Phase-matching of high-order harmonic generation in the extreme ultraviolet region with orbital angular momentum.

High-order harmonics can generate vortex beams with orbital angular momentum (OAM) in the extreme ultraviolet region. However, experimental research on their phase-matching (PM) characteristics is limited. In this study, vortex high-order harmonic generation (HHG) in the extreme ultraviolet region was generated with Ar gas. Phase-matched HHG with OAM was obtained by optimizing the focus position, laser energy, and gas pressure. The dependence of the PM characteristics on these parameters was analyzed. In addition, we conducted an experimental analysis of the dimensional properties of vortex harmonics under PM conditions. This study is a contribution towards the intense vortex high-order harmonic light sources and their applications.

Resonance absorption of the inner shell during high-order harmonic generation.

In this work, we report the observation of resonance absorption of the inner shell during the high-order harmonic generation (HHG) from xenon (Xe) and krypton (Kr). The absorption peaks show a periodic variation with the change of carrier-envelope phase of driving laser pulses and the delay of two-color laser field, which indicates the absorption peaks come from the collective multielectron effects during the HHG. With the increase of gas pressure, the depth of absorption peak will continue to increase, while due to the phase matching effect, there will be an optimal pressure for the intensity of harmonic signal. Our experimental results pave the way to uncover the physical mechanism of the collective multielectron effects involving inner-shell electrons in the HHG process.

The AWPM-19 Family Protein OsPM1 Mediates Abscisic Acid Influx and Drought Response in Rice.

Abscisic acid (ABA) regulates plant stress responses and development. However, how the ABA signal is transmitted in response to stresses remains largely unclear, especially in monocots. In this study, we found that rice (Oryza sativa) OsPM1 (PLASMA MEMBRANE PROTEIN1), encoded by a gene of AWPM-19 like family, mediates ABA influx through the plasma membrane. OsPM1 is predominantly expressed in vascular tissues, guard cells, and mature embryos. Phenotypic analysis of overexpression, RNA interference (RNAi), and knockout (KO) lines showed that OsPM1 is involved in drought responses and seed germination regulation. 3H-(±)ABA transport activity and fluorescence resonance energy transfer assays both demonstrated that OsPM1 facilitates ABA uptake into cells. The physiological isomer of ABA, (+)-ABA, is the preferred substrate of OsPM1. Higher ABA accumulation and faster stomatal closure in response to ABA treatment were observed in the overexpression lines compared with the wild-type control. Many ABA-responsive genes were upregulated more in the OsPM1-overexpression lines but less in the RNAi lines compared with wild-type plants. Further investigation revealed that OsPM1 expression is regulated by the AREB/ABF family transcription factor OsbZIP46. Our results thus revealed that OsPM1 is an ABA influx carrier that plays an important role in drought responses.

New perspectives on the plant PARP family: Arabidopsis PARP3 is inactive, and PARP1 exhibits predominant poly (ADP-ribose) polymerase activity in response to DNA damage.

BACKGROUND: Poly (ADP-ribosyl) ation (PARylation) is an important posttranslational modification that regulates DNA repair, gene transcription, stress responses and developmental processes in multicellular organisms. Poly (ADP-ribose) polymerase (PARP) catalyzes PARylation by consecutively adding ADP-ribose moieties from NAD+ to the amino acid receptor residues on target proteins. Arabidopsis has three canonical PARP members, and two of these members, AtPARP1 and AtPARP2, have been demonstrated to be bona fide poly (ADP-ribose) polymerases and to regulate DNA repair and stress response processes. However, it remains unknown whether AtPARP3, a member that is highly expressed in seeds, has similar biochemical activity to that of AtPARP1 and AtPARP2. Additionally, although both the phylogenetic relationships and structural similarities indicate that AtPARP1 and AtPARP2 correspond to animal PARP1 and PARP2, respectively, two previous studies have indicated that AtPARP2, and not AtPARP1, accounts for most of the PARP activity in Arabidopsis, which is contrary to the knowledge that PARP1 is the predominant PARP in animals. RESULTS: In this study, we obtained both in vitro and in vivo evidence demonstrating that AtPARP3 does not act as a typical PARP in Arabidopsis. Domain swapping and point mutation assays indicated that AtPARP3 has lost NAD+-binding capability and is inactive. In addition, our results showed that AtPARP1 was responsible for most of the PARP enzymatic activity in response to the DNA damage-inducing agents zeocin and methyl methanesulfonate (MMS) and was more rapidly activated than AtPARP2, which supports that AtPARP1 remains the predominant PARP member in Arabidopsis. AtPARP1 might first become activated by binding to damaged sites, and AtPARP2 is then poly (ADP-ribosyl) ated by AtPARP1 in vivo. CONCLUSIONS: Collectively, our biochemical and genetic analysis results strongly support the notion that AtPARP3 has lost poly (ADP-ribose) polymerase activity in plants and performs different functions from those of AtPARP1 and AtPARP2. AtPARP1, instead of AtPARP2, plays the predominant role in PAR synthesis in both seeds and seedlings. These data bring new insights into our understanding of the physiological functions of plant PARP family members.

