Transgenic expression of a putative calcium transporter affects the time of Arabidopsis flowering

The CALCINEURIN B-LIKE 4/CBL-INTERACTING PROTEIN 3 module degrades repressor JAZ5 during rose petal senescence

Flower senescence is genetically regulated and developmentally controlled. The phytohormone ethylene induces flower senescence in rose (Rosa hybrida), but the underlying signaling network is not well understood. Given that calcium regulates senescence in animals and plants, we explored the role of calcium in petal senescence. Here, we report that the expression of calcineurin B-like protein 4 (RhCBL4), which encodes a calcium receptor, is induced by senescence and ethylene signaling in rose petals. RhCBL4 interacts with CBL-interacting protein kinase 3 (RhCIPK3), and both positively regulate petal senescence. Furthermore, we determined that RhCIPK3 interacts with the jasmonic acid response repressor jasmonate ZIM-domain 5 (RhJAZ5). RhCIPK3 phosphorylates RhJAZ5 and promotes its degradation in the presence of ethylene. Our results reveal that the RhCBL4-RhCIPK3-RhJAZ5 module mediates ethylene-regulated petal senescence. These findings provide insights into flower senescence, which may facilitate innovations in postharvest technology for extending rose flower longevity.

Beyond NPK: Mineral Nutrient-Mediated Modulation in Orchestrating Flowering Time

Flowering time in plants is a complex process regulated by environmental conditions such as photoperiod and temperature, as well as nutrient conditions. While the impact of major nutrients like nitrogen, phosphorus, and potassium on flowering time has been well recognized, the significance of micronutrient imbalances and their deficiencies should not be neglected because they affect the floral transition from the vegetative stage to the reproductive stage. The secondary major nutrients such as calcium, magnesium, and sulfur participate in various aspects of flowering. Micronutrients such as boron, zinc, iron, and copper play crucial roles in enzymatic reactions and hormone biosynthesis, affecting flower development and reproduction as well. The current review comprehensively explores the interplay between microelements and flowering time, and summarizes the underlying mechanism in plants. Consequently, a better understanding of the interplay between microelements and flowering time will provide clues to reveal the roles of microelements in regulating flowering time and to improve crop reproduction in plant industries.

Integration of small RNAs and transcriptome sequencing uncovers a complex regulatory network during vernalization and heading stages of orchardgrass (Dactylis glomerata L.)

BACKGROUND

Flowering is a critical reproductive process in higher plants. Timing of optimal flowering depends upon the coordination among seasonal environmental cues. For cool season grasses, such as Dactylis glomerata, vernalization induced by low temperature provides competence to initiate flowering after prolonged cold. We combined analyses of the transcriptome and microRNAs (miRNAs) to generate a comprehensive resource for regulatory miRNAs and their target circuits during vernalization and heading stages.

RESULTS

A total of 3,846 differentially expressed genes (DEGs) and 69 differentially expressed miRNAs were identified across five flowering stages. The expression of miR395, miR530, miR167, miR396, miR528, novel_42, novel_72, novel_107, and novel_123 demonstrated significant variations during vernalization. These miRNA targeted genes were involved in phytohormones, transmembrane transport, and plant morphogenesis in response to vernalization. The expression patterns of DEGs related to plant hormones, stress responses, energy metabolism, and signal transduction changed significantly in the transition from vegetative to reproductive phases.

CONCLUSIONS

Five hub genes, c136110_g1 (BRI1), c131375_g1 (BZR1), c133350_g1 (VRN1), c139830_g1 (VIN3), and c125792_g2 (FT), might play central roles in vernalization response. Our comprehensive analyses have provided a useful platform for investigating consecutive transcriptional and post-transcriptional regulation of critical phases in D. glomerata and provided insights into the genetic engineering of flowering-control in cereal crops.

The contribution of organelles to plant intracellular Calcium signalling

Calcium (Ca2+) is among the most important intracellular messengers in living organisms. Understanding of the players and dynamics of Ca2+ signalling pathways in plants may help to unravel the molecular basis of their exceptional flexibility to respond and to adapt to different stimuli. In the present review we focus on new tools that have recently revolutionized our view of organellar Ca2+ signalling as well as on the current knowledge regarding the pathways mediating Ca2+ fluxes across intracellular membranes. The contribution of organelles and cellular subcompartments to the orchestrated response via Ca2+ signalling within a cell is also discussed, underlining the fact that one of the greatest challenges in the field is the elucidation of how influx and efflux Ca2+ transporters/channels are regulated in a concerted manner to translate specific information into a Ca2+ signature.

