Yueying Zhang

Melatonin acts as a crucial signaling and antioxidant molecule with multiple physiological functions in organisms. To explore effects of exogenous melatonin on postharvest browning and its possible mechanisms in litchi fruit, ‘Ziniangxi’ litchi fruits were treated with an aqueous solution of melatonin at 0.4 mM and then stored at 25 °C for 8 days. The results revealed that melatonin strongly suppressed pericarp browning and delayed discoloration during storage. Melatonin treatment reduced relative membrane-leakage rate and inhibited the generation of superoxide radicals (O2–·), hydrogen peroxide (H2O2), and malondialdehyde (MDA). Melatonin treatment markedly promoted the accumulation of endogenous melatonin; delayed loss of total phenolics, flavonoids, and anthocyanins; and enhanced the activities of antioxidant enzymes, including superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), ascorbate peroxidase (APX, EC 1.11.1.11), and glutathione reductase (GR, EC 1.6.4.2). By contrast, the activities of browning-related enzymes including polyphenoloxidase (PPO, EC 1.10.3.1) and peroxidase (POD, EC 1.11.1.7) were reduced. In addition, melatonin treatment up-regulated the expression of four genes encoding enzymes for repair of oxidized proteins, including LcMsrA1, LcMsrA2, LcMsrB1, and LcMsB2. These findings indicate that the delay of pericarp browning and senescence by melatonin in harvested litchi fruit could be attributed to the maintenance of redox homeostasis by the improvement of the antioxidant capacity and modulation of the repair of oxidatively damaged proteins.

Although the control of organ size is a fundamental question in developmental biology, little is known about the genetic and molecular mechanisms that determine the final size of seeds in plants. We previously demonstrated that the ubiquitin receptor DA1 acts synergistically with the E3 ubiquitin ligases DA2 and ENHANCER1 OF DA1 (EOD1)/BIG BROTHER to restrict seed growth in Arabidopsis thaliana. Here, we describe UBIQUITIN-SPECIFIC PROTEASE15 (UBP15), encoded by SUPPRESSOR2 OF DA1 (SOD2), which acts maternally to regulate seed size by promoting cell proliferation in the integuments of ovules and developing seeds. The sod2/ubp15 mutants form small seeds, while overexpression of UBP15 increases seed size of wild-type plants. Genetic analyses indicate that UBP15 functions antagonistically in a common pathway with DA1 to influence seed size, but does so independently of DA2 and EOD1. Further results reveal that DA1 physically associates with UBP15 in vitro and in vivo and modulates the stability of UBP15. Therefore, our findings establish a genetic and molecular framework for the regulation of seed size by four ubiquitin-related proteins DA1, DA2, EOD1, and UBP15 and suggest that they are promising targets for increasing seed size in crops.

Liquid-liquid phase separation plays an important role in a variety of cellular processes, including the formation of membrane-less organelles, the cytoskeleton, signalling complexes, and many other biological supramolecular assemblies. Studies on the molecular basis of phase separation in cells have focused on protein-driven phase separation. In contrast, there is limited understanding on how RNA specifically contributes to phase separation. Here, we described a phase-separation-like phenomenon that SHORT ROOT (SHR) RNA undergoes in cells. We found that an RNA G-quadruplex (GQ) forms in SHR mRNA and is capable of triggering RNA phase separation under physiological conditions, suggesting that GQs might be responsible for the formation of the SHR phase-separation-like phenomenon in vivo. We also found the extent of GQ-triggered-phase-separation increases on exposure to conditions which promote GQ. Furthermore, GQs with more G-quartets and longer loops are more likely to form phase separation. Our studies provide the first evidence that RNA can adopt structural motifs to trigger and/or maintain the specificity of RNA-driven phase separation.

BACKGROUND: Guanine-rich sequences are able to form complex RNA structures termed RNA G-quadruplexes in vitro. Because of their high stability, RNA G-quadruplexes are proposed to exist in vivo and are suggested to be associated with important biological relevance. However, there is a lack of direct evidence for RNA G-quadruplex formation in living eukaryotic cells. Therefore, it is unclear whether any purported functions are associated with the specific sequence content or the formation of an RNA G-quadruplex structure. RESULTS: Using rG4-seq, we profile the landscape of those guanine-rich regions with the in vitro folding potential in the Arabidopsis transcriptome. We find a global enrichment of RNA G-quadruplexes with two G-quartets whereby the folding potential is strongly influenced by RNA secondary structures. Using in vitro and in vivo RNA chemical structure profiling, we determine that hundreds of RNA G-quadruplex structures are strongly folded in both Arabidopsis and rice, providing direct evidence of RNA G-quadruplex formation in living eukaryotic cells. Subsequent genetic and biochemical analyses show that RNA G-quadruplex folding is able to regulate translation and modulate plant growth. CONCLUSIONS: Our study reveals the existence of RNA G-quadruplex in vivo and indicates that RNA G-quadruplex structures act as important regulators of plant development and growth.

Abstract Cellular RNAs are heterogeneous with respect to their alternative processing and secondary structures, but the functional importance of this complexity is still poorly understood. A set of alternatively processed antisense non-coding transcripts, which are collectively called COOLAIR , are generated at the Arabidopsis floral-repressor locus FLOWERING LOCUS C ( FLC ) 1 . Different isoforms of COOLAIR influence FLC transcriptional output in warm and cold conditions 2–7 . Here, to further investigate the function of COOLAIR , we developed an RNA structure-profiling method to determine the in vivo structure of single RNA molecules rather than the RNA population average. This revealed that individual isoforms of the COOLAIR transcript adopt multiple structures with different conformational dynamics. The major distally polyadenylated COOLAIR isoform in warm conditions adopts three predominant structural conformations, the proportions and conformations of which change after cold exposure. An alternatively spliced, strongly cold-upregulated distal COOLAIR isoform 6 shows high structural diversity, in contrast to proximally polyadenylated COOLAIR . A hyper-variable COOLAIR structural element was identified that was complementary to the FLC transcription start site. Mutations altering the structure of this region changed FLC expression and flowering time, consistent with an important regulatory role of the COOLAIR structure in FLC transcription. Our work demonstrates that isoforms of non-coding RNA transcripts adopt multiple distinct and functionally relevant structural conformations, which change in abundance and shape in response to external conditions.