TY - JOUR
AU - Minorsky, Peter V.
AB - Calcium Signaling and Sugar Homeostasis The calcium sensor Calcineurin B-like protein (CBL) participates in calcium signal transduction by interacting with CBL-interacting protein kinase (CIPK). CBL-CIPK pathways have been reported to participate in a range of biological processes. In Arabidopsis (Arabidopsis thaliana), there are 10 CBLs and 26 CIPKs. The diversities of expression patterns, subcellular localization, and the downstream targets of CBL-CIPK pathways make the networks flexible and complex. Deng et al. (pp. 236–249) now describe a CBL2-CIPK6-Tonoplast-Localized Sugar Transporter2 (TST2) molecular module in cotton (Gossypium hirsutum) that regulates plant sugar homeostasis, in particular Glc homeostasis. Sugars are the main carbon and energy sources for cellular activities, as well as substrates for cell wall construction and cellular solutes for osmotic balance. In plants, it is necessary to store sugars and distribute them by transporters to maintain a stable supply in sink tissues. In Arabidopsis, there are 79 sugar transporters, including 53 types of monosaccharide transporters (MSTs). Abundant sugar transporters from the three families are localized on the tonoplast membrane to regulate sugar homeostasis. The authors show that GhCIPK6 is recruited to the tonoplast by GhCBL2 and interacts with GhTST2. Overexpression of either GhCBL2, GhCIPK6, or GhTST2 was sufficient to promote sugar accumulation in transgenic cotton, whereas RNA interference-mediated knockdown of GhCIPK6 expression or CRISPR-Cas9-mediated knockout of GhTST2 resulted in significantly decreased Glc content. Mutation of GhCBL2 or GhTST2 in GhCIPK6-overexpressing cotton reinstated sugar content comparable to that of the wild type. This enhanced knowledge of the molecular players behind plant sugar homeostasis may be exploited to improve sugar content and abiotic stress resistance in plants. Hydrogen Sulfide and Cadmium Stress Cadmium (Cd) negatively impacts plant yield by causing growth inhibition, chlorosis, or even the death of entire plants. Cd interferes with the uptake and translocation of other ions, damages protein and DNA/RNA functions, and increases the accumulation of reactive oxygen species (ROS). An increasing number of reports indicate that hydrogen sulfide (H2S), a plant gasotransmitter, functions in plant responses to Cd stress. For example, exogenous application of NaHS, which releases H2S, alleviates the toxic effects of Cd in plants. Two enzymes, l-CYS DESULFHYDRASE (LCD) and d-CYS DESULFHYDRASE (DCD), degrade l-Cys and d-Cys, respectively, into H2S, pyruvate, and NH3. Both of these enzymes could potentially be involved in mitigating Cd toxicity. Indeed, lcd mutants are more sensitive to Cd stress than is the wild type, and plants overexpressing LCD are insensitive to Cd stress. Whether DCD plays a role in plant Cd tolerance is unknown. Zhang et al. (pp. 345–357) now report that DCD-mediated H2S production enhances Cd tolerance in Arabidopsis. When subjected to Cd stress, a dcd mutant accumulated more Cd and ROS and showed increased Cd sensitivity, whereas transgenic lines overexpressing DCD had decreased Cd and ROS levels and were more tolerant to Cd stress compared to wild-type plants. Furthermore, the expression of DCD was stimulated by Cd stress, and this up-regulation was mediated by a Cd-induced transcription factor, WRKY13, which binds to the DCD promoter. The higher Cd sensitivity of the wrky13-3 mutant was rescued by the overexpression of DCD. These results suggest that Cd-induced WRKY13 activates DCD expression to increase the production of H2S, leading to higher Cd tolerance in plants. New Insights into Tomato Ripening Fruit ripening results in changes in pigmentation, enhanced aromas and flavors, and flesh softening. The ripening process involves massive changes in gene expression patterns. RIPENING INHIBITOR (RIN) is a transcription factor that plays a major role in regulating fruit ripening in tomato (Solanum lycopersicum). The rin mutant has received substantial attention because it shows a distinct nonripening phenotype and has been commercially used for breeding cultivars with an extended shelf life. The rin mutant fruits never turn red and do not soften, a phenotype that lasts for several months or longer. Several recent findings have revealed that the role of RIN is more complex than originally imagined. Contrary to earlier views, RIN is indispensable for full ripening but not for the induction of ripening. Moreover, the rin mutation is not a null mutation but instead converts the encoded transcriptional activator into a repressor. Ito et al. (pp. 80–95) have revealed new aspects of RIN function by characterizing a series of allelic mutations within the RIN locus that were produced by CRISPR/Cas9. Unlike the nonripening phenotype of the original rin mutant allele, two genome-edited alleles of the RIN locus, a knockout allele and an allele that removes the C-terminal domain from the encoded protein (rinG2), showed small increases in lycopene accumulation. However, the alleles showed opposite effects with respect to flesh softening, with the knockout allele exhibiting aberrant softening in excess of that observed in the wild type and the allele without the C-terminal domain (i.e. rinG2) exhibiting extended shelf life equivalent to that of the rin mutant. Based on these results, the authors propose a model that accounts for its diverse modes of transcriptional regulation during tomato ripening. Cineole Synthesis in Kiwifruit Over 80 volatile organic compounds have been described in ripe kiwifruit (Actinidia spp.), with the most important odor-active compounds typically being straight-chain esters and C6 aldehydes and alcohols. Another odorant proposed to be important in the