Stages Of Leaf Development Research Articles

Developmental stages of delayed-greening leaves inferred from measurements of chlorophyll content and leaf growth.

Chlorophyll (Chl) accumulation and leaf growth were analysed in delayed-greening leaves of Theobroma cacao (L.) to examine whether these parameters are correlated during leaf development and can be used as non-destructive indicators of leaf developmental stages. There was a clear correlation between Chl content and leaf relative growth rate (RGR) and between Chl content and percentage of full leaf expansion (%FLE) under different growth conditions. Five distinct developmental phases were defined according to the correlation between these parameters and corroborated by data from the analyses of leaf growth (epidermal cell size and specific leaf area) or photosynthetic properties (maximal PSII efficiency, CO2 assimilation and non-structural carbohydrate contents). The five phases were characterised by rapid leaf expansion by cell division (I), pronounced cell expansion (II), development of photosynthetic capacity concomitant with reinforcement of leaf structure (III), and maturation (IV and V). The transition from cell division to cell expansion happened uniformly across the leaf lamina between phase I and II; the sink-to-source transition was found between phase III and IV. These results demonstrate coordinated development of photosynthetic machinery and leaf structure in delayed-greening leaves and provide a simple and non-invasive method for estimation of leaf developmental stages in T. cacao.

Spectral reflectance indices and pigment functions during leaf ontogenesis in six subtropical landscape plants

Pigment combinations are regulated during leaf ontogenesis. To better understand pigment function, alterations in chlorophyll, carotenoid and anthocyanin concentrations were investigated during different leaf development stages in six subtropical landscape plants, namely Ixora chinensis Lam, Camellia japonica Linn, Eugenia oleina Wight, Mangifera indica L., Osmanthus fragrans Lowr and Saraca dives Pierre. High concentrations of anthocyanin were associated with reduced chlorophyll in juvenile leaves. As leaves developed, the photosynthetic pigments (chlorophyll and carotenoid) of all six species increased while anthocyanin concentration declined. Chlorophyll fluorescence imaging of ΦPSII (effective quantum yield of PSII) and of NPQ (non-photochemical fluorescence quenching) and determination of electron transport rate-rapid light curve (RLC) showed that maximum ETR (leaf electron transport rate), ΦPSII and the saturation point in RLC increased during leaf development but declined as they aged. Juvenile leaves displayed higher values of NPQ and Car/Chl ratios than leaves at other developmental stages. Leaf reflectance spectra (400–800 nm) were measured to provide an in vivo non-destructive assessment of pigments in leaves during ontogenesis. Four reflectance indices, related to pigment characters, were compared with data obtained quantitatively from biochemical analysis. The results showed that the ARI (anthocyanin reflectance index) was linearly correlated to anthocyanin concentration in juvenile leaves, while a positive correlation of Chl NDI (chlorophyll normalized difference vegetation index) to chlorophyll a concentration was species dependent. Photosynthetic reflectance index was not closely related to Car/Chl ratio, while a structural-independent pigment index was not greatly altered by leaf development or species. Accordingly, it is suggested that the high concentration of anthocyanin, higher NPQ and Car/Chl ratio in juvenile leaves are important functional responses to cope with high radiation when the photosynthetic apparatus is not fully developed. Another two leaf reflectance indices, ARI and Chl NDI, are valuable for in vivo pigment evaluation during leaf development.

