Discovery of microRNAs in Pyrus stigma exudates opens new research avenues in Horticulture

In many plant species, flower stigma secretions are important in early stages of sexual reproduction. Previous chemical analysis and proteomic characterization of these exudates provided insights into their biological function. Nevertheless, the presence of nucleic acids in the stigma exudates has not been previously reported. Here, we studied the stigma exudates of Pyrus communis, Pyrus pyrifolia, and Pyrus syriaca and showed them to harbor extracellular RNAs of various sizes. RNA sequencing revealed, for the first time, the presence of known Rosaceae mature microRNAs (miRs), also abundant in the stigma source tissue. Predicted targets of the exudate miRs in the Arabidopsis thaliana genome include genes involved in various biological processes. Several of these genes are pollen transcribed, suggesting possible involvement of exudate miRs in transcriptional regulation of the pollen. Moreover, extracellular miRs can potentially act across kingdoms and target genes of stigma interacting organisms/microorganisms, thus opening novel applicative avenues in Horticulture.

Role of salts and solvents on the defibrillation of food dye "sunset yellow" induced hen egg white lysozyme amyloid fibrils

Several food dyes are known to induce amyloid fibrillation when interacting with proteins. Here, we studied the role of sunset yellow (SY) in the amyloid fibrillation of hen egg white lysozyme (HEWL) and characterized the changes using spectroscopy techniques. Turbidity results showed that SY dye induces aggregation in HEWL in concentrations dependent manner. The aggregation induced by SY dye is kinetically very fast, no lag phase was detected, and the kinetics process follows an isodesmic kinetics pathway. The SY-dye induce aggregates have cross-β secondary structure confirmed by far-UV CD measurements. The effect of salts and solvents was also seen on SY-induced aggregates. Turbidity, far-UV CD, and kinetics results suggest that certain concentrations of NaCl and (NH₄)₂SO₄ solubilize the SY-induce amyloid fibrils, but (NH₄)₂SO₄ is more effective. Similarly, solvents are also solubilized the SY-induces HEWL amyloid fibrillation but the order of defibrillation is as follows: Isopropanol> ethanol > methanol which signified that isopropanol is more effective than other solvents. The salts and solvents data suggest that the electrostatic, as well as hydrophobic interaction, is responsible for SY-induced amyloid fibrillation. These conformational changes should be examined, more seriously for the purpose of food safety.

Metamitron, a Photosynthetic Electron Transport Chain Inhibitor, Modulates the Photoprotective Mechanism of Apple Trees

Chemical thinning of apple fruitlets is an important practice as it reduces the natural fruit load and, therefore, increases the size of the final fruit for commercial markets. In apples, one chemical thinner used is Metamitron, which is sold as the commercial product Brevis^® (Adama, Ashdod, Israel). This thinner inhibits the electron transfer between Photosystem II and Quinone-b within light reactions of photosynthesis. In this study, we investigated the responses of two apple cultivars-Golden Delicious and Top Red-and photosynthetic light reactions after administration of Brevis^®. The analysis revealed that the presence of the inhibitor affects both cultivars' energetic status. The kinetics of the photoprotective mechanism's sub-processes are attenuated in both cultivars, but this seems more severe in the Top Red cultivar. State transitions of the antenna and Photosystem II repair cycle are decreased substantially when the Metamitron concentration is above 0.6% in the Top Red cultivar but not in the Golden Delicious cultivar. These attenuations result from a biased absorbed energy distribution between photochemistry and photoprotection pathways in the two cultivars. We suggest that Metamitron inadvertently interacts with photoprotective mechanism-related enzymes in chloroplasts of apple tree leaves. Specifically, we hypothesize that it may interact with the kinases responsible for the induction of state transitions and the Photosystem II repair cycle.

RabA2b Overexpression Alters the Plasma-Membrane Proteome and Improves Drought Tolerance in Arabidopsis

