A cargo sorting receptor mediates chloroplast protein trafficking through the secretory pathway.

Nucleus-encoded chloroplast proteins can be transported via the secretory pathway. The molecular mechanisms underlying the trafficking of chloroplast proteins between the intracellular compartments are largely unclear, and a cargo sorting receptor has not previously been identified in the secretory pathway. Here we report a cargo sorting receptor that is specifically present in Viridiplantae and mediates the transport of cargo proteins to the chloroplast. Using a forward genetic analysis, we identified a gene encoding a transmembrane protein (MtTP930) in barrel medic (Medicago truncatula). Mutation of MtTP930 resulted in impaired chloroplast function and a dwarf phenotype. MtTP930 is highly expressed in the aerial parts of the plant and is localized to the ER exit sites (ERESs) and Golgi. MtTP930 contains typical cargo sorting receptor motifs, interacts with Sar1, Sec12 and Sec24, and participates in coat protein II (COPII) vesicular transport. Importantly, MtTP930 can recognize the cargo proteins plastidial N-glycosylated nucleotide pyrophosphatase/ phosphodiesterase (MtNPP) and α-carbonic anhydrase (MtCAH) in the ER, and then transport them to the chloroplast via the secretory pathway. Mutation of a homolog of MtTP930 in Arabidopsis (Arabidopsis thaliana) resulted in a similar dwarf phenotype. Furthermore, MtNPP-GFP failed to localize to chloroplasts when transgenically expressed in Attp930 protoplasts, implying that these cargo sorting receptors are conserved in plants. These findings fill a gap in our understanding of the mechanism by which chloroplast proteins are sorted and transported via the secretory pathway.

Transverse mode switchable mode-locked laser with narrow bandwidth.

Transverse mode switchable ultrashort optical pulses with narrow bandwidths can create potential for exploring what we believe are new physical effects. We demonstrate the generation of transverse mode switchable ultrashort pulses with narrow bandwidths in an all-fiber mode-locked laser by exploring a mode-selective photonic lantern (MSPL). The laser cavity serves not only as a ring resonator but also as an intrinsic spectral filter. For mode-locking with the LP01, LP11a, and LP11b modes, the bandwidths are 3.0 nm, 86.7 pm and 101.7 pm, respectively. The narrowband pulses with higher-order modes are generated by an intrinsic spectral filter due to the spectral-domain intermodal interference. Mode-locked pulses with a signal-to-noise ratio better than 60 dB for LP01, LP11a, and LP11b modes are independently generated, i.e., transverse mode switchable by changing the input port of the MSPL. The mode-locked wavelength can be tuned for the LP11a mode and LP11b mode by adjusting the state of polarization. Furthermore, our experimental results also show that, the slope efficiency of LP11a and LP11b modes can be improved, by the use of LP11 mode pump scheme. We anticipate that, narrowband pulses with complex mode profiles can be generated by simultaneously phase-locked transverse and longitudinal modes.

The jasmonate pathway promotes nodule symbiosis and suppresses host plant defense in Medicago truncatula.

Root nodule symbiosis (RNS) between legume and rhizobia is a major source of nitrogen in agricultural systems. Effective symbiosis requires precise regulation of plant defense responses. The role of the defense hormone jasmonic acid in the immune response has been extensively studied. The current research shows that JA can play either a positive or negative regulatory role in RNS depending on its concentration, while the molecular mechanisms remain to be elucidated. Here, we found that inoculation with rhizobia Sm1021 induced the JA pathway response in Medicago truncatula, and blocking JA pathway significantly reduced the number of infection threads. Mutations in the MtMYC2 gene, a JA signaling master transcription factor, significantly inhibited rhizobia infection, terminal differentiation, and symbiotic cell formation. Combining RNA-seq and ChIP-seq, we discovered that MtMYC2 regulates the expression of nodule-specific MtDNF2, MtNAD1, and MtSymCRK to suppress host defense. MtMYC2 activates MtDNF1 expression to regulate the maturation of MtNCRs, which in turn promotes bacteroid formation. More importantly, MtMYC2 promotes the expression of MtIPD3 to participate in symbiotic signaling transduction. Notably, the MtMYC2-MtIPD3 transcriptional regulation module is specifically present in legumes. Additionally, The Mtmyc2 mutants exhibits a susceptible phenotype to Rhizoctonia solani. Collectively, our findings reveal the molecular mechanisms of the JA pathway in RNS and further broaden the understanding of JA in the plant-microbe interaction network.