OsERF101, an ERF family transcription factor, regulates drought stress response in reproductive tissues.

KEY MESSAGE: An ERF transcription factor OsERF101 is predominantly expressed in rice reproductive tissues and plays an important role in improving rice seed setting rate under drought stress. Drought reduces grain yield due to the cumulative damage effects to plant vegetative and reproductive developmental processes. However, the genes involved in these processes are still not completely understood. In this study, we identified a gene named OsERF101 as an important positive regulator in the adaptive responses to dehydration stress during the reproductive and vegetative stages. This gene encodes a member of APETALA2/Ethylene-Responsive Element Binding Protein (AP2/EREBP) family. OsERF101 was predominantly expressed in flowers, particularly in the tapetum and microspores under normal growth conditions. It was induced by drought, PEG6000 and abscisic acid (ABA) in leaves. During the vegetative stage, OsERF101-overexpression plants were more resistant to osmotic stress caused by PEG6000 compared to the control plants. They also had higher survival and seed setting rates than wild type when subjected to reproductive-stage drought stress. Further physiological analysis revealed that the pollen fertility was improved in the overexpression lines, while the knockout mutant and RNAi lines showed reduced pollen fertility and compromised drought tolerance during the reproductive stage. The increased proline content and peroxidase activity in OsERF101-overexpression plants might contribute to the improved drought-tolerance of plants. In addition, OsERF101-overexpression plants displayed ABA susceptible phenotype, in which the expression levels of ABA-responsive genes RD22, LEA3, and PODs were up-regulated. Taken together, our results indicate that OsERF101 is a gene that regulates dehydration responses during the vegetative and reproductive stages.

MID1 plays an important role in response to drought stress during reproductive development.

Drought during rice reproductive development results in yield loss. It is important to understand the functions of drought-responsive genes in reproductive tissues for improving rice yield under water-deficit conditions. We show here that MID1 (MYB Important for Drought Response1), encoding a putative R-R-type MYB-like transcription factor, can improve rice yield under drought. MID1 was primarily expressed in root and leaf vascular tissues, with low level in the tapetum, and was induced by drought and other abiotic stresses. Compared with wild type, MID1-overexpressing plants were more tolerant to drought at both vegetative and reproductive stages and produced more grains under water stress. MID1-overexpressing plants exhibited less severe anther defects such as deformed anther locules, abnormal tapetum, degenerated microspores and expanded middle layer, with improved pollen fertility and higher seed setting rate. MID1 was localized to the nucleus and could activate gene expression in yeast, and its homologs were identified in many other plants with high levels sequence similarity. In addition, candidate MID1-regulated genes were analyzed using RNA-seq and qRT-PCR, including genes crucial for stress responses and anther development, with altered expressions in the florets of MID1-overexpressing plants and RNAi lines. Furthermore, MID1 could bind to the promoters of two drought-related genes (Hsp17.0 and CYP707A5) and one anther developmental gene (KAR) according to ChIP-qPCR data. Our findings suggest that MID1 is a transcriptional regulator that promotes rice male development under drought by modulating the expressions of drought-related and anther developmental genes and provide valuable information for crop improvement.

Poly(ADP-ribose) polymerases regulate cell division and development in Arabidopsis roots.

