The angiosperm Rafflesia exhibits a unique biology, including a growth strategy that involves endophytic parasitism of a specific host, with only the gigantic flower externally visible. The Rafflesia possesses many unique evolutionary, developmental and morphological features that are rooted in yet-to-be-explained physiological processes. Although studies on the molecular biology of Rafflesia are limited by sampling difficulties due to its rarity in the wild and the short life span of its flower, current advances in high-throughput sequencing technology have allowed for the genome- and transcriptome-level dissection of the molecular mechanisms behind the unique characteristics of this parasitic plant. In this review, we summarize major findings on the cryptic biology of Rafflesia and provide insights into future research directions. The wealth of data obtained can improve our understanding of Rafflesia species and contribute toward the conservation strategy of this endangered plant.

Coral bleaching is primarily caused by high sea surface temperatures, and nutrient enrichment of reefs is associated with lower resilience to thermal stress and ecological degradation. Excess inorganic nitrogen relative to phosphate has been proposed to sensitize corals to thermal bleaching. We assessed the physiological and proteomic responses of cultures of the dinoflagellate coral symbiont Symbiodinium microadriaticum to elevated temperature under low-nutrient, high-nutrient and phosphate-limited conditions. Elevated temperature induced reductions of many chloroplast proteins, particularly the light-harvesting complexes, and simultaneously increased the abundance of many chaperone proteins. Proteomes were similar when the N:P ratio was near the Redfield ratio, regardless of absolute N and P concentrations, but were strongly affected by phosphate limitation. Very high N:P inhibited Symbiodinium cell division while increasing the abundance of chloroplast proteins. The proteome response to phosphate limitation was greater than that to elevated temperature, as measured by the number of differentially abundant proteins. Increased physiological sensitivity to high temperatures under high nutrients or imbalanced N:P ratios was not apparent; however, oxidative stress response proteins were enriched among proteins responding to thermal stress under imbalanced N:P ratios. These data provide a detailed catalog of the effects of high temperatures and nutrients on a coral symbiont proteome.

Arbuscular mycorrhizal (AM) fungi establish mutualistic symbiosis with a wide range of terrestrial plants, including rice. However, the mechanisms underlying the initiation of AM symbiosis are yet to be elucidated, particularly in nonleguminous plants. We previously demonstrated that chitin elicitor receptor kinase 1 (OsCERK1), a lysin motif receptor-like kinase essential for chitin-triggered immunity, also plays a key role in AM symbiosis in rice. However, the mechanisms underlying the regulation of switching between immunity and symbiosis by OsCERK1 are yet to be fully elucidated. SYMBIOSIS RECEPTOR-LIKE KINASE (SYMRK)/DOES NOT MAKE INFECTIONS 2 (DMI2) is a leucine-rich repeat receptor-like kinase associated with both root nodule symbiosis and AM symbiosis in legumes. The homolog of SYMRK in rice, OsSYMRK, has a shorter form than that in legumes because OsSYMRK lacks a malectin-like domain (MLD). The MLD reportedly contributes to symbiosis in Lotus japonicus; however, the contribution of OsSYMRK to AM symbiosis in rice remains unclear. Phylogenetic analyses indicated that the MLD of SYMRK/DMI2 is widely conserved even in mosses and ferns but absent in commelinids, including rice. To understand the function of OsSYMRK, we produced an Ossymrk knockout mutant using genome editing technology. AM colonization was mostly abolished in Ossymrk with a more severe phenotype than Oscerk1. Ca2+ spiking against chitin tetramer was also diminished in Ossymrk. In contrast, comparable defense responses against chitin heptamer to the wild type were observed in Ossymrk. Bimolecular fluorescence complementation studies demonstrating an interaction between OsSYMRK and OsCERK1 indicate that OsSYMRK may play an important role in switching from immunity to symbiosis through the interaction with OsCERK1 in rice.