The S-acylation cycle of transcription factor MtNAC80 influences cold stress responses in Medicago truncatula.

S-acylation is a reversible post-translational modification catalyzed by protein S-acyltransferases (PATs), and acyl protein thioesterases (APTs) mediate de-S-acylation. Although many proteins are S-acylated, how the S-acylation cycle modulates specific biological functions in plants is poorly understood. In this study, we report that the S-acylation cycle of transcription factor MtNAC80 is involved in the Medicago truncatula cold stress response. Under normal conditions, MtNAC80 localized to membranes through MtPAT9-induced S-acylation. In contrast, under cold stress conditions, MtNAC80 translocated to the nucleus through de-S-acylation mediated by thioesterases such as MtAPT1. MtNAC80 functions in the nucleus by directly binding the promoter of the glutathione S-transferase gene MtGSTU1 and promoting its expression, which enables plants to survive under cold stress by removing excess malondialdehyde and H2O2. Our findings reveal an important function of the S-acylation cycle in plants and provide insight into stress response and tolerance mechanisms.

Liquid-liquid phase separation in plants: Advances and perspectives from model species to crops.

Membraneless biomolecular condensates play important roles in both normal biological activities and responses to environmental stimuli in living organisms. Liquid‒liquid phase separation (LLPS) is an organizational mechanism that has emerged in recent years to explain the formation of biomolecular condensates. In the past decade, advances in LLPS research have contributed to breakthroughs in disease fields. By contrast, although LLPS research in plants has progressed over the past 5 years, it has been concentrated on the model plant Arabidopsis, which has limited relevance to agricultural production. In this review, we provide an overview of recently reported advances in LLPS in plants, with a particular focus on photomorphogenesis, flowering, and abiotic and biotic stress responses. We propose that many potential LLPS proteins also exist in crops and may affect crop growth, development, and stress resistance. This possibility presents a great challenge as well as an opportunity for rigorous scientific research on the biological functions and applications of LLPS in crops.

The PtdIns3P phosphatase MtMP promotes symbiotic nitrogen fixation via mitophagy in Medicago truncatula.

Symbiotic nitrogen fixation is a complex process in which legumes interact with rhizobia under nitrogen starvation. In this study, we found that myotubularin phosphatase (MtMP) is mainly expressed in roots and nodules in Medicago truncatula. MtMP promotes autophagy by dephosphorylating PtdIns3P on autophagosomes. The mp mutants inoculated with rhizobia showed a significant reduction in nitrogenase activity and significantly higher number of mitochondria than those of wild-type plants under nitrogen starvation, indicating that MtMP is involved in mitophagy of the infection zone. Mitophagy may provide carbon skeletons and nitrogen for the development of bacteroids and the reprogramming of infected cells. In conclusion, we found, for the first time, that myotubularin phosphatase is involved in autophagy in plants. MtMP-involved autophagy plays an active role in symbiotic nitrogen fixation. These results deepen our understanding of symbiotic nitrogen fixation.

Legume-specific SnRK1 promotes malate supply to bacteroids for symbiotic nitrogen fixation.

Nodulation is an energy-expensive behavior driven by legumes by providing carbon sources to bacteroids and obtaining nitrogen sources in return. The energy sensor sucrose nonfermenting 1-related protein kinase 1 (SnRK1) is the hub of energy regulation in eukaryotes. However, the molecular mechanism by which SnRK1 coordinates the allocation of energy and substances during symbiotic nitrogen fixation (SNF) remains unknown. In this study, we identified the novel legume-specific SnRK1α4, a member of the SnRK1 family that positively regulates SNF. Phenotypic analysis showed that nodule size and nitrogenase activity increased in SnRK1α4-overexpressing plants and decreased significantly in snrk1α4 mutants. We demonstrated that a key upstream kinase involved in nodulation, Does Not Make Infection 2 (DMI2), can phosphorylate SnRK1α4 at Thr175 to cause its activation. Further evidence clarified that SnRK1α4 phosphorylates the malate dehydrogenases MDH1/2 to promote malate production in the cytoplasm, supplying carbon sources to bacteroids. Therefore, our findings reveal an essential role of the DMI2-SnRK1α4-MDH pathway in supplying carbon sources to bacteroids for SNF and provide a new module for constructing cereal crops with SNF.

The thioesterase APT1 is a bidirectional-adjustment redox sensor.