The Glycosyltransferase QUA1 Regulates Chloroplast-Associated Calcium Signaling During Salt and Drought Stress in Arabidopsis

Cytoplasmic Ca2+ ([Ca2+]cyt) elevation induced by various signals is responsible for appropriate downstream responses. Through a genetic screen of Arabidopsis thaliana mutants defective in stress-induced [Ca2+]cyt elevation, the glycosyltransferase QUASIMODO1 (QUA1) was identified as a regulator of [Ca2+]cyt in response to salt stress. Compared with the wild type, the qua1-4 mutant exhibited a dramatically greater increase in [Ca2+]cyt under NaCl treatment. Functional analysis showed that QUA1 is a novel chloroplast protein that regulates cytoplasmic Ca2+ signaling. QUA1 was detected in chloroplast thylakoids, and the qua1-4 mutant exhibited irregularly stacked grana. The observed greater increase in [Ca2+]cyt was inhibited upon recovery of chloroplast function in the qua1-4 mutant. Further analysis showed that CAS, a thylakoid-localized calcium sensor, also displayed irregularly stacked grana, and the chloroplasts of the qua1-4 cas-1 double mutant were similar to those of cas-1 plants. In QUA1-overexpressing plants, the protein level of CAS was decreased, and CAS was readily degraded under osmotic stress. When CAS was silenced in the qua1-4 mutant, the large [Ca2+]cyt increase was blocked, and the higher expression of PLC3 and PLC4 was suppressed. Under osmotic stress, the qua1-4 mutant showed an even greater elevation in [Ca2+]cyt and was hypersensitive to drought stress. However, this sensitivity was inhibited when the increase in [Ca2+]cyt was repressed in the qua1-4 mutant. Collectively, our data indicate that QUA1 may function in chloroplast-dependent calcium signaling under salt and drought stresses. Additionally, CAS may function downstream of QUA1 to mediate these processes.

Ion Channels in Plant Bioenergetic Organelles, Chloroplasts and Mitochondria: From Molecular Identification to Function

Recent technical advances in electrophysiological measurements, organelle-targeted fluorescence imaging, and organelle proteomics have pushed the research of ion transport a step forward in the case of the plant bioenergetic organelles, chloroplasts and mitochondria, leading to the molecular identification and functional characterization of several ion transport systems in recent years. Here we focus on channels that mediate relatively high-rate ion and water flux and summarize the current knowledge in this field, focusing on targeting mechanisms, proteomics, electrophysiology, and physiological function. In addition, since chloroplasts evolved from a cyanobacterial ancestor, we give an overview of the information available about cyanobacterial ion channels and discuss the evolutionary origin of chloroplast channels. The recent molecular identification of some of these ion channels allowed their physiological functions to be studied using genetically modified Arabidopsis plants and cyanobacteria. The view is emerging that alteration of chloroplast and mitochondrial ion homeostasis leads to organelle dysfunction, which in turn significantly affects the energy metabolism of the whole organism. Clear-cut identification of genes encoding for channels in these organelles, however, remains a major challenge in this rapidly developing field. Multiple strategies including bioinformatics, cell biology, electrophysiology, use of organelle-targeted ion-sensitive probes, genetics, and identification of signals eliciting specific ion fluxes across organelle membranes should provide a better understanding of the physiological role of organellar channels and their contribution to signaling pathways in plants in the future.

Transport Across Chloroplast Membranes: Optimizing Photosynthesis for Adverse Environmental Conditions

Chloroplasts are central to solar light harvesting and photosynthesis. Optimal chloroplast functioning is vitally dependent on a very intensive traffic of metabolites and ions between the cytosol and stroma, and should be attuned for adverse environmental conditions. This is achieved by an orchestrated regulation of a variety of transport systems located at chloroplast membranes such as porines, solute channels, ion-specific cation and anion channels, and various primary and secondary active transport systems. In this review we describe the molecular nature and functional properties of the inner and outer envelope and thylakoid membrane channels and transporters. We then discuss how their orchestrated regulation affects thylakoid structure, electron transport and excitation energy transfer, proton-motive force partition, ion homeostasis, stromal pH regulation, and volume regulation. We link the activity of key cation and anion transport systems with stress-specific signaling processes in chloroplasts, and discuss how these signals interact with the signals generated in other organelles to optimize the cell performance, with a special emphasis on Ca(2+) and reactive oxygen species signaling.