Arabidopsis MBF1s Control Leaf Cell Cycle and its Expansion

Multiprotein bridging factor 1 (MBF1) is known as a transcriptional co-activator that enhances transcription of its target genes by bridging between transcription factors and TATA-box-binding protein in eukaryotes. Arabidopsis thaliana has three MBF1 genes: AtMBF1a-AtMBF1c. However, details of the functions of AtMBF1 remain unclear. For this study, transgenic Arabidopsis overexpressing AtMBF1 fused to an active transcriptional repression domain (SRDX) was constructed. The chimeric protein putatively functions as a transcriptional co-repressor and as a suppressor of functions of endogenous AtMBF1 in transgenic plants. Transgenic Arabidopsis overexpressing AtMBF1-SRDX (AtMBF1-SRDX(OE)) showed an extremely small leaf phenotype under a continuous white light condition. Its leaf cells-especially those around vascular tissues, where strong expression of endogenous AtMBF1s is observed-were much smaller than those from the wild type (WT). In addition, a lower cell number was observed in leaves from AtMBF1-SRDX(OE) plants. Time course analysis of cell size revealed that cell expansion of leaves of AtMBF1-SRDX(OE) plants was dramatically suppressed during the late leaf developmental stage (cell expansion stage), when endogenous AtMBF1b is strongly expressed in the WT. The results show that ploidy levels of leaves from AtMBF1-SRDX(OE) plants were dramatically lower than those from the WT; moreover, expression levels of several negative regulators of endoreduplication were more elevated in AtMBF1s-SRDX(OE) plants than those in the WT. These observations suggest that AtMBF1-SRDX interacts with regulators of endoreduplication. Therefore, AtMBF1s are considered to affect not only leaf cell expansion but also regulation of the ploidy level in leaf cells during the leaf expansion stage.

A cross-species transcriptomics approach to identify genes involved in leaf development.

BackgroundWe have made use of publicly available gene expression data to identify transcription factors and transcriptional modules (regulons) associated with leaf development in Populus. Different tissue types were compared to identify genes informative in the discrimination of leaf and non-leaf tissues. Transcriptional modules within this set of genes were identified in a much wider set of microarray data collected from leaves in a number of developmental, biotic, abiotic and transgenic experiments.ResultsTranscription factors that were over represented in leaf EST libraries and that were useful for discriminating leaves from other tissues were identified, revealing that the C2C2-YABBY, CCAAT-HAP3 and 5, MYB, and ZF-HD families are particularly important in leaves. The expression of transcriptional modules and transcription factors was examined across a number of experiments to select those that were particularly active during the early stages of leaf development. Two transcription factors were found to collocate to previously published Quantitative Trait Loci (QTL) for leaf length. We also found that miRNA family 396 may be important in the control of leaf development, with three members of the family collocating with clusters of leaf development QTL.ConclusionThis work provides a set of candidate genes involved in the control and processes of leaf development. This resource can be used for a wide variety of purposes such as informing the selection of candidate genes for association mapping or for the selection of targets for reverse genetics studies to further understanding of the genetic control of leaf size and shape.

Comparing two versions of a non-linear model for simulating leaf number and developmental stages in maize based on air temperature

The Wang and Engel (WE) model simulates crop development considering the non-linear response of plant development to temperature. Daily air temperature is the input for the temperature response function [f(T)] in the WE model, and because there are several approaches for computing daily temperatures, there are several ways to calculate the f(T). The objective of this study was to compare two versions of the WE model for simulating leaf number and developmental stages in maize, considering two approaches for imputing daily air temperature (daily mean air temperature and daily minimum/maximum air temperature). A two-year field experiment with the maize variety BRS Missões sown in several sowing dates was conducted in Santa Maria, Rio Grande do Sul State, Brazil, during the 2005-2006 and 2006-2007 growing seasons. The f(T) in the WE model was calculated using daily mean air temperature calculated as the arithmetic average of daily minimum (TN) and maximum (TX) air temperatures (WE Tmean), and calculating an f(T) using TN and an f(T) using TX and then averaging the two f(T)s (WE Tmm). Ligule and tip leaf number, and silking and physiological maturity developmental stages measured in the 2005-2006 growing season were used to estimate model coefficients and the ones measured in the 2006-2007 growing season were used as independent data sets to evaluate models. Predictions of ligule and tip leaf number, silking and physiological maturity of the maize variety BRS Missões were better with the WE Tmm model than with the WE Tmean model.

