Ascorbate–glutathione pathways mediated by cytokinin regulate H2O2 levels in light-controlled rose bud burst

Rosebush (Rosa “Radrazz”) plants are an excellent model to study light control of bud outgrowth since bud outgrowth only arises in the presence of light and never occurs in darkness. Recently, we demonstrated high levels of hydrogen peroxide (H₂O₂) present in the quiescent axillary buds strongly repress the outgrowth process. In light, the outgrowing process occurred after H₂O₂ scavenging through the promotion of Ascorbic acid–Glutathione (AsA–GSH)-dependent pathways and the continuous decrease in H₂O₂ production. Here we showed Respiratory Burst Oxidase Homologs expression decreased in buds during the outgrowth process in light. In continuous darkness, the same decrease was observed although H₂O₂ remained at high levels in axillary buds, as a consequence of the strong inhibition of AsA–GSH cycle and GSH synthesis preventing the outgrowth process. Cytokinin (CK) application can evoke bud outgrowth in light as well as in continuous darkness. Furthermore, CKs are the initial targets of light in the photocontrol process. We showed CK application to cultured buds in darkness decreases bud H₂O₂ to a level that is similar to that observed in light. Furthermore, this treatment restores GSH levels and engages bud burst. We treated plants with buthionine sulfoximine, an inhibitor of GSH synthesis, to solve the sequence of events involving H₂O₂/GSH metabolisms in the photocontrol process. This treatment prevented bud burst, even in the presence of CK, suggesting the sequence of actions starts with the positive CK effect on GSH that in turn stimulates H₂O₂ scavenging, resulting in initiation of bud outgrowth.

Light-induced bud outgrowth in rosebush results from cytokinin-mediated peroxide scavenging and glutathione metabolism stimulation.     

Branching responses of a plagiotropic clonal herb to localised incidence of light simulating that reflected from vegetation

Plants sense the presence of neighbouring vegetation through phytochrome photoreceptors perceiving a lowered red to far-red ratio (R:FR) of light reflected from such vegetation. We hypothesised that it would be advantageous for the grassland clonal herb, Trifolium repens, to have an inhibitory branching response to perception by leaves of light reflected from neighbouring vegetation (i.e. light with lowered R:FR ratio) but have no response to interception of such light by the plagiotropic stem. We tested whether photoreception of reflected light by plagiotropic stems resulted in a different branching response to photoreception by leaves and whether leaf ontogeny influenced the response. To simulate light reflected from vegetation, FR light-emitting-diodes were used to supplement controlled environment room light so that the R:FR ratio, but not the photosynthetic photon fluence rate, of light incident at the stem or leaf of a phytomer of T. repens was lowered from 1.20 to 0.25. The plagiotropic stems were unresponsive to light simulating that reflected from vegetation. This response differs from that of stems of orthotropic species, indicating that plagiotropic stems have evolved an organ-specific photobiology. Treatment of the mature leaf with light of lowered FR ratio reduced phytomer production only of the branch in the axil of the treated leaf. Similar treatment of the immature leaf retarded, in addition, branching at basal phytomers on the same side of the primary stem axis. Thus the response to light simulating that reflected from neighbouring vegetation depended upon whether the light was incident at the stem or the leaf and on the stage of leaf development. We argue that such responses improve the performance and fitness of T. repens within grassland habitats by allowing axillary buds on plagiotropic stems to branch freely when stems are in receipt of light reflected from vegetation while leaves are in full light.     

Effect of Light Quality (Red:Far-red Ratio) at the Apical Bud of the Main Stolon on Morphogenesis of Trifolium repens L.