Calcium-dependent regulation of photosynthesis

The understanding of calcium as a second messenger in plants has been growing intensively over the last decades. Recently, attention has been drawn to the organelles, especially the chloroplast but focused on the stromal Ca2+ transients in response to environmental stresses. Herein we will expand this view and discuss the role of Ca2+ in photosynthesis. Moreover we address of how Ca2+ is delivered to chloroplast stroma and thylakoids. Thereby, new light is shed on the regulation of photosynthetic electron flow and light-dependent metabolism by the interplay of Ca2+, thylakoid acidification and redox status. This article is part of a Special Issue entitled: Chloroplast biogenesis.

ROW1 maintains quiescent centre identity by confining WOX5 expression to specific cells

The quiescent centre (QC) in the Arabidopsis root apical meristem is essential for stem cell organization. Here we show that the loss of REPRESSOR OF WUSCHEL1 (ROW1), a PHD domain-containing protein, leads to QC failure, defects in cell differentiation and ectopic expression of WUSCHEL-RELATED HOMEOBOX 5 (WOX5) in cells that normally express ROW1. The wox5-1/row1-3 double mutants show similar phenotypes to wox5-1 indicating that WOX5 is epistatic to ROW1. ROW1 specifically binds trimethylated histone H3 lysine 4 (H3K4me3) in the WOX5 promoter region to repress its transcription. QC expression of ROW1 results in a wox5-1-like phenotype with undetectable WOX5 transcripts. We propose that ROW1 is essential for QC maintenance and for stem cell niche development through the repression of WOX5 in the proximal meristem.

The quiescent centre controls stem cell differentiation at the root apical meristem. Here Zhang et al. propose that ROW1 maintains meristem cell identity by repressing expression of the WOX5 transcription factor in the proximal meristem zone, thus confining its activity to the quiescent centre.

Toxicity and translocation of graphene oxide in Arabidopsis thaliana

We investigated the possible safety property and translocation of graphene oxide (GO) in the range of μg/L in Arabidopsis. GO exposure did not obviously influence germination, seed development, shoot and root development of seedlings, and flowering time. Meanwhile, GO exposure could not induce severe H2O2 production, increase in malondialdehyde content, formation of oxidative stress, and altered activities of antioxidant enzymes. Moreover, GO exposure did not change expression patterns of examined genes required for germination, photomorphogenesis, root development, and transition from vegetative to reproductive development. In the seedling, we did not observe severe GO accumulation in mesophyll and parenchyma cells of leaf or stem, and in sieve element in leaf, stem, or root. In contrast, we observed the severe GO accumulation in root hair and root parenchyma cells. Our results provide the physiological basis for safety property of GO at the examined concentrations in Arabidopsis plants. Furthermore, our data imply that although GO was absorbed by Arabidopsis plants through root hairs, plants might still have strong ability to be against GO translocation into stem or leaves. In addition, we found that cotyledon might serve as an important site for GO distribution during the early development.

Calcium signaling in plant endosymbiotic organelles: mechanism and role in physiology

Recent studies have demonstrated that chloroplasts and mitochondria evoke specific Ca(2+) signals in response to biotic and abiotic stresses in a stress-dependent manner. The identification of Ca(2+) transporters and Ca(2+) signaling molecules in chloroplasts and mitochondria implies that they play roles in controlling not only intra-organellar functions, but also extra-organellar processes such as plant immunity and stress responses. It appears that organellar Ca(2+) signaling might be more important to plant cell functions than previously thought. This review briefly summarizes what is known about the molecular basis of Ca(2+) signaling in plant mitochondria and chloroplasts.

Plant organellar calcium signalling: an emerging field

This review provides a comprehensive overview of the established and emerging roles that organelles play in calcium signalling. The function of calcium as a secondary messenger in signal transduction networks is well documented in all eukaryotic organisms, but so far existing reviews have hardly addressed the role of organelles in calcium signalling, except for the nucleus. Therefore, a brief overview on the main calcium stores in plants-the vacuole, the endoplasmic reticulum, and the apoplast-is provided and knowledge on the regulation of calcium concentrations in different cellular compartments is summarized. The main focus of the review will be the calcium handling properties of chloroplasts, mitochondria, and peroxisomes. Recently, it became clear that these organelles not only undergo calcium regulation themselves, but are able to influence the Ca(2+) signalling pathways of the cytoplasm and the entire cell. Furthermore, the relevance of recent discoveries in the animal field for the regulation of organellar calcium signals will be discussed and conclusions will be drawn regarding potential homologous mechanisms in plant cells. Finally, a short overview on bacterial calcium signalling is included to provide some ideas on the question where this typically eukaryotic signalling mechanism could have originated from during evolution.