Endoplasmic reticulum (ER) stress is caused by the stress-induced accumulation of unfolded proteins in the ER. Several compounds are used to induce the unfolded protein response (UPR) in animals, with different modes of action, but which ER stress-inducing drugs induce ER stress in microalgae or land plants is unclear. In this study, we examined the effects of seven chemicals that were reported to induce ER stress in animals on the growth, UPR gene expression and fatty acid profiles of Chlamydomonas reinhardtii (Chlamydomonas) and Arabidopsis thaliana (Arabidopsis): 2-deoxyglucose, dithiothreitol (DTT), tunicamycin (TM), thapsigargin, brefeldin A (BFA), monensin (MON) and eeyarestatin I. In both model photosynthetic organisms, DTT, TM, BFA and MON treatment induced ER stress, as indicated by the induction of spliced bZIP1 and bZIP60, respectively. In Chlamydomonas, DTT, TM and BFA treatment induced the production of transcripts related to lipid biosynthesis, but MON treatment did not. In Arabidopsis, DTT, TM, BFA and MON inhibited seed germination and seedling growth with the activation of bZIP60. These findings lay the foundation for using four types of ER stress-inducing drugs in photosynthetic organisms, and they help uncover the mode of action of each compound.

Maltodextrin metabolism is thought to be involved in both starch initiation and degradation. In this study, potato tuber discs from transgenic lines containing antisense constructs against the plastidial and cytosolic isoforms of α-glucan phosphorylase and phosphoglucomutase were used to evaluate their influences on the conversion of externally supplied glucose-1-phosphate into soluble maltodextrins, as compared to wild-type potato tubers (Solanum tuberosum L. cv. Desiree). Relative maltodextrin amounts analyzed by capillary electrophoresis with laser-induced fluorescence revealed that tuber discs could immediately uptake glucose-1-phosphate and use it to produce maltooligosaccharides with a degree of polymerization of up to 30, as opposed to tubers repressing the plastidial glucan phosphorylase. The results presented here support previous indications that a specific transporter for glucose-1-phosphate may exist in both the plant cells and the plastidial membranes, thereby allowing a glucose-6-phosphate-independent transport. Furthermore, it confirms that the plastidial glucan phosphorylase is responsible for producing longer maltooligosaccharides in the plastids by catalyzing a glucosyl polymerization reaction when glucose-1-phosphate is available. All these findings contribute to a better understanding of the role of the plastidial phosphorylase as a key enzyme directly involved in the synthesis and degradation of glucans and their implication on starch metabolism.

Jasmonic acid (JA) regulates plant growth, development and stress responses. Coronatine insensitive 1 (COI1) and jasmonate zinc-finger inflorescence meristem-domain (JAZ) proteins form a receptor complex for jasmonoyl-l-isoleucine, a biologically active form of JA. Three COIs (OsCOI1a, OsCOI1b and OsCOI2) are encoded in the rice genome. In the present study, we generated mutants for each rice COI gene using genome editing to reveal the physiological functions of the three rice COIs. The oscoi2 mutants, but not the oscoi1a and oscoi1b mutants, exhibited severely low fertility, indicating the crucial role of OsCOI2 in rice fertility. Transcriptomic analysis revealed that the transcriptional changes after methyl jasmonate (MeJA) treatment were moderate in the leaves of oscoi2 mutants compared to those in the wild type or oscoi1a and oscoi1b mutants. MeJA-induced chlorophyll degradation and accumulation of antimicrobial secondary metabolites were suppressed in oscoi2 mutants. These results indicate that OsCOI2 plays a central role in JA response in rice leaves. In contrast, the assessment of growth inhibition upon exogenous application of JA to seedlings of each mutant revealed that rice COIs are redundantly involved in shoot growth, whereas OsCOI2 plays a primary role in root growth. In addition, a co-immunoprecipitation assay showed that OsJAZ2 and OsJAZ5 containing divergent Jas motifs physically interacted only with OsCOI2, whereas OsJAZ4 with a canonical Jas motif interacts with all three rice COIs. The present study demonstrated the functional diversity of rice COIs, thereby providing clues to the mechanisms regulating the various physiological functions of JA.