A controlled environment experiment investigated whether thered:far-red (R:FR) ratio of light at the apical bud of the mainstolon could alter plant morphogenesis in clonal cuttings ofwhite clover (Trifolium repens L.) The apical bud included theapical meristem, five to six developing leaf primordia withassociated axillary bud primordia and stipules and the firstemerged folded leaf until development was greater than 0·3on the Carlson scale. Three light regimes were imposed on theapical bud by collimating light from R or FR light-emittingdiodes so that the R:FR ratio of light incident at the apicalbud was set at 0·25, 1·6 or 2·1, withoutsignificantly altering photosynthetically active radiation.The effect of these light regimes on white clover seedling growthwas also tested. At a low R:FR ratio seedling hypocotyl and cotyledon lengthswere significantly longer. However, with the cuttings, the lighttreatments did not alter node appearance rate or internode lengthof the main stolon, petiole length, area of leaves or totalshoot dry matter. There was one significant photomorphogeneticresponse in the cuttings, a delay of 0·5 of a phyllochronin the appearance of branches from axillary buds in the lowR:FR ratio treatment relative to the other treatments. Wherebranch appearance was delayed plants had fewer branches. Thisdifference could be ascribed solely to a delay in branch appearanceas there were no significant treatment effects on either theinitiation of axillary bud primordia within the apical bud,the probability of branching or on the rate of growth of branchesafter appearance. Because treatment of the apical bud inducedonly one of the many previously observed responses of whiteclover to a decrease in the R:FR ratio of light, we concludethat other plant organs must also sense the quality of incidentlight.Copyright 1994, 1999 Academic Press White clover, Trifolium repens, apical bud, light quality, red:far-red ratio, light-emitting diode, branching, axillary buds, photomorphogenesis     

Dual effect of light on flowering and sprouting of rose shoots

Shade, caused by a dense leaf canopy in the light conditions of a normal greenhouse, reduced sprouting of the third axillary bud (from the top) on decapitated rose branches ( Rosa hybrida cv. Marimba) in comparison to less shaded buds on branches protruding above the canopy and sparsely spaced. Flowering of the third young shoot on shaded branches bearing 3 lateral shoots was totally inhibited. Mixed fluorescent and incandescent light in a growth chamber reduced sprouting of the third bud on decapitated rose branches in comparison to decapitated branches on rose plants held in fluorescent light of similar photon flux density. This was attributed to the higher R:FR ratio in fluorescent vs mixed light that reached the third bud, and in exposed vs shaded branches. Flowering of the third shoot was promoted by several factors: high photon flux density, 0.5 m M gibberellic acid (GA) or 0.2 m M benzyladenine (BA). BA was the most effective treatment. Treatments promoting flowering of the third shoot did not reduce growth or flowering of the upper shoots. However, spraying the uppermost shoot with BA suppressed the growth of the shoots below. It is concluded that light affects flowering in two ways. The effect on bud sprouting is related mainly to R:FR ratios, while the effect on flower development is related mainly to photon flux density. Cytokinins may substitute for the light effect on flower development.     

A developmentally based categorization of branching in Trifolium repens L.: influence of nodal roots

This study describes the successive stages of development of branches from axillary buds in fully rooted plants of Trifolium repens grown in near optimal conditions, and the way in which this developmental pathway differs when nodal root formation is prevented as plants grow out from a rooted base. Cuttings of a single genotype were established in a glasshouse with nodal root systems on the two basal phytomers and grown on so that nodal rooting was either permitted (+R) or prevented (-R). In +R plants, axillary tissues could be assigned to one of four developmental categories: unemerged buds, emerged buds, unbranched lateral branches or secondarily branched lateral branches. In -R plants, branch development was retarded, with the retardation becoming increasingly pronounced as the number of -R phytomers on the primary stolon increased. Retarded elongation of the internodes of lateral shoots on -R plants resulted in the formation of a distinct fifth developmental category: short shoots (defined as branches with two or more leaves but with mean internode length equal to, or less than, 10% of that of the immediately proximal internode on the parent stolon) which had reduced phytomer appearance rates but retained the potential to develop into lateral branches. Transfer of +R plants to -R conditions, and vice versa, after 66 d demonstrated that subsequent branch development was wholly under the control of the youngest nodal root present, regardless of the age and number of root systems proximal to it.     