Root organogenesis involves cell division, differentiation and expansion. The molecular mechanisms regulating root development are not fully understood. In this study, we identified poly(adenosine diphosphate (ADP)-ribose) polymerases (PARPs) as new players in root development. PARP catalyzes poly(ADP-ribosyl)ation of proteins by repeatedly adding ADP-ribose units onto proteins using nicotinamide adenine dinucleotide (NAD+ ) as the donor. We found that inhibition of PARP activities by 3-aminobenzomide (3-AB) increased the growth rates of both primary and lateral roots, leading to a more developed root system. The double mutant of Arabidopsis PARPs, parp1parp2, showed more rapid primary and lateral root growth. Cyclin genes regulating G1-to-S and G2-to-M transition were up-regulated upon treatment by 3-AB. The proportion of 2C cells increased while cells with higher DNA ploidy declined in the roots of treated plants, resulting in an enlarged root meristematic zone. The expression level of PARP2 was very low in the meristematic zone but high in the maturation zone, consistent with a role of PARP in inhibiting mitosis and promoting cell differentiation. Our results suggest that PARPs play an important role in root development by negatively regulating root cell division.

Frequency modulation of high-order harmonic generation in an orthogonally polarized two-color laser field.

We have experimentally investigated the frequency modulation of high-order harmonics in an orthogonally polarized two-color laser field consisting of a mid-infrared 1800nm fundamental pulse and its second harmonic pulse. It is demonstrated that the high harmonic spectra can be fine-tuned as we slightly change the relative delay of the two-color laser pulses. By analyzing the relative frequency shift of each harmonic at different two-color delays, the nonadiabatic spectral shift induced by the rapid variation of the intensity-dependent intrinsic dipole phase can be distinguished from the blueshift induced by the change of the refractive index during self-phase modulation (SPM). Our comprehensive analysis shows that the frequency modulation pattern is a reflection of the average emission time of high-order harmonic generation (HHG), thus offering a simple method to fine-tune the spectra of the harmonics on a sub-cycle time scale.

Effect of nuclear motion on spectral broadening of high-order harmonic generation.

High-order harmonic generation (HHG) in molecular targets is experimentally investigated in order to reveal the role of the nuclear motion played in the harmonic generation process. An obvious broadening in the harmonic spectrum from the H2 molecule is observed in comparison with the harmonic spectrum generated from other molecules with relatively heavy nuclei. We also find that the harmonic yield from the H2 molecule is much weaker than the yield from those gas targets with the similar ionization potentials, such as Ar atom and N2 molecule. The yield suppression and the spectrum broadening of HHG can be attributed to the vibrational motion of nuclear induced by the driving laser pulse. Moreover, the one-dimensional (1D) time-dependent Schrödinger equation (TDSE) with the non-Born-Oppenheimer (NBO) treatment is numerically solved to provide a theoretical support to our explanation.

Ultrafast Excitation of an Inner-Shell Electron by Laser-Induced Electron Recollision.

Extreme ultraviolet attosecond pulses, generated by a process known as laser-induced electron recollision, are a key ingredient for attosecond metrology, providing a tool to precisely initiate and probe subfemtosecond dynamics in atoms, molecules, and solids. However, extending attosecond metrology to scrutinize the dynamics of the inner-shell electrons is a challenge, that is because of the lower efficiency in generating the required soft x-ray (ℏω>300 eV) attosecond bursts. A way around this problem is to use the recolliding electron to directly initiate the desired inner-shell process, instead of using the currently low flux x-ray attosecond sources. Such an excitation process occurs in a subfemtosecond time scale, and may provide the necessary "pump" step in a pump-probe experiment. Here we used a few cycle infrared (λ_{0}≈1800 nm) source and observed direct evidence for inner-shell excitations through the laser-induced electron recollision process. It is the first step toward time-resolved core-hole studies in the keV energy range with subfemtosecond time resolution.

Enhanced high-order harmonic generation from spatially prepared filamentation in argon.