Juvenile Rhus glabra leaves have higher temperatures and lower gas exchange rates than mature leaves when compared in the field during periods of high irradiance

We sought to test the hypothesis that stomatal development determines the timing of gas exchange competency, which then influences leaf temperature through transpirationally driven leaf cooling. To test this idea, daily patterns of gas exchange and leaflet temperature were obtained from leaves of two distinctively different developmental stages of smooth sumac (Rhus glabra) grown in its native habitat. Juvenile and mature leaves were also sampled for ultrastructural studies of stomatal development. When plants were sampled in May-June, the hypothesis was supported: juvenile leaflets were (for part of the day) from 1.4 to 6.0 degrees C warmer than mature leaflets and as much as 2.0 degrees C above ambient air temperature with lower stomatal conductance and photosynthetic rates than mature leaflets. When measurements were taken from July to October, no significant differences were observed, although mature leaflet gas exchange rates declined to the levels of the juvenile leaves. The gas exchange data were supported by the observations that juvenile leaves had approximately half the number of functional stomata on a leaf surface area basis as did mature leaves. It was concluded that leaf temperature and stage of leaf development in sumac are strongly linked with the higher surface temperatures observed in juvenile leaflets in the early spring possibly being involved in promoting photosynthesis and leaf expansion when air temperatures are cooler.

A Bioinformatics Approach to Investigating Leaf Development

The question of how leaf form is established has long intrigued plant biologists and has important ramifications in both natural and agricultural systems. Heterochrony refers to developmental changes in the timing of events that lead to changes in the size and/or shape of an organ. Heterochronic genes control the temporal dimension of development, in contrast with homeotic genes, which control spatial patterns of development (Slack and Ruvkun, 1997). Chronological changes during leaf morphogenesis often are monitored using anatomical markers, such as trichomes, guard cells, and vascular cells. However, in mutants lacking these cell types it can be difficult to distinguish between factors affecting the progress of leaf maturation (heterochrony) and cell fate specification. Efroni et al. (pages 2293–2306) introduce a bioinformatics approach to quantifying the stages of leaf development based on gene expression profiles. The authors make use of publicly available gene expression data to develop an algorithm for defining a leaf differentiation score, termed the digital differentiation index, which is based on expression patterns that change with leaf age. This approach is found to work surprisingly well on samples from different labs and different ecotypes, despite the limitations inherent in inferring physiological or morphological status solely from gene expression data. The authors proceed to use this approach to investigate the role of CINCINNATA (CIN)related TCP transcription factors in the control of leaf morphogenesis. CIN-TCPs have been implicated in regulation of surface curvature in Antirrhinum and in formation of tomato compound leaves (Nath, et al., 2003; Ori et al., 2007). To find a unifying theme for these functions, the authors used the digital differentiation index to examine Arabidopsis leaves in which the activity of all eight CIN-TCPs was downregulated by overexpression of microRNAs (miR319, which targets five of the eight TCPs [Palatnik et al., 2003], and an artificial microRNA designed to target the remaining three). Analysis of young transgenic lines with this approach implicated regulation of leaf differentiation as a prime role of these TCPs. In mature mutant plants, cell size and number was altered dramatically, suggesting that further stages of leaf differentiation were also delayed or inhibited in the absence of TCP activity (see figure). This work provides a new tool for the investigation of heterochronic pathways operating in plant organs or at the whole-shoot level and highlights the role of CIN-TCPs as heterochronic regulators of leaf development.

Trade‐offs between direct and indirect defences of lima bean (Phaseolus lunatus)

Summary Plant defence theory predicts trade‐offs among defence traits as a result of resource limitation or pleiotropic effects. Although theoretically widely accepted, empirical demonstrations of such trade‐offs are surprisingly scarce and mechanistic explanations are usually lacking. We quantified cyanogenesis (the release of hydrogen cyanide (HCN)) as a direct defence and the emission of volatile organic compounds (VOCs) as an indirect defence against herbivores. To elucidate whether the trade‐offs occur at the genetic or phenotypic level we investigated cultivated and wild‐type accessions of lima bean (Fabaceae: Phaseolus lunatus L.) and compared different leaf developmental stages. Genetic relationships among the accessions were studied using amplified fragment length polymorphism (AFLP) analysis. Cyanogenesis and the release of VOCs differed significantly among the accessions and were negatively correlated: high cyanogenic accessions released low amounts of VOCs and vice versa. The same remained true for the ontogenetic stages, since primary leaves of all accessions hardly ever produced HCN at all, yet regularly showed high release rates of VOCs. Low and high cyanogenic accessions of lima bean formed distinct clades in an AFLP‐based dendrogram, while wild‐types and cultivars did not separate. The first pattern indicates that the underlying defensive syndromes are genetically conserved, while the latter is likely to be caused by a multiple origin of cultivated lima beans or an extensive gene flow among cultivated and wild plants. Synthesis. Trade‐offs between cynogenesis and VOC release were obvious both between accessions and at the ontogenetic level, and thus cannot be explained by pleiotropy. We contend that allocation restrictions and/or adaptations to different enemy pressures are most likely to explain why lima bean can invest into cyanogenesis or VOCs, but not both.