Jasmonates Inhibit Flowering in Short-Day Plant Pharbitis nil

The role of jasmonates in the photoperiodic flower induction of short-day plant Pharbitis nil was investigated. The plants were grown in a special cycle: 72 h of darkness, 24 h of white light with lowered intensity, 24-h long inductive night, 14 days of continuous light. At 4 h of inductive night the cotyledons of non-induced plants contained about two times the amount of endogenous jasmonates (JA/JA-Me) compared to those induced. A 15-min long pulse of far red light (FR) applied at the end of a 24-h long white light phase inhibited flowering of P. nil. The concentration of jasmonates at 2 and 4 h of inductive night in the cotyledons of the plants treated with FR was similar. Red light (R) could reverse the effect of FR. R light applied after FR light decreased the content of jasmonates by about 50%. Methyl jasmonate (JA-Me) applied to cotyledons, shoot apices and cotyledon petioles of P. nil inhibited the formation of flower buds during the first half of a 24-h long inductive or 14-h long subinductive night. Application of JA-Me to the cotyledons was the most effective. None of the plants treated with JA-Me on the cotyledons in the middle of the inductive night formed terminal flower buds. The aspirin, ibuprofen and phenidone, jasmonates biosynthesis inhibitors partially reversed the effect of FR, stimulating the formation of axillary and terminal flower buds. Thus, the results obtained suggests that phytochrome system control both the photoperiodic flower induction and jasmonates metabolism. Jasmonates inhibit flowering in P. nil.     

Effects of fertilization on the growth,photosynthesis, and biomass accumulation in juvenile plants of three coffee (<Emphasis Type="Italic">Coffea arabica</Emphasis> L.) cultivars

We carried out a field experiment in order to study effects of fertilization in juvenile plants of three coffee (Coffea arabica) cultivars in Yunnan, SW China. Fertilization treatments included a control without fertilizer (CK), combinations of three NPK fertilization rates [high fertilization (F_H), medium fertilization (F_M), and low fertilization (F_L) with 135, 90, and 45 g per plant per year, respectively], and at two N:P₂O₅:K₂O ratios (R₁, 1:0.5:0.8; R₂, 1:0.8:0.5). The growth in juvenile plants was not altered by fertilization, with two clear growth peaks being observed in both the height and stem growth rates (RGRs) throughout a year. Both F_M and F_H resulted in significantly higher RGRs in both height and stem diameter compared to F_L and CK in all three cultivars. At the same fertilization rate, the leaf area, branch number, longest branch length, internode number, and biomass of R₂ were higher than those of R₁, and P significantly affected the root biomass and root to shoot ratio. Compared to the F_L treatment, both F_M and F_H treatments resulted in higher net photosynthetic rates and stomatal conductance across seasons, and in higher intrinsic water-use efficiency during the dry season and at the middle of the wet season. Photosynthetic nitrogen-use efficiency at R₂ was higher than that at R₁, but no significant differences were observed between the different fertilization rates. Among the three coffee cultivars, Caturra exhibited the highest height, stem diameter, longest branch length, and internode number. Our results indicated that the optimal N:P₂O₅:K₂O ratio was 1:0.8:0.5 for the juvenile growth of coffee plants. Both F_M and F_H could help optimize the growth and photosynthetic rate of coffee plants, but F_M is suitable for the ecological friendly agriculture and economic sustainability at coffee plantations.     

Effect of light quality (red: far-red ratio) and defoliation treatments applied at a single phytomer on axillary bud outgrowth in Trifolium repens L.