Occurrence of the transition of apical architecture and expression patterns of related genes during conversion of apical meristem identity in G2 pea

G2 pea exhibits an apical senescence delaying phenotype under short-day (SD) conditions; however, the structural basis for its apical development is still largely unknown. In the present study, the apical meristem of SD-grown G2 pea plants underwent a transition from vegetative to indeterminate inflorescence meristem, but the apical meristem of long-day (LD)-grown G2 pea plants would be further converted to determinate floral meristem. Both SD signal and GA(3) treatment enhanced expression of the putative calcium transporter PPF1, and pea homologs of TFL1 (LF and DET), whereas LD signal suppressed their expression at 60 d post-flowering compared with those at 40 d post-flowering. Both PPF1 and LF expressed at the vegetative and reproductive phases in SD-grown apical buds, but floral initiation obviously increased the expression level of PPF1 compared with the unchanged expression level of LF from 40 to 60 d post-flowering. In addition, although the floral initiation significantly enhanced the expression levels of PPF1 and DET, DET was mainly expressed after floral initiation in SD-grown apical buds. Therefore, the main structural difference between LD- and SD-grown apical meristem in G2 pea lies in whether their apical indeterminate inflorescence meristem could be converted to the determinate structure.

Shaping the calcium signature

In numerous plant signal transduction pathways, Ca2+ is a versatile second messenger which controls the activation of many downstream actions in response to various stimuli. There is strong evidence to indicate that information encoded within these stimulus-induced Ca2+ oscillations can provide signalling specificity. Such Ca2+ signals, or 'Ca2+ signatures', are generated in the cytosol, and in noncytosolic locations including the nucleus and chloroplast, through the coordinated action of Ca2+ influx and efflux pathways. An increased understanding of the functions and regulation of these various Ca2+ transporters has improved our appreciation of the role these transporters play in specifically shaping the Ca2+ signatures. Here we review the evidence which indicates that Ca2+ channel, Ca2+-ATPase and Ca2+ exchanger isoforms can indeed modulate specific Ca2+ signatures in response to an individual signal.

Mutation of Arabidopsis BARD1 causes meristem defects by failing to confine WUSCHEL expression to the organizing center

Stem cell fate in the Arabidopsis thaliana shoot apical meristem (SAM) is controlled by WUSCHEL (WUS) and CLAVATA. Here, we examine BARD1 (for BRCA1-associated RING domain 1), which had previously been implicated in DNA repair functions; we find that it also regulates WUS expression. We observed severe SAM defects in the knockout mutant bard1-3. WUS transcripts accumulated >238-fold in bard1-3 compared with the wild type and were located mainly in the outermost cell layers instead of the usual organizing center. A specific WUS promoter region was recognized by nuclear protein extracts obtained from wild-type plants, and this protein-DNA complex was recognized by antibodies against BARD1. The double mutant (wus-1 bard1-3) showed prematurely terminated SAM structures identical to those of wus-1, indicating that BARD1 functions through regulation of WUS. BARD1 overexpression resulted in reduced WUS transcript levels, giving a wus-1-like phenotype. Either full-length BARD1 or a clone that encoded the C-terminal domain (BARD1:C-ter;bard1-3) was sufficient to complement the bard1-3 phenotype, indicating that BARD1 functions through its C-terminal domain. Our data suggest that BARD1 regulates SAM organization and maintenance by limiting WUS expression to the organizing center.

A plastid protein crucial for Ca2+-regulated stomatal responses

* Guard cell movements are regulated by environmental cues including, for example, elevations in extracellular Ca(2+) concentration. Here, the subcellular localization and physiological function of the Ca(2+)-sensing receptor (CAS) protein was investigated. * CAS protein localization was ascertained by microscopic analyses of green fluorescent protein (GFP) fusion proteins and biochemical fractionation assays. Comparative guard cell movement investigations were performed in wild-type and cas loss-of-function mutant lines of Arabidopsis thaliana. Cytoplasmic Ca(2+) dynamics were addressed in plants expressing the yellow cameleon reporter protein YC3.6. * This study identified CAS as a chloroplast-localized protein that is crucial for proper stomatal regulation in response to elevations of external Ca(2+). CAS fulfils this role through modulation of the cytoplasmic Ca(2+) concentration. * This work reveals a novel role of the chloroplast in cellular Ca(2+) signal transduction.