Transcription factors regulating leaf senescence in Arabidopsis thaliana

Senescence is a highly regulated process, eventually leading to cell and tissue disintegration: a physiological process associated with nutrient (e.g. nitrogen) redistribution from leaves to reproductive organs. Senescence is not observed in young leaves, indicating that repressors efficiently act to suppress cell degradation during early leaf development and/or that senescence activators are switched on when a leaf ages. Thus, massive regulatory network re-wiring likely constitutes an important component of the pre-senescence process. Transcription factors (TFs) have been shown to be central elements of such regulatory networks. Here, we used quantitative real-time polymerase chain reaction (qRT-PCR) analysis to study the expression of 1880 TF genes during pre-senescence and early-senescence stages of leaf development, using Arabidopsis thaliana as a model. We show that the expression of 185 TF genes changes when leaves develop from half to fully expanded leaves and finally enter partial senescence. Our analysis identified 41 TF genes that were gradually up-regulated as leaves progressed through these developmental stages. We also identified 144 TF genes that were down-regulated during senescence. A considerable number of the senescence-regulated TF genes were found to respond to abiotic stress, and salt stress appeared to be the major factor controlling their expression. Our data indicate a peculiar fine-tuning of developmental shifts during late-leaf development that is controlled by TFs.

Cytokinin deficiency causes distinct changes of sink and source parameters in tobacco shoots and roots

Cytokinin deficiency causes pleiotropic developmental changes such as reduced shoot and increased root growth. It was investigated whether cytokinin-deficient tobacco plants, which overproduce different cytokinin oxidase/dehydrogenase enzymes, show changes in different sink and source parameters, which could be causally related to the establishment of the cytokinin deficiency syndrome. Ultrastructural analysis revealed distinct changes in differentiating shoot tissues, including an increased vacuolation and an earlier differentiation of plastids, which showed partially disorganized thylakoid structures later in development. A comparison of the ploidy levels revealed an increased population of cells with a 4C DNA content during early stages of leaf development, indicating an inhibited progression from G2 to mitosis. To compare physiological characteristics of sink leaves, source leaves and roots of wild-type and cytokinin-deficient plants, several photosynthetic parameters, content of soluble sugars, starch and adenylates, as well as activities of enzymes of carbon assimilation and dissimilation were determined. Leaves of cytokinin-deficient plants contained less chlorophyll and non-photochemical quenching of young leaves was increased. However, absorption rate, photosynthetic capacity (Fv/Fm and JCO2max) and efficiency (ΦCO2app), as well as the content of soluble sugars, were not strongly altered in source leaves, indicating that chlorophyll is not limiting for photoassimilation and suggesting that source strength did not restrict shoot growth. By contrast, shoot sink tissues showed drastically reduced contents of soluble sugars, decreased activities of vacuolar invertases, and a reduced ATP content. These results strongly support a function of cytokinin in regulating shoot sink strength and its reduction may be a cause of the altered shoot phenotype. Roots of cytokinin-deficient plants contained less sugar compared with wild-type. However, this did not negatively affect glycolysis, ATP content, or root development. It is suggested that cytokinin-mediated regulation of the sink strength differs between roots and shoots.