We studied the effects of light quality and defoliation on the rate of phytomer appearance and axillary bud outgrowth in white clover. The treatments were applied to one phytomer, a phytomer being defined as the structural unit comprising a node, internode, axillary bud, subtending leaf and two nodal root primordia. Light of a low red:far-red (R:FR) ratio (0.27) was applied to a target phytomer either (i) within the apical bud and then to the axillary bud after emergence of the phytomer from the apical bud, or (ii) to the axillary bud only after emergence. The light conditions were directed to these specific parts of the plant by collimating light from small FR light-emitting diodes; with this technique we were able to change the light quality without any change in the level of photosynthetically active radiation. The subtending leaf of the target phytomer was retained or defoliated when it had emerged from the apical bud. FR light applied from the time the phytomer was within the apical bud caused a delay in branch appearance at the target phytomer. In contrast, direct treatment of the axillary bud with FR light after it had emerged from the apical bud did not result in any delay in branch appearance. As the light treatment of the apical bud may have changed the light environment of any of the organs contained in the bud we were unable to ascribe the delay in branch appearance to light perception by any particular organ. However, indirect evidence leads to the conclusion that the likely site of light perception was the developing leaf subtending the axillary bud while it was the outermost phytomer within the apical bud. These results do not support the hypothesis that the R:FR ratio of light incident at an axillary bud site is the environmental factor that controls bud development. Defoliation of the unfolding leaf reduced the rate of phytomer appearance on the main stolon but had no immediate effect on branch appearance. As a consequence there was a reduction in the number of phytomers between the stolon apical meristem and the first phytomer with a branch. This is frequently taken to indicate a relaxation of apical dominance, but in this case was found not to involve a direct effect on bud activity. A current model of white clover growth suggests that there is integration of activity between apical meristems but independence of activity and response to the local micro-environment by axillary buds. In contrast, we found that (i) defoliation reduced phytomer appearance only at the main stolon apical meristem and not at all the meristems in the plant and (ii) that a change in the local light environment of an axillary bud had no discernible effect on bud activity once the bud had emerged from the apical bud but could delay branching if applied before emergence. These results are at variance with the predictions of the model.     

Characterization of Shade Avoidance Responses in Lotus japonicus

Sessile plants must continuously adjust their growth and development to optimize photosynthetic activity under ever-fluctuating light conditions. Among such light responses in plants, one of the best-characterized events is the so-called shade avoidance, for which a low ratio of the red (R):far-red (FR) light intensities is the most prominent stimulus. Such shade avoidance responses enable plants to overtop their neighbors, thereby enhancing fitness and competitiveness in their natural habitat. Considerable progress has been achieved during the last decade in understanding the molecular mechanisms underlying the shade avoidance responses in the model rosette plant, Arabidopsis thaliana. We characterize here the fundamental aspects of the shade avoidance responses in the model legume, Lotus japonicus, based on the fact that its phyllotaxis (or morphological architecture) is quite different from that of A. thaliana. It was found that L. japonicus displays the characteristic shade avoidance syndrome (SAS) under defined laboratory conditions (a low R:FR ratio, low light intensity, and low blue light intensity) that mimic the natural canopy. In particular, the outgrowth of axillary buds (i.e., both aerial and cotyledonary shoot branching) was severely inhibited in L. japonicus grown in the shade. These results are discussed with special emphasis on the unique aspects of SAS observed with this legume.     

Characterization of shade avoidance responses in Lotus japonicus

Sessile plants must continuously adjust their growth and development to optimize photosynthetic activity under ever-fluctuating light conditions. Among such light responses in plants, one of the best-characterized events is the so-called shade avoidance, for which a low ratio of the red (R):far-red (FR) light intensities is the most prominent stimulus. Such shade avoidance responses enable plants to overtop their neighbors, thereby enhancing fitness and competitiveness in their natural habitat. Considerable progress has been achieved during the last decade in understanding the molecular mechanisms underlying the shade avoidance responses in the model rosette plant, Arabidopsis thaliana. We characterize here the fundamental aspects of the shade avoidance responses in the model legume, Lotus japonicus, based on the fact that its phyllotaxis (or morphological architecture) is quite different from that of A. thaliana. It was found that L. japonicus displays the characteristic shade avoidance syndrome (SAS) under defined laboratory conditions (a low R:FR ratio, low light intensity, and low blue light intensity) that mimic the natural canopy. In particular, the outgrowth of axillary buds (i.e., both aerial and cotyledonary shoot branching) was severely inhibited in L. japonicus grown in the shade. These results are discussed with special emphasis on the unique aspects of SAS observed with this legume.     

