligand tethered calcium indicator that can be applied for living plant cells.Here, we will report the development of second-generation indicator based on the detailed examination on physical properties of the first-generation to improve the drawbacks. Through the examination, we also found that cell permeability of the probe is strongly affected by their aggregation properties. Our findings will provide new strategy for the design of synthetic indicators.Download high-res image (116KB)Download full-size image

Affinity labeling of a target protein is a powerful method for chemical biology studies. However, it is still difficult to label intracellular proteins efficiently in living cells. We propose the novel design strategy of a reactive group-embedded affinity labeling reagent for efficient protein labeling. With FKBP12 as the model target protein, the ligand binding pocket-oriented labeling reagent could label intracellular protein, whereas protein surface-oriented reagent was ineffective for labeling in living cells, partially because of the intracellular protein fluctuation under the macromolecular crowding effects. These results provide new insight for efficient intracellular protein labeling.Download high-res image (100KB)Download full-size image

Coronatine (COR, 1) is a phytotoxin and structural mimic of the plant hormone (+)-7-iso-jasmonoyl-L-isoleucine (2). COR (1) functions as a ligand of the COI1–JAZ co-receptor, which is the exclusive receptor of 2. Recently, a new role for 1 as a plant virulence factor for Pseudomonas syringae has attracted the attention of plant scientists. Bacteria invade the plant apoplast through stomatal pores. The host plant then responds to the bacterial invasion by closing the stomatal pores (stomatal defense). COR (1) functions as a bacterial chemical weapon that secures the path of infection by reopening the closed stomata. The mechanism is thought to involve inhibition of abscisic acid-signaling through the COI1–JAZ pathway. Thus, 1 plays an important role in plant–microbe interactions by abrogating the plant immune response. In this study, we synthesized seven analogues of 1 with naturally occurring α-amino acids and assessed their effect on stomata in a model plant, Arabidopsis thaliana. Structure–activity relationship studies of the analogues coupled with genetic studies and in silico docking analyses with COI1–JAZ strongly suggested that stomatal reopening induced by 1 may not rely on the COI1–JAZ signaling pathway. Our results suggest that stomatal reopening is triggered by 1 in conjunction with the conventional COI1–JAZ mode of action.

In plant biology, calcium ions are involved in a variety of intriguing biological phenomena as a secondary messenger. However, most conventional calcium indicators are not applicable for plant cells because of the difficulty with their localization control in plant cells. We here introduce a method to monitor spatiotemporal Ca2+ dynamics in living plant cells based on linking the synthetic calcium indicator Calcium Green-1 to a natural product-based protein ligand. In a proof-of-concept study using cultured BY-2 cells overexpressing the target protein for the ligand, the ligand-tethered probe accumulated in the cytosol and nucleus, and enabled real-time monitoring of the cytosolic and nucleus Ca2+ dynamics under the physiological condition. The present strategy using ligand-tethered fluorescent sensors may be successfully applied to reveal the spatiotemporal dynamics of calcium ions in living plant cells.

12-O-β-D-Glucopyranosyljasmonic acid (JAG, 1) induces nyctinastic leaf-folding of Samanea saman. The SAR studies of 1 revealed the unique role of its glycone moiety. Biological activity and the target affinity of 1 were affected by the stereochemistry of the glycone moiety. JAG belongs to a unique class of ligands in which the structure of the glycone moiety is involved in the molecular recognition by the target protein.

In methods employing molecular probes to explore the targets of bioactive small molecules, long or rigid linker moieties are thought to be critical factors for efficient tagging of target protein. We previously reported the synthesis of a jasmonate glucoside probe with a highly rigid linker consisting of a triazoyl–phenyl (TAzP) moiety, and this probe demonstrated effective target tagging. Here we compare the TAzP probe with other rigid or flexible probes with respect to target tagging efficiency, hydrophobic parameters, aqueous solubility, and dihedral angles around the biaryl linkage by a combination of empirical and calculation methods. The rigid biaryl linkage of the TAzP probe has a skewed conformation that influences its aqueous solubility. Such features that include rigidness and good aqueous solubility resulted in highly efficient target tagging. These findings provide a promising guideline toward designing of better linkers for improving molecular probe performance.

We report on the design, synthesis and assessment of a novel biaryl-linked (BArL) molecular probe for the exploration of low-abundant target proteins for bioactive compounds based on the activity based protein profiling (ABPP) approach. Surprisingly, the performance of the BArL probe was better than that of the stepwise tagging approach that is considered to be the most effective method used in ABPP study.

Affinity purification using immunoprecipitation (IP) is an extremely useful method for target profiling of bioactive natural products. We examined IP purification of CMetE, which is a molecular target for potassium isolespedezate (1), a leaf-opening factor of Cassia plant. We studied IP efficiency using a panel of FLAG-connected molecular probes (2–8), including probes with varying structures and lengths of the linker moiety. The results suggest that not only the length, but the chemical nature of the linker moiety, strongly affect the IP efficiency. 3XFLAG, a tag combined with a linker moiety of charged amino acids, gave the best results and was most useful for IP purification of the molecular target.

We report on the design, synthesis and assessment of a novel biaryl-linked (BArL) molecular probe for the exploration of low-abundant target proteins for bioactive compounds based on the activity based protein profiling (ABPP) approach. Surprisingly, the performance of the BArL probe was better than that of the stepwise tagging approach that is considered to be the most effective method used in ABPP study.

12-O-β-D-Glucopyranosyljasmonic acid (JAG, 1) induces nyctinastic leaf-folding of Samanea saman. The SAR studies of 1 revealed the unique role of its glycone moiety. Biological activity and the target affinity of 1 were affected by the stereochemistry of the glycone moiety. JAG belongs to a unique class of ligands in which the structure of the glycone moiety is involved in the molecular recognition by the target protein.