Nitrogen Recycling and Remobilization Are Differentially Controlled by Leaf Senescence and Development Stage in Arabidopsis under Low Nitrogen Nutrition

Five recombinant inbred lines (RILs) of Arabidopsis (Arabidopsis thaliana), previously selected from the Bay-0 x Shahdara RIL population on the basis of differential leaf senescence phenotypes (from early senescing to late senescing) when cultivated under nitrogen (N)-limiting conditions, were analyzed to monitor metabolic markers related to N assimilation and N remobilization pathways. In each RIL, a decrease of total N, free amino acid, and soluble protein contents with leaf aging was observed. In parallel, the expression of markers for N remobilization such as cytosolic glutamine synthetase, glutamate dehydrogenase, and CND41-like protease was increased. This increase occurred earlier and more rapidly in early-senescing lines than in late-senescing lines. We measured the partitioning of (15)N between sink and source leaves during the vegetative stage of development using (15)N tracing and showed that N remobilization from the source leaves to the sink leaves was more efficient in the early-senescing lines. The N remobilization rate was correlated with leaf senescence severity at the vegetative stage. Experiments of (15)N tracing at the reproductive stage showed, however, that the rate of N remobilization from the rosettes to the flowering organs and to the seeds was similar in early- and late-senescing lines. At the reproductive stage, N remobilization efficiency did not depend on senescence phenotypes but was related to the ratio between the biomasses of the sink and the source organs.

Identification of early senescence-associated genes in rice flag leaves

Leaf senescence is one of the key stages of plant leaf development. It is a highly complex but ordered process involving expression of large scale senescence associated genes, and its molecular mechanisms still remain unclear. By using suppression subtractive hybridization, 815 ESTs that are up-regulated at the onset of rice flag leaf senescence have been isolated. A total of 533 unigenes have been confirmed by macroarray detection and sequencing. 183 of these unigenes have GO annotations, involved in macromolecule metabolism, protein biosynthesis regulation, energy metabolism, gene expression regulations, detoxification, pathogenicity and stress, cytoskeleton organization and flower development. Another 121 unigenes co-localized with previously reported known stay-green QTLS. RT-PCR analysis on the other novel genes indicated that they can be up-regulated in natural early senescence and induced by hormone. Our results indicate that senescence is closely related to various metabolic pathways, thus providing new insight into the onset of leaf senescence mechanism.

Characterization of KNOX genes in Medicago truncatula

We isolated three class I and three class II KNOX genes in Medicago truncatula. The predicted amino acid sequences suggested a possible orthology to the Arabidopsis homeodomain proteins STM, KNAT1/BP, KNAT3 and KNAT7 that was confirmed by phylogenetic and conserved structural domain analyses. Moreover, the STM-like MtKNOX1 and MtKNOX6 proteins were shown to retain the capability to interact with the Arabidopsis BELL protein partners of STM and KNAT1/BP. Amino acid residues that characterize the different classes of KNOX proteins were identified. Gene expression studies revealed organ-specificity, possible cytokinin-dependent transcriptional activation of two MtKNOXs and expression of one STM-like and a BP/KNAT1-like MtKNOX in roots. Interestingly, mRNA localization studies carried out on class I MtKNOX genes revealed important differences with previously characterised legume KNOXs. M. truncatula transcripts were not down-regulated in leaf primordia and early stages of leaf development, features shared with the more distant compound-leaved species Solanum lycopersicum.

Metabolism of 14C-glycine as a substrate for photorespiration of the leaf at different developmental stages of ephemeroides