PPF1 may suppress plant senescence via activating TFL1 in transgenic Arabidopsis plants

Senescence, a sequence of biochemical and physiological events, constitutes the final stage of development in higher plants and is modulated by a variety of environmental factors and internal factors. PPF1 possesses an important biological function in plant development by controlling the Ca2+ storage capacity within chloroplasts. Here we show that the expression of PPF1 might play a pivotal role in negatively regulating plant senescence as revealed by the regulation of overexpression and suppression of PPF1 on plant development. Moreover, TFL1, a key regulator in the floral repression pathway, was screened out as one of the downstream targets for PPF1 in the senescence-signaling pathway. Investigation of the senescence-related phenotypes in PPF1(-) tfl1-1 and PPF1(+) tfl1-1 double mutants confirmed and further highlighted the relation of PPF1 with TFL1 in transgenic plants. The activation of TFL1 expression by PPF1 defines an important pathway possibly essential for the negative regulation of plant senescence in transgenic Arabidopsis.

Evidence for chloroplast control of external Ca2+-induced cytosolic Ca2+ transients and stomatal closure

The role of guard cell chloroplasts in stomatal function is controversial. It is usually assumed that stomatal closure is preceded by a transient increase in cytosolic free Ca(2+) concentration ([Ca(2+)](cyt)) in the guard cells. Here, we provide the evidence that chloroplasts play a critical role in the generation of extracellular Ca(2+) ([Ca(2+)](ext))-induced [Ca(2+)](cyt) transients and stomatal closure in Arabidopsis. CAS (Ca(2+) sensing receptor) is a plant-specific putative Ca(2+)-binding protein that was originally proposed to be a plasma membrane-localized external Ca(2+) sensor. In the present study, we characterized the intracellular localization of CAS in Arabidopsis with a combination of techniques, including (i) in vivo localization of green fluorescent protein (GFP) fused gene expression, (ii) subcellular fractionation and fractional analysis of CAS with Western blots, and (iii) database analysis of thylakoid membrane proteomes. Each technique produced consistent results. CAS was localized mainly to chloroplasts. It is an integral thylakoid membrane protein, and the N-terminus acidic Ca(2+)-binding region is likely exposed to the stromal side of the membrane. The phenotype of T-DNA insertion CAS knockout mutants and cDNA mutant-complemented plants revealed that CAS is essential for stomatal closure induced by external Ca(2+). In contrast, overexpression of CAS promoted stomatal closure in the absence of externally applied Ca(2+). Furthermore, using the transgenic aequorin system, we showed that [Ca(2+)](ext)-induced [Ca(2+)](cyt) transients were significantly reduced in CAS knockout mutants. Our results suggest that thylakoid membrane-localized CAS is essential for [Ca(2+)](ext)-induced [Ca(2+)](cyt) transients and stomatal closure.

Systematic studies of 12S seed storage protein accumulation and degradation patterns during Arabidopsis seed maturation and early seedling germination stages

Seed storage proteins (SSPs) are important for seed germination and early seedling growth. We studied the accumulation and degradation profiles of four major Arabidopsis 12S SSPs using a 2-DE scheme combined with mass spectrometric methods. On the 2-DE map of 23 dpa (days post anthesis) siliques, 48 protein spots were identified as putative full-length or partial alpha, beta subunits. Only 9 of them were found in 12 dpa siliques with none in younger than 8 dpa siliques, indicating that the accumulation of 12S SSPs started after the completion of cell elongation processes both in siliques and in developing seeds. The length and strength of transcription activity for each gene determined the final contents of respective SSP. At the beginning of imbibition, 68 SSP spots were identified while only 2 spots were found at the end of the 4 d germination period, with alpha subunits degraded more rapidly than the beta subunits. The CRC alpha subunit was found to degrade from its C-terminus with conserved sequence motifs. Our data provide an important basis for understanding the nutritional value of developing plant seeds and may serve as a useful platform for other species.

