Molecular analysis of natural leaf senescence in Arabidopsis thaliana

Using artificial canopies, several authors have shown that horizontally propagated and overall propagated radiation beneath the canopy differ substantially in spectral distribution in the red (R) and far red (FR) wavelengths. Given the lack of information about light quality under real crop canopies, the R:FR ratio of vertical and horizontal radiation beneath field-grown maize, soybean and wheat was monitored until leaf area index (LAI) reached 4, 2.5 and 6.9, respectively.
A Li-Cor 1800 spectroradiometer with a remote cosine receptor fitted with a quartz fibre-optic light-guide was used. To isolate radiation coming from a given direction, a black coated tube was fitted to the cosine receptor. The viewing angle was 15°. In open conditions, the values of R:FR from the upper hemisphere were between 1.07 and 1.20. For vertically and horizontally-propagated light, average values were 1.22 and 0.75 respectively.
Beneath the canopy, both R:FR and photosynthetic photon flux density (PPFD) from the entire upper hemisphere decreased in relation to LAI and crop height. R:FR of the horizontal component were found to be generally much lower than the vertical, which decreased significantly only in the later measurements.
The lowest R:FR values were recorded under wheat and soybean canopies. Even the very low LAIs present at early development stages were enough to cause a sharp decrease of R:FR in the horizontal fluxes. Referring to the entire upper hemisphere, PPFD transmittance and R:FR as a percentage of the external references appeared well correlated.     

Far-red enrichment and photosynthetically active radiation level influence leaf senescence in field-grown sunflower

Basal leaves frequently senesce before anthesis in high population density crops. This paper evaluates the hypothesis that quantitative and qualitative changes in the light environment associated with a high leaf area index (LAI) trigger leaf senescence in sunflower ( Helianthus annuus L.) canopies. Mean leaf duration (LD, time from achievement of maximum leaf area) of leaf 8 was significantly ( P < 0.05) reduced from 51 to 19 days as crop population density was increased from 0.47 to 4.76 plants m⁻². High compared to low plant population density was associated with earlier reduction in the photosynthetically active radiation (PAR) and red/far-red ratio (R/FR) reaching the target leaf. However the changes in R/FR preceded those in PAR. When the light environment of individual leaves of isolated plants growing under field conditions was manipulated using filters and FR-reflecting mirrors, LD was positively and linearly related with the mean daily PAR (MDR) received in the FR- (no FR enrichment) treatments. FR enrichment of light reaching the abaxial surface of the leaf significantly ( P < 0.05) reduced LD by 9 days at intermediate PAR levels with respect to FR-controls, but did not affect LD at the maximum PAR used in these experiments. However, when light reaching both leaf surfaces was enriched with FR, LD (for leaves receiving maximum PAR) was 13 days shorter than that of the FR- control. These results show that basal leaf senescence in sunflower is enhanced both by a decrease in PAR and by a decrease in R/FR.     

Dual effect of phytochrome A on hypocotyl growth under continuous red light

The role of phytochrome A in the control of hypocotyl growth under continuous red light (Rc) was investigated using phyA and phyB mutants of Arabidopsis thaliana, which lack phytochrome A (phyA) or phytochrome B (phyB), respectively, and transgenic seedlings of Nicotiana tabacum overexpressing Avena phyA, compared to the corresponding wild type (WT). In WT seedlings of A. thaliana, hypocotyl growth inhibition showed a biphasic response to the fluence rate of Rc, with a brake at 10^?2μmol m^?2 s^?1. At equal total fluence rate, hourly pulses of red light caused slightly more inhibition than Rc. The response to very low fluences of continuous or pulsed red light was absent in the phyA and phyA phyB mutants and present in the phyB mutant. The second part of the response was steeper in the phyA mutant than in the WT but was absent in the phyB mutant. In WT tobacco the response to Rc was biphasic. Overexpression of Avena phyA enhanced the response only at very low fluence rates of Rc (< 10^?2μmol m^?2 s^?1). In both species, the effect of hourly pulses of far-red light was similar to the maximum inhibition observed in the first phase of the response to Rc. Using reciprocity failure (i.e. higher inhibition under continuous than pulsed light) as the operational criterion, a ‘true’ high-irradiance reaction occurred under continuous far-red light but not under Rc or red plus far-red light mixtures. Native and overexpressed phyA are proposed to mediate very low fluence responses under Rc. In WT A. thaliana, this effect is counteracted by a negative action of phyA on phyB-mediated low-fluence responses.     