The metabolism of 14C-glycine (a substrate for photorespiration) was studied in the light and in darkness under natural CO2 concentration (0.03%) in the leaves of ephemeroides Scilla sibirica Haw. and Ficaria verna Huds. at different developmental stages. Using one and the same sample, potential photosynthesis (at 1% CO2), true photosynthesis (at 0.03% CO2), and leaf respiratory capacity were measured by the radiometric and manometric methods, respectively. All measurements were performed at 15°C, an average temperature during ephemer growth. It was found that, in the white zone of the Scilla leaf, the rate of CO2 evolution resulting from metabolization of exogenous 14C-glycine was similar in the light and in darkness. In the green zone of the Scilla leaf and in the green leaf of Ficaria, both 14C-glycine absorption and 14CO2 evolution were lower in the light as compared with darkness, which is explained by CO2 reassimilation. In all treatments of both plant species, a specific inhibitor of glycine decarboxylase complex (GDC), aminoacetonitrile (5 mM) suppressed CO2 evolution by 20–40%. It was concluded that in ephemeroides mitochondrial GDC, responsible for CO2 evolution in photorespiration, is formed at the earliest stage of leaf development. This indicates that photorespiration can occur simultaneously with the development of the leaf photosynthetic activity. On the basis of the assumption that carbon losses in the form of CO2 evolved during photorespiration comprise 25% of true photosynthesis, it was calculated that, in ephemer leaves, the highest rates of photorespiration and photosynthesis were attained during flowering when the leaf area was the largest and the rate of dark respiration was reduced by 1.5–2.0 times. The highest rates of dark respiration were observed in the beginning of growth. In senescing leaves by the end of the plant vegetation, potential photosynthesis and true photosynthesis were reduced, whereas dark respiration remained essentially unchanged. It is concluded that the high rates of potential and true photosynthesis are characteristic of ephemeroides when they complete their short developmental program in early spring (at 15°C); theoretically, photorespiration also occurs at a high rate during this period, when this process provides for a defense against the threat of photoinhibition at low temperature and high insolation.

Chapter Seven: Pattern of Leaf Development in Schistochilaceae

Early stages of leaf development were studied in Schistochila appendiculata (Hook.) Dumort. ex Trevis., S. nobilis (Hook.) Trevis., Pachyschistochila subhyalina (R. M. Schust.) R. M. Schust. & J. J. Engel, and Gottschea conchophylla (E. A. Hodgs. & Allison) Grolle & Zijlstra, using transverse microtome sections. Like other groups of leafy liverworts, all four species have apical cells with three cutting faces and two primary leaf initials from which bilobed leaves are initiated. Apical cell geometry and early patterns of merophyte development are the same for all taxa studied, including those without underleaves. The wing of the leaf develops very early, shortly after the leaf lobes begin to form. The initial sign of wing development is the outward expansion and protrusion of a row of cells along the keel and the basiscopic or posterior margin of the ventral lobe. These cells undergo subsequent divisions so that transverse sections of the young leaves appear Y-shaped.

Study of Early Leaf Senescence in Arabidopsis thaliana by Quantitative Proteomics Using Reciprocal 14N/15N Labeling and Difference Gel Electrophoresis

Leaf senescence represents the final stage of leaf development and is associated with fundamental changes on the level of the proteome. For the quantitative analysis of changes in protein abundance related to early leaf senescence, we designed an elaborate double and reverse labeling strategy simultaneously employing fluorescent two-dimensional DIGE as well as metabolic (15)N labeling followed by MS. Reciprocal (14)N/(15)N labeling of entire Arabidopsis thaliana plants showed that full incorporation of (15)N into the proteins of the plant did not cause any adverse effects on development and protein expression. A direct comparison of DIGE and (15)N labeling combined with MS showed that results obtained by both quantification methods correlated well for proteins showing low to moderate regulation factors. Nano HPLC/ESI-MS/MS analysis of 21 protein spots that consistently exhibited abundance differences in nine biological replicates based on both DIGE and MS resulted in the identification of 13 distinct proteins and protein subunits that showed significant regulation in Arabidopsis mutant plants displaying advanced leaf senescence. Ribulose 1,5-bisphosphate carboxylase/oxygenase large and three of its four small subunits were found to be down-regulated, which reflects the degradation of the photosynthetic machinery during leaf senescence. Among the proteins showing higher abundance in mutant plants were several members of the glutathione S-transferase family class phi and quinone reductase. Up-regulation of these proteins fits well into the context of leaf senescence since they are generally involved in the protection of plant cells against reactive oxygen species which are increasingly generated by lipid degradation during leaf senescence. With the exception of one glutathione S-transferase isoform, none of these proteins has been linked to leaf senescence before.