Density-dependent regulation of ramet recruitment by the red:far-red ratio of solar radiation: a field evaluation with the bunchgrass Schizachyrium scoparium

Depressions in the red to far-red ratio (R:FR) of solar radiation arising from the selective absorption of R (600–700 nm) and scattering of FR (700–800 nm) by chlorophyll within plant canopies may function as an environmental signal directly regulating axillary bud growth and subsequent ramet recruitment in clonal plants. We tested this hypothesis in the field within a single cohort of parental ramets in established clones of the perennial bunchgrass, Schizachyrium scoparium. The R:FR was modified near leaf sheaths and axillary buds at the bases of individual ramets throughout the photoperiod without increasing photosynthetic photon flux density (PPFD) by either (1) supplementing R beneath canopies to raise the naturally low R:FR or (2) supplementing FR beneath partially defoliated canopies to suppress the natural R:FR increase following defoliation. Treatment responses were assessed by simultaneously monitoring ramet recruitment, PPFD and the R:FR beneath individual clone canopies at biweekly intervals over a 12-week period. Neither supplemental R nor FR influenced the rate or magnitude of ramet recruitment despite the occurrence of ramet recruitment in all experimental clones. In contrast, defoliation with or without supplemental FR beneath clone canopies reduced ramet recruitment 88% by the end of the experiment. The hypothesis stating that the R:FR signal directly regulates ramet recruitment is further weakened by evidence demonstrating that (1) the low R:FR-induced suppression of ramet recruitment is only one component of several architectural modifications exhibited by ramets in response to the R:FR signal (2) immature leaf blades, rather than leaf sheaths or buds, function as sites of R:FR perception on individual ramets, and (3) increases in the R:FR at clone bases following partial canopy removal are relatively transient and do not override the associated constraints on ramet recruitment resulting from defoliation. A depressed R:FR is probably of greater ecological significance as a signal of competition for light in vegetation canopies than as a density-dependent signal which directly regulates bud growth and ramet recruitment.     

Chilling injury and recovery in detached and attached leaves measured by chlorophyll fluorescence

Smillie, R. M., Nott, R., Hetherington, S. E. and Öyustt, G. 1987. Chilling injury and recovery in detached and attached leaves measured by chlorophyll fluorescence Chilling injury was compared in detached and attached leaves chilled at 0 or 0.5°C by measuring the decrease in induced chlorophyll fluorescence in vivo. The fluorescence parameter measured was F_R, the maximal rate of rise of induced chlorophyll fluorescence emission after irradiating dark-adapted leaves. The plants used were bean, Phaseolus vulgaris L. cv. Pioneer, and maize, Zea mays L. cvs hybrid GH 390 and Northern Belle. Leaves were detached and placed on wet paper and covered with thin polyethylene film to prevent water loss during chilling. Leaves left attached on plants were treated similarly. When chilled in this way at 100% relative humidity, the chilling-induced decrease in F_R was the same in detached and attached leaves. For the attached leaves, the same result was obtained whether just a single leaf was chilled or the whole plant. Expression of chilling injury was greatest in fully turgid leaves and comparisons can be invalid unless the water status of the detached and attached leaves are the same. Problems arising from diurnal fluctuations in water potential of plants grown in a glasshouse were circumvented by placing leaves on the wet filter paper under polyethylene film prior to chilling, which allowed high water potentials to be regained, or mist sprays in the glasshouse were employed. Determinations of the time course for changes in F_R of maize (cv. Northern Belle) during chilling at 0°C showed that F_R decreased exponentially, at the same rate (time to 50% decrease in F_R was 9.3 h) in detached and attached leaves. Chilling injury was largely reversible for the first 20 h of chilling stress as both detached and attached leaves recovered their pre-chilling values of F_R after a further 20 h at 20°C in darkness. Leaves chilled for 48 h showed partial recovery, while those chilled for 72 h did not recover. Recovery was impeded by light. Inability to recover from chilling as indicated by measurements of F_R was paralleled by the incidence of visible symptoms of injury. It is concluded that detached and attached leaves behave similarly during chilling and short-term recovery, provided a similarity in treatments is rigorously maintained.     