Rab proteins are small GTPases that are important in the regulation of vesicle trafficking. Through data mining, we identified RabA2b to be stress responsive, though little is known about the involvement of RabA in plant responses to abiotic stresses. Analysis of the RabA2b native promoter showed strong activity during osmotic stress, which required the stress hormone Abscisic acid (ABA) and was restricted to the vasculature. Sequence analysis of the promoter region identified predicted binding motifs for several ABA-responsive transcription factors. We cloned RabA2b and overexpressed it in Arabidopsis. The resulting transgenic plants were strikingly drought resistant. The reduced water loss observed in detached leaves of the transgenic plants could not be explained by stomatal aperture or density, which was similar in all the genotypes. Subcellular localization studies detected strong colocalization between RabA2b and the plasma membrane (PM) marker PIP2. Further studies of the PM showed, for the first time, a distinguished alteration in the PM proteome as a result of RabA2b overexpression. Proteomic analysis of isolated PM fractions showed enrichment of stress-coping proteins as well as cell wall/cuticle modifiers in the transgenic lines. Finally, the cuticle permeability of transgenic leaves was significantly reduced compared to the wild type, suggesting that it plays a role in its drought resistant properties. Overall, these data provide new insights into the roles and modes of action of RabA2b during water stresses, and indicate that increased RabA2b mediated PM trafficking can affect the PM proteome and increase drought tolerance.

Salt induced programmed cell death in rice: evidence from chloroplast proteome signature

Soil salinity, depending on its intensity, drives a challenged plant either to death, or survival with compromised productivity. On exposure to moderate salinity, plants can often survive by sacrificing some of their cells 'in target' following a route called programmed cell death (PCD). In animals, PCD has been well characterised, and involvement of mitochondria in the execution of PCD events has been unequivocally proven. In plants, mechanistic details of the process are still in grey area. Previously, we have shown that in green tissues of rice, for salt induced PCD to occur, the presence of active chloroplasts and light are equally important. In the present work, we have characterised the chloroplast proteome in rice seedlings at 12 and 24 h after salt exposure and before the time point where the signature of PCD was observed. We identified almost 100 proteins from chloroplasts, which were divided in to 11 categories based on the biological functions in which they were involved. Our results concerning the differential expression of chloroplastic proteins revealed involvement of some novel candidates. Moreover, we observed maximum phosphorylation pattern of chloroplastic proteins at an early time point (12 h) of salt exposure.

Cyclin B1;1 activity is observed in lateral roots but not in the primary root during lethal salinity and salt stress recovery

Earlier, we reported that Arabidopsis young lateral roots (LR) exhibited improved lethal salinity tolerance as compared with the primary root (PR). We have shown that cell death processes which take place in the PR during salt stress are postponed in the LR. Still, very little is known about the regulation of cell survival mechanisms in the LR during salinity stress. Here we used transgenic Arabidopsis plants expressing Cyclin B1;1:GUS, to further study the responses to salinity in the PR and LR positions. We found strong Cyclin B1;1:GUS activity in young budding LR of salt stressed and stress recovered plants. The Cyclin B1;1:GUS activity dropped significantly in long LR and was almost completely abolished in the PR. Our data provides another line of evidence that position-dependent response occurs in Arabidopsis roots during lethal salinity. The possible roles Cyclin B1;1 plays in the young LR during the response to lethal salinity are discussed.

Differential cell persistence is observed in the Arabidopsis female gametophyte during heat stress

The central cell withstands heat stress better than the egg and antipodal cells. Insilco analysis of transcriptomic data identified several heat responsive genes which are central cell specific. Crop damage due to heat stress (HS) is a major cause of yield lost. Plants are particularly susceptible to negative effects of HS during gametophyte development and fertilization. Extensive studies have been performed on the male gametophyte under HS, but how the female gametophyte copes with HS is largely unknown. To learn how the different cell types of the female gametophyte reacts to HS, we studied unfertilized CDC123::H2B:YFP ovules. We found that the YFP-specific florescent signal persisted in the central cell during HS significantly more than the egg cell. We also found that the fluorescent signal persistence was the lowest in the antipodal cells. This finding suggests that the reaction of the female gametophyte to HS is rather unique and differentially mediated according to the cell's identity. In addition, mining through published transcriptomic datasets we found that several important heat stress responsive genes which are extremely upregulated during HS (more than 64-fold) are specifically expressed in the CC but not in the EC. Further research such as comparative transcriptomics and cell biology will likely shed more light on the phenomena reported here and increase our basic understandings about the ways sexual reproduction processes are affected by heat stress.