The adjustment of cellular redox homeostasis is essential in when responding to environmental perturbations, and the mechanism by which cells distinguish between normal and oxidized states through sensors is also important. In this study, we found that acyl-protein thioesterase 1 (APT1) is a redox sensor. Under normal physiological conditions, APT1 exists as a monomer through S-glutathionylation at C20, C22 and C37, which inhibits its enzymatic activity. Under oxidative conditions, APT1 senses the oxidative signal and is tetramerized, which makes it functional. Tetrameric APT1 depalmitoylates S-acetylated NAC (NACsa), and NACsa relocates to the nucleus, increases the cellular glutathione/oxidized glutathione (GSH/GSSG) ratio through the upregulation of glyoxalase I expression, and resists oxidative stress. When oxidative stress is alleviated, APT1 is found in monomeric form. Here, we describe a mechanism through which APT1 mediates a fine-tuned and balanced intracellular redox system in plant defence responses to biotic and abiotic stresses and provide insights into the design of stress-resistant crops.

The histone methyltransferase SUVR2 promotes DSB repair via chromatin remodeling and liquid-liquid phase separation.

Maintaining genomic integrity and stability is particularly important for stem cells, which are at the top of the cell lineage origin. Here, we discovered that the plant-specific histone methyltransferase SUVR2 maintains the genome integrity of the root tip stem cells through chromatin remodeling and liquid-liquid phase separation (LLPS) when facing DNA double-strand breaks (DSBs). The histone methyltransferase SUVR2 (MtSUVR2) has histone methyltransferase activity and catalyzes the conversion of histone H3 lysine 9 monomethylation (H3K9me1) to H3K9me2/3 in vitro and in Medicago truncatula. Under DNA damage, the proportion of heterochromatin decreased and the level of DSB damage marker γ-H2AX increased in suvr2 mutants, indicating that MtSUVR2 promotes the compaction of the chromatin structure through H3K9 methylation modification to protect DNA from damage. Interestingly, MtSUVR2 was induced by DSBs to phase separate and form droplets to localize at the damage sites, and this was confirmed by immunofluorescence and fluorescence recovery after photobleaching experiments. The IDR1 and low-complexity domain regions of MtSUVR2 determined its phase separation in the nucleus, whereas the IDR2 region determined the interaction with the homologous recombinase MtRAD51. Furthermore, we found that MtSUVR2 drove the phase separation of MtRAD51 to form "DNA repair bodies," which could enhance the stability of MtRAD51 proteins to facilitate error-free homologous recombination repair of stem cells. Taken together, our study reveals that chromatin remodeling-associated proteins participate in DNA repair through LLPS.

Ultra-broadband LP11 mode converter with high purity based on long-period fiber grating and an integrated Y-junction.

We report an ultra-broadband LP11 mode converter with high purity based on integrated two shunt-wound long-period fiber gratings (LPFGs) and an adiabatic Y-junction, together with a high-order-mode bandpass filter. Two shunt-wound LPFGs are inscribed by CO2 laser in a two-mode fiber to achieve a 10 dB bandwidth of 50 nm and 51 nm at resonance wavelengths of 1530 nm and 1570 nm, respectively. Meanwhile, the Y-junction fabricated by lithography can be operated over S + C+L band to combine the converted LP11 mode. The presented ultra-broadband mode converter is able to achieve a mode conversion efficiency of 95%, together with a wavelength-dependent loss of less than 3 dB over the S + C+L band. This device has low modal crosstalk of 17 dB between the LP01 and LP11 modes, because most of the residual LP01 mode is further filtered by a high-order-mode bandpass filter at the output port of the Y-junction. The insertion loss of mode converter is estimated to be lower than 2.7 dB, due to the use of low loss polymer material during the fabrication. The proposed ultra-broadband LP11 mode converter with high purity is promising for the application of ultra-broadband mode-division-multiplexing transmission systems.

All-optical light manipulation based on graphene-embedded side-polished fiber.

We present a study of all-optical light manipulation arising in a graphene-embedded side-polished fiber (SPF) with a Norland Optical Adhesives (NOA)-coated structure. With the help of the Pauli blocking effect, such an all-fiber device serves to manage the loss of transverse-electric-polarized light when the control light and the signal light are polarized along the direction parallel to the graphene surface. The insertion loss of this device can be effectively reduced with the NOA coating. An enhanced interaction between the graphene and the propagated light can be achieved via the strong evanescent field of the SPF and longer interaction length. This results in effective all-optical manipulation of light with a modulation depth of 10.4 dB (or modulation efficiency of ∼91%) and a modulation slope of ∼1.3, where the required control power is only about 14 dBm. The device has broadband operation wavelength. The insertion loss for both the signal light and the control light are only about 0.6 dB. The experimental results are well-fitting with the simulation study. Such an all-fiber device has the potential for all-optical signal processing.