Uncoupling light quality from light irradiance effects in Helianthus annuus shoots: putative roles for plant hormones in leaf and internode growth

An attempt has been made to uncouple the effects of the two primary components of shade light, a reduced red to far-red (R/FR) ratio and low photosynthetically active radiation (PAR), on the elongation of the youngest internode of sunflower (Helianthus annuus) seedlings. Maximal internode growth (length and biomass) was induced by a shade light having a reduced R/FR ratio (0.85) under the low PAR of 157 micromol m(-2) s(-1). Reducing the R/FR ratio under normal PAR (421 micromol m(-2) s(-1)) gave similar growth trends, albeit with a reduced magnitude of the response. Leaf area growth showed a rather different pattern, with maximal growth occurring at the higher (normal) PAR of 421 micromol m(-2) s(-1)), but with variable effects being seen with changes in light quality. Reducing the R/FR ratio (by enrichment with FR) gave significant increases in gibberellin A(1) (GA(1)) and indole-3-acetic acid (IAA) contents in both internodes and leaves. By contrast, a lower PAR irradiance had no significant effect on GA(1) and IAA levels in internodes or leaves, but did increase the levels of other GAs, including two precursors of GA(1). Interestingly, both leaf and internode hormone content (GAs, IAA) are positively and significantly correlated with growth of the internode, as are leaf levels of abscisic acid (ABA). However, changes in these three hormones bear little relationship to leaf growth. By implication, then, the leaf may be the major source of GAs and IAA, at least, for the rapidly elongating internode. Several other hormones were also assessed in leaves for plants grown under varying R/FR ratios and PARs. Leaf ethylene production was not influenced by changes in R/FR ratio, but was significantly reduced under the normal (higher) PAR, the irradiance treatment which increased leaf growth. Levels of the growth-active free base and riboside cytokinins were significantly increased in leaves under a reduced R/FR ratio, but only at the higher (normal) PAR irradiance; other light quality treatments evoked no significant changes. Taken in toto, these results indicate that both components of shade light can influence the levels of a wide range of endogenous hormones in internodes and leaves while evoking increased internode elongation and biomass accumulation. However, it is light quality changes (FR enrichment) which are most closely tied to increased hormone content, and especially with increased GA and IAA levels. Finally, the increases seen in internode and leaf GA content with a reduced R/FR ratio are consistent with FR enrichment inducing an overall increase in sunflower seedling GA biosynthesis.     

不同光照质量和刈割强度对小糠草无性繁殖特性的影响

本实验于1996年在美国德克萨斯理工大学草原,野生动物和渔类系的温室和实验室中进行,实验材料均为盆栽糠草,实验设计为在自然光照下（R／FR＝1．1）,以不同留茬高度（10,15,20cm和不刈割）作为不同刈割强度,用不同红光／远红光比例（R／FR＝1．3和0．7）处理不同刈割强度的小糠草,观测不同处理对小糠草无性繁殖特性的影响,结果表明,小糠草净增蘖数随刈割强度增加而减少,除不刈割处理外,光照质量变化（R／FR的比例）对分蘖性能影响不大,所有刈割强度下都不存在分蘖的补偿性生长效应,但幼叶伸长速度随刈割强度的增大而加快,可以认为小糠草应适度利用,强度利用或不利均不利于该草生长发育。     

Differential response of Scots pine seedlings to variable intensity and ratio of red and far‐red light