Execution of programmed cell death by singlet oxygen generated inside the chloroplasts of Arabidopsis thaliana

Absorption of excess excitation energy induces overproduction of singlet oxygen (¹O₂) in plants. The major sources of singlet oxygen production are chlorophyll and its intermediates located in the chloroplast. Over-accumulation of the chlorophyll biosynthetic intermediate protochlorophyllide by the exogenous application of 5-aminolevulinic acid (ALA), the precursor of tetrapyrrole, induced singlet oxygen production in the plastidic membranes. Over-expression of protochlorophyllide oxidoreductase C (PORC) in Arabidopsis thaliana resulted in efficient light-induced photo-transformation of protochlorophyllide to chlorophyllide that limited the accumulation of protochlorophyllide. Consequently, the ¹O₂ generation decreased in the PORC overexpressors (PORCx) and their cell death was minimal. Conversely, porC-2 over-accumulated protochlorophyllide in response to ALA treatment and generated higher amounts of ¹O₂ in light and had highest cell death as monitored by Evans blue staining. The protoplasts isolated from PORCx plants, when treated with ALA, generated minimal amounts of ¹O₂ as revealed by singlet oxygen sensor green (SOSG) fluorescence emission from chloroplasts. Conversely, the protoplasts of porC-2 mutants under identical conditions generated the maximum SOSG fluorescence in their chloroplasts and cytosol surrounding the chloroplasts most likely due to the leakage from the organelle. The membrane blebbing, a hallmark of programmed cell death, was clearly visible in WT and porC-2 protoplasts. Similarly, the nick end labelling (TUNEL) assay revealed nicks in the DNA. The TUNEL-positive nuclei after 30 min of light exposure were highest in porC-2 and lowest in PORCx protoplasts. The results demonstrate that higher amounts of singlet oxygen produced in the chloroplasts play an important role in programmed cell death.

Photo-modulation of programmed cell death in rice leaves triggered by salinity

In this paper we provide evidence for involvement of chloroplast as alternate organelle for initiating PCD in plants under light and abiotic stress. In animals, mitochondria are the major source of reactive oxygen species (ROS) and key executioner of programmed cell death (PCD). In plants, however, the primary site of generation of ROS is chloroplast and yet its involvement in PCD has not been worked out in details. We found by Evans blue staining that salt (150 mM NaCl)-treated protoplasts obtained from green seedlings had higher rate of cell death than protoplasts obtained from etiolated seedlings. This indicated that cell death induced by NaCl is accentuated by light. Imposition of salt-stress to green protoplasts generated H₂O₂. Known hallmarks of PCD i.e., blebbing of cell membrane, loabing in nucleus, nick in DNA were observed in light-exposed salt-treated protoplasts and seedlings. TUNEL-FACS assay demonstrate several DNA nicks in the salt-treated green protoplasts exposed to light. Conversely, salt-treated etiolated protoplasts kept in dark had only a few TUNEL-positive nuclei. Similarly, a substantial numbers of TUNEL positive nuclei were observed in green seedlings due to salt treatment in light. However, salt-treated etiolated seedlings kept in dark had very few TUNEL positive nuclei. Addition of Caspase 3 inhibitor (DAVD-CHO) rescued (~50 %) green protoplasts from salt-stress induced cell death suggesting an involvement of apoptosis like PCD (AL-PCD). Ultra structure studies of chloroplast, mitochondria and nucleus from the leaves obtained from salt treated seedlings at the time point that showed PCD signature, resulted to severe granal de-stacking in chloroplasts while structural integrity of mitochondria was maintained. These studies demonstrate the photo-modulation of salinity-induced PCD in photosynthetic tissues is mainly executed by chloroplasts.

Programmed cell death in plants: A chloroplastic connection

Programmed cell death (PCD) is an integral cellular program by which targeted cells culminate to demise under certain developmental and pathological conditions. It is essential for controlling cell number, removing unwanted diseased or damaged cells and maintaining the cellular homeostasis. The details of PCD process has been very well elucidated and characterized in animals but similar understanding of the process in plants has not been achieved rather the field is still in its infancy that sees some sporadic reports every now and then. The plants have 2 energy generating sub-cellular organelles- mitochondria and chloroplasts unlike animals that just have mitochondria. The presence of chloroplast as an additional energy transducing and ROS generating compartment in a plant cell inclines to advocate the involvement of chloroplasts in PCD execution process. As chloroplasts are supposed to be progenies of unicellular photosynthetic organisms that evolved as a result of endosymbiosis, the possibility of retaining some of the components involved in bacterial PCD by chloroplasts cannot be ruled out. Despite several excellent reviews on PCD in plants, there is a void on an update of information at a place on the regulation of PCD by chloroplast. This review has been written to provide an update on the information supporting the involvement of chloroplast in PCD process and the possible future course of the field.