Ultrasensitive temperature sensor and mode converter based on a modal interferometer in a two-mode fiber.

This paper presents an ultrasensitive temperature sensor and tunable mode converter based on an isopropanol-sealed modal interferometer in a two-mode fiber. The modal interferometer consists of a tapered two-mode fiber (TTMF) sandwiched between two single-mode fibers. The sensor provides high-sensitivity temperature sensing by taking advantages of TTMF, isopropanol and the Vernier-like effect. The TTMF provides a uniform modal interferometer with LP01 and LP11 modes as well as strong evanescent field on its surface. The temperature sensitivity of the sensor can be improved due to the high thermo-optic coefficient of isopropanol. The Vernier-like effect based on the overlap of two interference spectra is applied to magnify the sensing capabilities with a sensitivity magnification factor of 58.5. The temperature sensor is implemented by inserting the modal interferometer into an isopropanol-sealed capillary. The experimental and calculated results show the transmission spectrum exhibit blue shift with increasing ambient temperature. Experimental results show that the isopropanol-sealed modal interferometer provides a temperature sensitivity up to -140.5 nm/°C. The interference spectrum has multiple dips at which the input LP01 mode is converted to the LP11 mode. This modal interferometer acts as a tunable multi-channel mode converter. The mode converter that can be tuned by varying temperature and mode switch is realized.

Broadband mode-selective couplers based on tapered side-polished fibers.

We propose the broadband mode-selective coupler (MSC) formed with a side-polished six mode fiber (6MF) and a tapered side-polished small core single-mode fiber (SC-SMF) or an SMF. The MSCs are designed to allow the LP01 mode in the SC-SMF and SMF to completely couple to the LP01, LP11, LP21, LP02, LP31, LP12 modes in the 6MF over a broadband wavelength range. The phase-matching conditions of the MSCs are satisfied by tapering the SC-SMF and SMF to specific diameters. The tapered fibers are side-polished to designed residual fiber thickness using the wheel polishing technique. The effective indices of the side-polished fibers are measured with the prism coupling method. The MSCs provide high coupling ratio and high mode purity. High coupling efficiencies in excess of 81% for all the higher-order modes are obtained in the wavelength range 1530-1600 nm. For the LP01, LP11, LP21, LP02, LP31, LP12 MSCs at 1550 nm, the coupling ratios are 96.2%, 99.8%, 89.5%, 85.0%, 90.9%, 96.1%, respectively, and the mode purity of the MSCs is higher than 88.0%. The loss of the MSCs is lower than 1.8 dB in the wavelength range 1530-1600 nm. This device can be applied in broadband mode-division multiplexing transmission systems.

Highly efficient second harmonic generation of thin film lithium niobate nanograting near bound states in the continuum.

Bound states in the continuum (BICs) are ubiquitous physical phenomena where such states occur due to strong coupling between leaky modes in side lossy systems. BICs in meta-optics and nanophotonics enable optical mode confinement to strengthen local field enhancement in nonlinear optics. In this study, we numerically investigate second-harmonic generation (SHG) in the vicinity of BICs with a photonic structure comprising one-dimensional nanogratings and a slab waveguide made of lithium niobate (LiNbO3, LN). By breaking the symmetry of LN nanogratings, BICs transition to quasi-BICs, which enable strong local field confinement inside LN slab waveguide to be supported, thereby resulting in improving SHG conversion with lower pump power of fundamental frequency (FW). With a peak intensity of 1.33 GW cm-2at the FW, our structure features a second-harmonic conversion efficiency up to 8.13 × 10-5at quasi-BICs. We believe that our results will facilitate the application of LN in integrated nonlinear nanophotonic.

Autophagy in Plant Abiotic Stress Management.

Plants can be considered an open system. Throughout their life cycle, plants need to exchange material, energy and information with the outside world. To improve their survival and complete their life cycle, plants have developed sophisticated mechanisms to maintain cellular homeostasis during development and in response to environmental changes. Autophagy is an evolutionarily conserved self-degradative process that occurs ubiquitously in all eukaryotic cells and plays many physiological roles in maintaining cellular homeostasis. In recent years, an increasing number of studies have shown that autophagy can be induced not only by starvation but also as a cellular response to various abiotic stresses, including oxidative, salt, drought, cold and heat stresses. This review focuses mainly on the role of autophagy in plant abiotic stress management.