We investigated the response to increasing intensity of red (R) and far‐R (FR) light and to a decrease in R:FR ratio in Pinus sylvestris L. (Scots pine) seedling. The results showed that FR high‐irradiance response for hypocotyl elongation may be present in Scots pine and that this response is enhanced by increasing light intensity. However, both hypocotyl inhibition and pigment accumulation were more strongly affected by the R light compared with FR light. This is in contrast to previous reports in Arabidopsis thaliana (L.) Heynh. In the angiosperm, A. thaliana R light shows an overall milder effect on inhibition of hypocotyl elongation and on pigment biosynthesis compared with FR suggesting conifers and angiosperms respond very differently to the different light regimes. Scots pine shade avoidance syndrome with longer hypocotyls, shorter cotyledons and lower chlorophyll content in response to shade conditions resembles the response observed in A. thaliana. However, anthocyanin accumulation increased with shade in Scots pine, which again differs from what is known in angiosperms. Overall, the response of seedling development and physiology to R and FR light in Scots pine indicates that the regulatory mechanism for light response may differ between gymnosperms and angiosperms.     

The interaction of light quality and irradiance with gibberellins, cytokinins and auxin in regulating growth of Helianthus annuus hypocotyls

A reduced red to far-red (R/FR) light ratio and low photosynthetically active radiation (PAR) irradiance are both strong signals for inducing etiolation growth of plant stems. Under natural field conditions, plants can be exposed to either a reduced R/FR ratio or lower PAR, or to a combination of both. We used Helianthus annuus L., the sunflower, to study the effect of reduced R/FR ratio, low PAR or their combination on hypocotyl elongation. To accomplish this, we attempted to uncouple light quality from light irradiance as factors controlling hypocotyl elongation. We measured alterations in the levels of endogenous gibberellins (GAs), cytokinins (CKs) and the auxin indole-3-acetic acid (IAA), and the effect of exogenous hormones on hypocotyl growth. As expected, both reduced R/FR ratio and lower PAR can significantly promote sunflower hypocotyl elongation when given separately. However, providing the reduced R/FR ratio at a low PAR resulted in the greatest hypocotyl growth, and this was accompanied by significantly higher levels of endogenous IAA, GA1, GA8, GA20 and of a wide range of CKs. Providing a reduced R/FR ratio under normal PAR also significantly increased growth and again gave significantly higher levels of endogenous IAA, GAs and CKs. However, only under the de-etiolating influence of a normal R/FR ratio did lowering PAR significantly increase levels of GA1, GA8 and GA20. We thus conclude that light quality (e.g. the R/FR ratio) is the most important component of shade for controlling hypocotyl growth and elevated growth hormone content.     

The effect of light quantity and quality during development on the photosynthetic characteristics of six Australian rainforest tree species

Summary Seedlings of six subtropical rainforest tree species representing early (Omalanthus populifolius, Solanum aviculare), middle (Duboisia myoporoides, Euodia micrococca) and late (Acmena ingens, Argyrodendron actinophyllum) successional stages in forest development were grown in a glasshouse, under four levels of neutral shade (60%, 15%, 5%, 1% of photosynthetically active radiation (PAR) in incident sunlight) and three levels of selectively filtered shade (producing 15%, 5%, 1% of PAR). This design served to analyse the interactions between reduced photon flux density (PFD) and reduced red/far-red (R/FR) ratio in their effects on selected photosynthetic characteristics of each species. The light-saturated rate of photosynthesis was significantly influenced by growth irradiance in five of the six species, with all of these showing a non-linear decrease in maximum assimilation rate from 60% down to 1% PAR. The degree of acclimation to this range was not clearly related to the successional status of the species. Dark respiration was more sensitive to growth irradiance in the early- and mid-stage species than in the late-stage species. Although levels of dark respiration were clearly greater in leaves of early- and mid-stage species from the highest light levels, differences between successional groups were negligible at 1% PAR. Growth in filtered shade, typical of that beneath a closed canopy, resulted in lower photosynthetic capacities and quantum yields in those species which did respond. Although dark respiration rates were more sensitive to filtered shade in the early-stage than in the late-stage species, there was no evidence from other gas exchange characteristics to suggest that overall sensitivity to light quality (as characterised by the R/FR ratio) is greater in early successional-stage species.     