We experimentally demonstrate enhanced high-order harmonic generation (HHG) from spatially prepared filamentation in Argon. Upon shifting the focus position of an elliptically polarized laser pulse over the filament induced by a linearly polarized laser pulse, an obvious enhancement of harmonic yield by nearly one order of magnitude is observed. The result could be interpreted in terms of the double contributions from both the excited states of target atom and the phase-matching effect of harmonic beam. In contrast to the enhancement phenomena, an obvious suppression of harmonic yield is also presented, which could be attributed to both the ground-state depletion and the plasma effect.

The ammonium/nitrate ratio is an input signal in the temperature-modulated, SNC1-mediated and EDS1-dependent autoimmunity of nudt6-2 nudt7.

AtNUDT7 was reported to be a negative regulator of EDS1-mediated immunity in Arabidopsis. However, the underlying molecular and genetic mechanism of the AtNUDT7-regulated defense pathway remains elusive. Here we report that AtNUDT7 and its closest paralog AtNUDT6 function as novel negative regulators of SNC1, a TIR-NB-LRR-type R gene. SNC1 is upregulated at transcriptional and possibly post-transcriptional levels in nudt6-2 nudt7. The nudt6-2 nudt7 double mutant exhibits autoimmune phenotypes that are modulated by temperature and fully dependent on EDS1. The nudt6-2 nudt7 mutation causes EDS1 nuclear accumulation shortly after the establishment of autoimmunity caused by the temperature shift. We found that a low ammonium/nitrate ratio in growth media leads to a higher level of nitrite-dependent nitric oxide (NO) production in nudt6-2 nudt7, and NO acts in a positive feedback loop with EDS1 to promote the autoimmunity. The low ammonium/nitrate ratio also enhances autoimmunity in snc1-1 and cpr1, two other autoimmune mutants in Arabidopsis. Our study indicates that Arabidopsis senses the ammonium/nitrate ratio as an input signal to determine the amplitude of the EDS1-mediated defense response, probably through the modulation of NO production.

CEP-controlled supercontinuum generation during filamentation with mid-infrared laser pulse.

With carrier-envelope phase (CEP) stabilized mid-infrared (MIR) laser pulse, the CEP-controlled supercontinuum generation can be distinctly observed in a very small distance range when the focus of the laser pulse closes to the exit surface of the fused silica (FS). This CEP effect will be gradually weakened and finally disappears if the laser focus moves out of this range. With numerical simulation, we find that although the CEP effect starts from the tunneling ionization of the electron, it can be observed only when the supercontinuum mainly comes from the self-phase modulation (SPM) and self-steepening of the laser pulse and too much electrons will make it ambiguous.

Carrier-envelope phase effects on the spatial coherence of high-order harmonics.

In this work, we report the Carrier-Envelope Phase Effects on the Spatial Coherence of High-order Harmonics driven by phase-stabilized few-cycle mid-infrared laser pulses. The degree of coherence varies with carrier-envelope phase periodically with a period of π. At the same time, as the harmonic orders increase, the extreme points on the curve of coherence degree vs. carrier-envelope phase shift toward the direction of carrier-envelope phase increasing. Through theoretical analysis, we find that the ionization induced frequency chirp plays an important role in the Carrier-Envelope Phase Effects on the Spatial Coherence. This effect suggests a possible method to optimize the spatial coherence of harmonics by tuning the carrier-envelope phase of the driving field.

Spatially and spectrally resolved quantum-path tracing in high-order harmonic generation.

We experimentally demonstrate the macroscopic evolution of quantum-path distributions in harmonic emission with spatial and spectral resolution from an argon gas jet, and obviously observe that the spatial profiles of harmonics are gradually split into two components (the red and blue shifts) when the driving laser intensity is increased. Moreover, the red and blue shifts in quantum-path distributions are experimentally traced and clarified in the spatial and spectral domain by choosing the focal position. These results give a more comprehensive understanding and therefore a better control of harmonic emission.

The rice OsDIL gene plays a role in drought tolerance at vegetative and reproductive stages.