Multigene editing reveals that MtCEP1/2/12 redundantly control lateral root and nodule number in Medicago truncatula.

The multimember CEP (C-terminally Encoded Peptide) gene family is a complex group that is involved in various physiological activities in plants. Previous studies demonstrated that MtCEP1 and MtCEP7 control lateral root formation or nodulation, but these studies were based only on gain of function or artificial miRNA (amiRNA)/RNAi approaches, never knockout mutants. Moreover, an efficient multigene editing toolkit is not currently available for Medicago truncatula. Our quantitative reverse transcription-PCR data showed that MtCEP1, 2, 4, 5, 6, 7, 8, 9, 12, and 13 were up-regulated under nitrogen starvation conditions and that MtCEP1, 2, 7, 9, and 12 were induced by rhizobial inoculation. Treatment with synthetic MtCEP peptides of MtCEP1, 2, 4, 5, 6, 8, and 12 repressed lateral root emergence and promoted nodulation in the R108 wild type but not in the cra2 mutant. We optimized CRISPR/Cas9 [clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9] genome editing system for M. truncatula, and thus created single mutants of MtCEP1, 2, 4, 6, and 12 and the double mutants Mtcep1/2C and Mtcep5/8C; however, these mutants did not exhibit significant differences from R108. Furthermore, a triple mutant Mtcep1/2/12C and a quintuple mutant Mtcep1/2/5/8/12C were generated and exhibited more lateral roots and fewer nodules than R108. Overall, MtCEP1, 2, and 12 were confirmed to be redundantly important in the control of lateral root number and nodulation. Moreover, the CRISPR/Cas9-based multigene editing protocol provides an additional tool for research on the model legume M. truncatula, which is highly efficient at multigene mutant generation.

A CEP Peptide Receptor-Like Kinase Regulates Auxin Biosynthesis and Ethylene Signaling to Coordinate Root Growth and Symbiotic Nodulation in Medicago truncatula.

Because of the large amount of energy consumed during symbiotic nitrogen fixation, legumes must balance growth and symbiotic nodulation. Both lateral roots and nodules form on the root system, and the developmental coordination of these organs under conditions of reduced nitrogen (N) availability remains elusive. We show that the Medicago truncatula COMPACT ROOT ARCHITECTURE2 (MtCRA2) receptor-like kinase is essential to promote the initiation of early symbiotic nodulation and to inhibit root growth in response to low N. C-TERMINALLY ENCODED PEPTIDE (MtCEP1) peptides can activate MtCRA2 under N-starvation conditions, leading to a repression of YUCCA2 (MtYUC2) auxin biosynthesis gene expression, and therefore of auxin root responses. Accordingly, the compact root architecture phenotype of cra2 can be mimicked by an auxin treatment or by overexpressing MtYUC2, and conversely, a treatment with YUC inhibitors or an MtYUC2 knockout rescues the cra2 root phenotype. The MtCEP1-activated CRA2 can additionally interact with and phosphorylate the MtEIN2 ethylene signaling component at Ser643 and Ser924, preventing its cleavage and thereby repressing ethylene responses, thus locally promoting the root susceptibility to rhizobia. In agreement with this interaction, the cra2 low nodulation phenotype is rescued by an ein2 mutation. Overall, by reducing auxin biosynthesis and inhibiting ethylene signaling, the MtCEP1/MtCRA2 pathway balances root and nodule development under low-N conditions.

The Chromosome-Level Genome Sequence of the Autotetraploid Alfalfa and Resequencing of Core Germplasms Provide Genomic Resources for Alfalfa Research.

Alfalfa (Medicago sativa) is one of the most important forage crops in the world; however, its molecular genetics and breeding research are hindered due to the lack of a high-quality reference genome. Here, we report a de novo assembled 816-Mb high-quality, chromosome-level haploid genome sequence for 'Zhongmu No.1' alfalfa, a heterozygous autotetraploid. The contig N50 is 3.92 Mb, and 49 165 genes are annotated in the genome. The alfalfa genome is estimated to have diverged from M. truncatula approximately 8 million years ago. Genomic population analysis of 162 alfalfa accessions revealed high genetic diversity, weak population structure, and extensive gene flow from wild to cultivated alfalfa. Genome-wide association studies identified many candidate genes associated with important agronomic traits. Furthermore, we showed that MsFTa2, a Flowering Locus T homolog, whose expression is upregulated in salt-resistant germplasms, may be associated with fall dormancy and salt resistance. Taken together, these genomic resources will facilitate alfalfa genetic research and agronomic improvement.