塞罕坝华北落叶松针叶光响应指标变化规律及其影响因素

以河北省塞罕坝机械林场华北落叶松人工林为研究对象,比较潜在最大光能利用效率(LUEmax)、潜在最有效光照强度(PARe)、光补偿点(LCP)和光饱和点(LSP)在树冠不同垂直位置和不同物候期的差异,并分析其主要影响因素.结果表明:LUEmax在树冠垂直方向随着冠层深度的增加而增大,PARe、LCP和LSP则随冠层深度...     

Dependence of catalase photoinactivation in rye leaves on light intensity and quality and characterization of a chloroplast-mediated inactivation in red light

In green or etiolated rye leaves catalase was most efficiently inactivated by blue light absorbed by its prosthetic heme. Red light was ineffective at low intensity but induced marked inactivation in green leaves at higher photon flux, while far-red light was ineffective. At identical intensities of photosynthetically active radiation, Photosystem II (PS II) was equally inactivated by both blue and red light. Since catalase was insensitive to red light and no sensitizer for red light was detected in isolated peroxisomes, the inactivation of catalase observed in leaves in red light must result from photooxidative reactions initiated in the chloroplasts. In a simplified model system the inactivation of isolated catalase was induced by the presence of a suspension of either intact or broken chloroplasts in red light. This chloroplast-mediated inactivation of catalase in vitro was O₂-dependent. It was greatly retarded at low temperature, fully suppressed by the radic al scavenger Trolox, partially retarded by superoxide dismutase, but only little diminished by the singlet oxygen quencher histidine and not affected by dimethylsulfoxide, a hydroxyl radical scavenger. Chloroplast-mediated catalase inactivation in vitro was suppressed by suitable electron acceptors, in particular by methyl viologen. A comparison of the effects of inhibitors, donors, or acceptors for specific sites of the photosynthetic electron transport indicated that an overreduction of PS II and plastoquinone represented the major sources for the formation of O₂ and some unidentified radical that appeared to mediate the inactivation of catalase outside of the chloroplasts. Chloroplast-mediated catalase inactivation provides a means for the detection of a redox signalling system of chloroplasts that was postulated to indicate overreduction of plastoquinones. Similarly as in the in vitro system, catalase inactivation in red light was also in leaves temperature-dependent and stimulated by DBMIB (2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone). These results provide strong evidence that inactivation of catalase initiated by chloroplastic reactions in red light occurred also in leaves under identical conditions as in the model system in vitro.     

Effect of jasmonic acid on morphogenesis and photosynthetic pigment level in Arabidopsis seedlings grown under green light

The role of jasmonic acid (JA) in plant photomorhogenesis under green light (GL) was studied. The effect of GL of different intensity (8.1 and 18.1 W/m²) with or without 1 μM JA treatment on growth of plants and photosynthetic pigment level was compared for two types of Arabidopsis thaliana (L.) Heynh. Landsberg erecta ecotype plants: Ler, the wild type, and hy4, a mutant. A much more pronounced growth of hypocotyls and cotyledons of Ler plants in GL was observed compared to that of hy4 with suppressed cryptochrome 1 (cry1), a GL photoreceptor. Treatment with JA in GL caused retardation of hypocotyl and cotyledon growth of Ler plants; however, it stimulated their growth in hy4 plants. JA reduced the chlorophyll a and total carotenoids levels in cotyledons of Arabidopsis plants in GL. Blocked GL signal transduction due to the absence of cry1 in hy4, as well as the higher intensity of GL reduced the negative effect of exogenous JA on growth of cotyledons and photosynthetic pigments. The data obtained provide a basis for discussion of interaction between the JA and GL signals in the growth regulation controlled by cry1. Original Russian Text ? I.F. Golovatskaya, R.A. Karnachuk, 2008, published in Fiziologiya Rastenii, 2008, Vol. 55, No. 2, pp. 240–244.     