Quantifying Phenology and Maturity in Crisphead Lettuce

The Biologische Bundesanstalt, Bundessortenamt und Chemische Industrie (BBCH) identification key was adapted for crisphead lettuce (Lactuca sativa) to facilitate identification of phenological stages and decisions regarding field operations from seeding to harvest maturity. The original system described leaf development based on leaf count from stage 11 (1 leaf) to stage 19 (9 leaf), and head development based on percentage of expected head size reached at maturity from stage 41 to 49. The new coding leaf development stages range from 11 to 29, corresponding to the 1-leaf to 19-leaf stages. The head development stages also ranged from 41 to 49, but phenological stages near commercial maturity from 43 to 49 are now described as a function of head firmness. The important maturity traits of crisphead lettuce include head size and density. Head volume can be estimated from three diameters by using Currence's equation, which takes into account head geometry. The firmness index obtained by hand compression gave a more precise estimate of head density than the density estimate derived from Currence's equation or the sphere equation. Crisphead lettuce development stages and maturity traits can be easily quantified in the field for use in planning field operations and for experimental purposes.

Genetic control of early stages of leaf development

The results of studies of genetic regulation of the early leaf morphogenesis, demarcation of the future primordium and transition of cells to determination, have been reviewed. The genetic systems of control of these developmental stages were shown to be conservative and hypotheses of possible mechanisms underlying the evolution of leaf morphology on their basis have been considered.

Increased expression of the rice C-type cyclin-dependent protein kinase gene, Orysa; CDKC;1, in response to salt stress

Cyclin-dependent protein kinases (CDKs) form a conserved superfamily of eukaryotic serine/threonine protein kinases which require binding to a regulatory cyclin for activity. CDKs are organized in several gene families and are involved in different aspects of cell biology, such as gene transcription, cell proliferation and differentiation. In this study, we present the characteristics of the rice C-type CDK gene, Orysa; CDKC;1. The gene was expressed in all organs of the plant and relatively high amounts of its transcript were detected in mature leaves and panicles. This indicated a putative role during later stages of leaf and panicle development. Using yeast two-hybrid analysis, we identified Orysa; CYCT as an interactor with Orysa; CDKC;1. The expression profiles of the new Orysa; CDKC;1 in response to various factors from rice cells were compared with those of previously described Orysa; CDKA;1, and Orysa; CDKB2;1. The Orysa; CDKC;1 transcript was induced by salt stress and ABA treatments, but was unaffected by mannitol. In addition, the MBP kinase activity associated with Orysa;CDKC;1 immunocomplexes was induced under salt stress. We propose that Orysa; CDKC;1 may be involved in developmental programs, as well as in the salt- and ABA-signaling pathway.

Overexpression of a chromatin architecture‐controlling AT‐hook protein extends leaf longevity and increases the post‐harvest storage life of plants

Leaf senescence is the final stage of leaf development and is finely regulated via a complex genetic regulatory network incorporating both developmental and environmental factors. In an effort to identify negative regulators of leaf senescence, we screened activation-tagged Arabidopsis lines for mutants that exhibit a delayed leaf senescence phenotype. One of the mutants (ore7-1D) showed a highly significant delay of leaf senescence in the heterozygous state, leading to at least a twofold increase in leaf longevity. The activated gene (ORE7/ESC) encoded a protein with an AT-hook DNA-binding motif; such proteins are known to co-regulate transcription of genes through modification of chromatin architecture. We showed that ORE7/ESC, in addition to binding to a plant AT-rich DNA fragment, could also modify the chromatin architecture, as illustrated by an altered distribution of a histone-GFP fusion protein in the nucleus of the mutant. Globally altered gene expression, shown by microarray analysis, also indicated that activation of ORE7/ESC results in a younger condition in the mutant leaves. We propose that ectopically expressed ORE7/ESC is negatively regulating leaf senescence and suggest that the resulting chromatin alteration may have a role in controlling leaf longevity. Interestingly, activation of ORE7/ESC also led to a highly extended post-harvest storage life.