CML24, regulated in expression by diverse stimuli, encodes a potential Ca2+ sensor that functions in responses to abscisic acid, daylength, and ion stress

Changes in intracellular calcium (Ca(2+)) levels serve to signal responses to diverse stimuli. Ca(2+) signals are likely perceived through proteins that bind Ca(2+), undergo conformation changes following Ca(2+) binding, and interact with target proteins. The 50-member calmodulin-like (CML) Arabidopsis (Arabidopsis thaliana) family encodes proteins containing the predicted Ca(2+)-binding EF-hand motif. The functions of virtually all these proteins are unknown. CML24, also known as TCH2, shares over 40% amino acid sequence identity with calmodulin, has four EF hands, and undergoes Ca(2+)-dependent changes in hydrophobic interaction chromatography and migration rate through denaturing gel electrophoresis, indicating that CML24 binds Ca(2+) and, as a consequence, undergoes conformational changes. CML24 expression occurs in all major organs, and transcript levels are increased from 2- to 15-fold in plants subjected to touch, darkness, heat, cold, hydrogen peroxide, abscisic acid (ABA), and indole-3-acetic acid. However, CML24 protein accumulation changes were not detectable. The putative CML24 regulatory region confers reporter expression at sites of predicted mechanical stress; in regions undergoing growth; in vascular tissues and various floral organs; and in stomata, trichomes, and hydathodes. CML24-underexpressing transgenics are resistant to ABA inhibition of germination and seedling growth, are defective in long-day induction of flowering, and have enhanced tolerance to CoCl(2), molybdic acid, ZnSO(4), and MgCl(2). MgCl(2) tolerance is not due to reduced uptake or to elevated Ca(2+) accumulation. Together, these data present evidence that CML24, a gene expressed in diverse organs and responsive to diverse stimuli, encodes a potential Ca(2+) sensor that may function to enable responses to ABA, daylength, and presence of various salts.

Proteomics of calcium-signaling components in plants

Calcium functions as a versatile messenger in mediating responses to hormones, biotic/abiotic stress signals and a variety of developmental cues in plants. The Ca(2+)-signaling circuit consists of three major "nodes"--generation of a Ca(2+)-signature in response to a signal, recognition of the signature by Ca2+ sensors and transduction of the signature message to targets that participate in producing signal-specific responses. Molecular genetic and protein-protein interaction approaches together with bioinformatic analysis of the Arabidopsis genome have resulted in identification of a large number of proteins at each "node"--approximately 80 at Ca2+ signature, approximately 400 sensors and approximately 200 targets--that form a myriad of Ca2+ signaling networks in a "mix and match" fashion. In parallel, biochemical, cell biological, genetic and transgenic approaches have unraveled functions and regulatory mechanisms of a few of these components. The emerging paradigm from these studies is that plants have many unique Ca2+ signaling proteins. The presence of a large number of proteins, including several families, at each "node" and potential interaction of several targets by a sensor or vice versa are likely to generate highly complex networks that regulate Ca(2+)-mediated processes. Therefore, there is a great demand for high-throughput technologies for identification of signaling networks in the "Ca(2+)-signaling-grid" and their roles in cellular processes. Here we discuss the current status of Ca2+ signaling components, their known functions and potential of emerging high-throughput genomic and proteomic technologies in unraveling complex Ca2+ circuitry.

PPF1 inhibits programmed cell death in apical meristems of both G2 pea and transgenic Arabidopsis plants possibly by delaying cytosolic Ca2+ elevation

PPF1 encodes a putative calcium ion carrier that affects the flowering time of transgenic Arabidopsis by modulating Ca(2+) storage capacities in chloroplasts of a plant cell. In the current work, we found that differential expression of PPF1 might affect processes of programmed cell death (PCD) since DNA fragmentation was detected in senescencing apical buds of long day-grown G2 pea (Pisum sativum L.) plants, but was not in non-senescencing short day-grown counterparts at all growth stages. An animal inhibitor of caspase-activated DNase (ICAD) homologue was detected in short day-grown plant continuously throughout the whole experiment and only in early stages of long day-grown pre-floral G2 pea apical buds. DNA fragmentation was significantly inhibited in apical meristems of transgenic Arabidopsis that over-expressed the PPF1 gene when compared to that of either wild-type control or to PPF1 (-) plants. The expression of ICAD-like protein decreased to undetectable level at 45 dpg in apical tissues of PPF1 (-) Arabidopsis, which was much earlier than that found in PPF1 (+) or wild-type controls. In epidermal cells of PPF1 (-) plants, we recorded significantly earlier calcium transient prior to PCD. We suggest that the expression of PPF1, a chloroplast localized Ca(2+) ion channel may inhibit programmed cell death in apical meristems of flowering plants by keeping a low cytoplasmic calcium content that might inhibit DNA fragmentation in plant cells.