Drought is one of the critical factors limiting reproductive yields of rice and other crops globally. However, little is known about the molecular mechanism underlying reproductive development under drought stress in rice. To explore the potential gene function for improving rice reproductive development under drought, a drought induced gene, Oryza sativa Drought-Induced LTP (OsDIL) encoding a lipid transfer protein, was identified from our microarray data and selected for further study. OsDIL was primarily expressed in the anther and mainly responsive to abiotic stresses, including drought, cold, NaCl, and stress-related plant hormone abscisic acid (ABA). Compared with wild type, the OsDIL-overexpressing transgenic rice plants were more tolerant to drought stress during vegetative development and showed less severe tapetal defects and fewer defective anther sacs when treated with drought at the reproductive stage. The expression levels of the drought-responsive genes RD22, SODA1, bZIP46 and POD, as well as the ABA synthetic gene ZEP1 were up-regulated in the OsDIL-overexpression lines but the ABA degradation gene ABAOX3 was down-regulated. Moreover, overexpression of OsDIL lessened the down-regulation by drought of anther developmental genes (OsC4, CYP704B2 and OsCP1), providing a mechanism supporting pollen fertility under drought. Overexpression of OsDIL significantly enhanced drought resistance in transgenic rice during reproductive development, while showing no phenotypic changes or yield penalty under normal conditions. Therefore, OsDIL is an excellent candidate gene for genetic improvement of crop yield in adaption to unfavorable environments.

Observation of CEP effect via filamentation in transparent solids.

We report on the first direct observation of carrier-envelope-phase (CEP) effect during the interaction between few-cycle laser pulses and bulk solid materials. Using 2-cycle mid-infrared laser pulses with stabilized CEP, the CEP effect of tunneling ionization during the laser filamentation in a fused silica is revealed. The phase variation of the accompanying supercontinuum (SC) emission with filamentation at different CEPs of laser pulses can be measured by means of spectral interference technique, as a direct manifestation of the strong field tunneling ionization dynamics in transparent solids.

Laser filamentation induced air-flow motion in a diffusion cloud chamber.

We numerically simulated the air-flow motion in a diffusion cloud chamber induced by femtosecond laser filaments for different chopping rates. A two dimensional model was employed, where the laser filaments were treated as a heat flux source. The simulated patterns of flow fields and maximum velocity of updraft compare well with the experimental results for the chopping rates of 1, 5, 15 and 150 Hz. A quantitative inconsistency appears between simulated and experimental maximum velocity of updraft for 1 kHz repetition rate although a similar pattern of flow field is obtained, and the possible reasons were analyzed. Based on the present simulated results, the experimental observation of more water condensation/snow at higher chopping rate can be explained. These results indicate that the specific way of laser filament heating plays a significant role in the laser-induced motion of air flow, and at the same time, our previous conclusion of air flow having an important effect on water condensation/snow is confirmed.

Selective enhancement of a single harmonic emission in a driving laser field with subcycle waveform control.

We experimentally demonstrate a robust scheme to select a single high-order harmonic among the harmonic comb by using a driving laser field with subcycle waveform control, which is synthesized by the fundamental 800 nm laser pulse and two controlling laser pulses at 400 and 267 nm with perpendicular polarizations. By controlling the relative phase among the pulses of different colors, a single high-order harmonic is selectively enhanced while the adjacent harmonics are greatly suppressed with the intensity contrast increased by more than 1 order of magnitude and the peak intensity enhanced simultaneously by more than 2 orders of magnitude compared to the case by using only the fundamental 800 nm laser pulse. Such phenomena can be mainly attributed to the intra-atomic phase matching realized with the sub-cycle waveform controlled field.

Determination of Paraquat in Arabidopsis Tissues and Protoplasts by UHPLC-MS/MS.

Paraquat is a cost-effective herbicide, widely used in many countries, that can induce severe oxidative stress in photosynthetic tissues. Studying plant herbicide resistance or antioxidant stress mechanisms requires determining the cellular paraquat level when plants are treated by paraquat. The traditional isotopic labeling method has the potential risk to cause problems to both human health and the environment. For radioisotope manipulation, special operation spaces and strict environmental inspection are also required. In addition, the radiolabeled paraquat is increasingly hard to buy due to the extended production cycle. Here, we describe a nonradioactive method to determine the paraquat level in a small number of Arabidopsis tissues or protoplasts, using a high resolution ultra-high-performance liquid chromatography (UHPLC)-mass spectrometry (MS)/MS method. This method is highly selective and sensitive, and more environmentally compatible and technically feasible than the isotope detection method.