Organ correlations inchenopodium rabrum l. shoots studied by Means of32P Distribution

21-day old plants ofChenopodium rubrum L. ecotype 374 were used. Organ relationships in the shoots were investigated by³²P distribution, which indicated different organ correlations in plants grown in continuous light and in plants treated with flower-inducing and non-inducing dark periods. Dark periods were associated with a low³²P distribution in young leaves and a high one in axillary buds. In the following light period the high³²P distribution in axillary buds continued whereas the³²P distribution in the leaves on the main axis increased and was similar to that in plants grown in continuous light. The high³²P distribution in axillary buds was brought about by both, flower-inducing and non-inducing dark treatments. Decapitation resulted in a high³²P distribution in buds, in continuous light an increased³²P distribution was also found in leaves. These effects were not fully cancelled by IAA application. The results are discussed with respect to an assumption that decrease of apical dominance represents a step in a sequence of events leading to flowering.     

The Formation and Development of Adventitious Buds on Potato Sprouts

Potato tubers were pre-sprouted in trays at 10 C for 28 d.The scale leaves at the base of the sprouts were marked andtubers then planted in field soil. Twenty-eight days after plantingan average of three adventitious buds were present per sproutcompared with an average of six below-ground axillary buds.Ten per cent of adventitious buds formed stolons compared withalmost 100 per cent of axillary buds. Similar responses wereseen with tubers pre-sprouted either in the light or dark.     

Axillary Bud Development in White Clover in Relation to Defoliation and Shading Treatments

A period of growth under shade netting in the glasshouse allowedthe cultivation of white clover stolons with an accumulationof undeveloped axillary buds similar to that often found onstolons from grass/clover swards. The subsequent capacity ofthese nodes to develop branches under different circumstanceswas investigated in three experiments. Removal of the laminaeand petioles subtending sets of four buds along a stolon reducedthe rate at which branches were initiated from the buds. Treatmentsin which petioles, or petioles plus laminae, were retained initiatedbranches more quickly. Shading the stolons reduced both therate of initiation and the percentage of buds which developed,unless both petioles and laminae were retained. There was someevidence that conditions applied to individual buds may actin the same way as the same conditions applied to sets of fourbuds and that illuminated nodes may depress the performanceof neighbouring shaded notes. Fewer buds developed at older nodes than at younger nodes duringthe summer, but during the autumn younger buds initially developedmore slowly than older buds. This suggests that buds can developat a younger nodal age in summer than in winter. When leafless stolons were cut up into component internodesbuds developed faster than on intact stolons, provided the budwas located at the end of the internode nearest the main stolongrowing point. If the bud was at the other end, branch developmentwas slower than on intact stolons. The results are discussedin relation to clover growth in sward conditions. White clover, Trifolium repens, axillary bud development, branching, growing points, defoliation, shading     

Effets du rapport RC:RS sur les jeunes feuilles de trèfle blanc. Assimilation nette de CO2, activité photochimique et morphologie foliaire

Young leaves of white clover are subjected to low irradiance and low red to far-red (R:FR) ratio within canopies. The objectives were to investigate the consequences of low R:FR ratio on morphology, net CO₂ assimilation and photochemical activity of leaves developed under simulated light environment of canopy. We used far-red (FR) light emitting diodes to modify the R:FR ratio only at the developing leaf under a low irradiance. Net CO₂ assimilation rate, stomatal conductance and leaf morphology were not affected by low R:FR ratio. FR exposure slightly reduced the photochemical quantum yield of PSII but there were no consequences on electron flow through photosystem II. The carbon fixation by the leaf was therefore not modified by light quality. However, low R:FR ratio decreased the leaf chlorophyll content by 21 %. Those effects were only attributed to just unfolded leaves as they were not persistent in mature leaves and there were no consequences on plant biomass accumulation.     