flavor of ‘Hort16A’, one of three major cultivars of kiwifruit, is the terpene 1,8-cineole. Terpene volatiles are found in many important fruit crops but their relationship to flavor is poorly understood. By means of sensory descriptive and discriminant analysis, Zeng et al. (pp. 51–66) demonstrate that the monoterpene 1,8-cineole contributes a eucalyptus-like note to the aroma of ripe ‘Hort16A’ kiwifruit. The final step in terpene biosynthesis is catalyzed by the terpene synthase (TPS) enzyme family. A number of TPS genes have been shown to be responsible for volatile organic compound production in different kiwifruit tissues and developmental stages. However, definitive genetic and biochemical evidence for signature flavor production in kiwifruit cultivars has not been reported. In the current study, the authors report that transient overexpression of AcTPS1b in Actinidia eriantha fruit confirmed that the enzyme encoded by this gene plays a major role in 1,8-cineole production in Actinidia. Their results also highlight the complexity of terpene production in kiwifruit with multiple TPS genes from a single locus producing different terpene products in different tissues and at different times. This diversity can potentially be utilized in breeding novel flavored kiwifruit, and in understanding the structure-function relationships between different TPS enzymes. Small Secreted Peptides of a Model Legume: A Database Plant small secreted peptides (SSPs) have emerged as an important class of regulatory molecules involved in plant growth, development, plant-microbe interactions, and stress tolerance. SSPs are typically encoded within preproteins of 100–250 amino acids, which are subsequently processed into shorter bioactive peptides of ∼5–50 residues that act at very low, often nanomolar physiological concentrations. SSP-coding genes are frequently overlooked because genome annotation pipelines generally ignore small open reading frames, which are those most likely to encode SSPs. Also, SSP-coding small open reading frames are often expressed at low levels or only under specific conditions, and thus are often underrepresented. SSPs are of particular significance for legumes, since recent discoveries show that SSPs regulate symbiotic root nodulation and root development. Boschiero et al. (pp. 399–413) previously identified 4,439 SSP-encoding genes in the model legume Medicago truncatula. To support systematic characterization and annotation of these putative SSP-encoding genes, they have developed the M. truncatula Small Secreted Peptide Database (MtSSPdb). MtSSPdb currently hosts (1) a compendium of M. truncatula SSP candidates with putative function and family annotations; (2) a large-scale M. truncatula RNA-sequencing-based gene expression atlas integrated with various analytical tools, including differential expression, coexpression, and pathway enrichment analyses; (3) an online plant SSP prediction tool capable of analyzing protein sequences at the genome scale following the same protocol used for identification of SSP genes; and (4) information about a library of synthetic peptides and root and nodule phenotyping data from synthetic peptide screens in planta. MtSSPdb is an important resource for the plant scientific community and has the potential to become the most complete database of SSPs in plants. Chlamydomonas and Vitamin B12 Nutrient amendment experiments suggest that B12 limits phytoplankton growth in many aquatic ecosystems. Eukaryotic algae cannot synthesize B12 and must instead obtain it from certain B12-producing prokaryotes. In many algae, B12 is required as a cofactor for the B12-dependent Met synthase enzyme, although some algae encode a B12-independent isoform of this enzyme (METE) and thus do not require B12 for growth. The chlorophyte Chlamydomonas reinhardtii possesses a functional copy of METE and so is B12-independent. Previously, researchers associated with Bunbury et al. (pp. 167–178) reported that they had been able, over the course of ∼500 cell generations, to generate a METE mutant of C. reinhardtii (metE7) by experimental evolution under conditions of high vitamin B12 concentration, demonstrating that sustained levels of B12 in the environment can drive METE gene loss. metE7 provides a unique opportunity to study the physiology of a nascent B12 auxotroph. In the present work, the authors characterized the responses of metE7 to different vitamin B12 regimes and compared them with the responses of its ancestral B12-independent strain as well as those of a closely related, naturally B12-dependent alga Lobomonas rostrata. The authors demonstrate that B12 deprivation of metE7 disrupts C1 metabolism, causes an accumulation of starch and triacylglycerides, and leads to a decrease in photosynthetic pigments, proteins, and free amino acids. B12 deprivation also caused a substantial increase in ROS, which preceded rapid cell death. Further improvements in survival under B12 limitation and an increase in B12 use efficiency were found after metE7 underwent a further period of experimental evolution, this time in coculture with a B12-producing bacterium. Therefore, although an early B12-dependent alga would likely be poorly adapted to coping with B12 deprivation, association with B12-producers can ensure long-term survival while also providing a suitable environment for evolving mechanisms to better tolerate B12 limitation. Author notes www.plantphysiol.org/cgi/doi/10.1104/pp.20.00442 © 2020 American Society of Plant Biologists. All Rights Reserved. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)
TI - On the Inside
JF - Plant Physiology
DO - 10.1104/pp.20.00442
DA - 2020-05-08
UR - https://www.deepdyve.com/lp/oxford-university-press/on-the-inside-S57eUu0iPI
SP - 1
EP - 2
VL - 183
IS - 1
DP - DeepDyve
ER -