A modular approach to biomass allocation in an invasive annual (Microstegium vimineum; Poaceae)

The invasive grass Microstegium vimineum grows in low light beneath the canopy of eastern forests in North America by reiteration of modules (phytomers) along a tiller. Basal phytomers are vegetative; terminal phytomers produce a raceme of chasmogamous (CH) spikelets plus an axillary raceme of cleistogamous (CL) spikelets. Additional subterminal phytomers with CL racemes mature basipetally. Allocation to culms, leaves, and CH and CL within phytomers was examined in relation to light conditions for a population in New Jersey, USA. Plants were reared in a greenhouse from seed families of parents in deep shade (2-8% full sun) or sunny, edge habitats. Primary tillers were subdivided into phytomers, dried, and weighed. Tillers from field habitats were similarly treated. For vegetative and subterminal phytomers, allocation to leaves and CH was greatest for the shady habitat. CL allocation decreased from terminal to reproductively immature subterminal phytomers. CH and CL mass was positively correlated with leaf mass, suggesting that reproduction is determined by available photosynthate. CH mass showed a genetic correlation with leaf mass. Developmental plasticity in modular allocation allows Microstegium to maximize fitness when conditions are favorable (e.g., high light along forest edges) by continual maturation of CL caryopses on axillary racemes.     

Phytochrome A, phytochrome B and other phytochrome(s) regulate ATHB-2 gene expression in etiolated and green Arabidopsis plants

The expression of the Arabidopsis ATHB-2 gene is light-regulated both in seedlings and in adult plants. The gene is expressed at high levels in rapidly elongating etiolated seedlings and is down-regulated by a pulse of red light (R) through the action of a phytochrome other than phytochrome A or B, or by a pulse of far-red light (FR) through the action of phytochrome A. In green plants, the expression of the ATHB-2 gene is rapidly and strongly enhanced by lowering the R:FR ratio perceived by a phytochrome other than A or B. Returning the plant to a high R:FR ratio results in an equally rapid decrease of the ATHB-2 mRNA. Consistently, plants overproducing ATHB-2 show developmental phenotypes characteristic of plants grown in low R:FR: elongated petioles, reduced leaf area, early flowering, and reduced number of rosette leaves. Taken together, the data strongly suggest a direct involvement of ATHB-2 in light-regulated growth phenomena throughout Arabidopsis development.     

Accelerated leaf senescence takes part in enhanced resistance in cucumber mosaic virus inoculated pepper leaves

Altered photosynthetic reactions in cucumber mosaic virus (CMV) inoculated leaves of virus resistant lines L113 and L57 and susceptible pepper (Capsicum annuum L.) plants cv. Albena grown in controlled environment and in the field were investigated. The CMV inoculated leaves of virus resistant lines developed different symptoms—necrotic local lesions on L113 and chlorotic spots on L57 while the same leaves of susceptible cv. Albena were symptomless. The changes in Photosystem II (PSII) and PSI electron transport were evaluated by chlorophyll fluorescence, and far-red (FR) light induced leaf absorbance A _810–860. CMV infection caused a decrease in maximal PSII quantum yield, F _v/F _m, in susceptible leaves. Increased non-photochemical fluorescence quenching in CMV-inoculated leaves of both resistant lines were observed. In CMV-inoculated leaves of all tested plants FR light induced P700 oxidation was decreased. In the present study, the viral-infected pepper plants grown in controlled environment to avoid the effects of abiotic factors were used as model system that allow us to investigate the differences in leaf senescence in CMV-inoculated leaves of susceptible and resistant pepper lines expressing different symptoms. Earlier leaf falls of inoculated leaves as a result of accelerated leaf senescence is important for building successful secondary virus resistance strategy following fast responses such as hypersensitive reaction.