The impact of the phytochromes on photosynthetic processes

This review describes the phytochrome system in higher plants and cyanobacteria and its role in regulation of photosynthetic processes and stress protection of the photosynthetic apparatus. A relationship between the content of the different phytochromes, the changes in the ratios of the physiologically active forms of phytochromes to their total pool and the resulting influence on photosynthetic processes is reviewed. The role of the phytochromes in the regulation of the expression of genes encoding key photosynthetic proteins, antioxidant enzymes and other components involved in stress signaling is elucidated.     

Phytochrome modulation of calcium fluxes in wheat (Triticum aestivum L.) protoplasts

Employing the metallochromic dye murexide and by monitoring the uptake of radiolabelled calcium, photoreversible calcium fluxes were measured in wheat leaf protoplast suspensions. Results obtained by both methods were identical — red light promoted and subsequent far-red irradiation reversed an influx of Ca⁺⁺ ions into the protoplasts. These findings imply phytochrome regulation of Ca⁺⁺ fluxes across the plasma membrane. The influx of Ca⁺⁺ stimulated by 2 min red irradiation could be maintained in total darkness for the initial 16–18 min after illumination, after which a 6–8 min efflux process was triggered and the basal Ca⁺⁺ level restored. Verapamil, a calcium channel blocker, inhibited the red-promoted influx, whereas the far-red mediated efflux could be checked by the use of the ATPase inhibitor vanadate, and also by the calmodulin antagonist chlorpromazine, thus suggesting a role of ion channels and pumps in phytochrome-controlled Ca⁺⁺ fluxes. The possible involvement of phosphoinositides in phytochrome-modulated calcium fluxes was also investigated.Abbreviations A difference in absorbance - CPZ chlorpromazine - FR far-red (light) - MX murexide - PI phosphatidylinositol - PIP₂ phosphatidylinositol 4, 5-bisphosphate - PIPES piperazine-N,N-bis[2-ethanesulfonic acid] - POPOP 1, 4-bis [2-(5-phenyl-1, 3-oxazolyl)]-benzene - PPO 2, 5-diphenyl-1, 3-oxazole - R red (light) - SOV sodium orthovanadate     

The RNAi-mediated silencing of xanthine dehydrogenase impairs growth and fertility and accelerates leaf senescence in transgenic Arabidopsis plants

Xanthine dehydrogenase (XDH) is a ubiquitous enzyme involved in purine metabolism which catalyzes the oxidation of hypoxanthine and xanthine to uric acid. Although the essential role of XDH is well documented in the nitrogen-fixing nodules of leguminous plants, the physiological importance of this enzyme remains uncertain in non-leguminous species such as Arabidopsis. To evaluate the impact of an XDH deficiency on whole-plant physiology and development in Arabidopsis, RNA interference (RNAi) was used to generate transgenic lines of this species in which AtXDH1 and AtXDH2, the two paralogous genes for XDH in this plant, were silenced simultaneously. The nearly complete reduction in the total XDH protein levels caused by this gene silencing resulted in the dramatic overaccumulation of xanthine and a retarded growth phenotype in which fruit development and seed fertility were also affected. A less severe silencing of XDH did not cause these growth abnormalities. The impaired growth phenotype was mimicked by treating wild-type plants with the XDH inhibitor allopurinol, and was reversed in the RNAi transgenic lines by exogenous supplementation of uric acid. Inactivation of XDH is also associated with precocious senescence in mature leaves displaying accelerated chlorophyll breakdown and by the early induction of senescence-related genes and enzyme markers. In contrast, the XDH protein levels increase with the aging of the wild-type leaves, supporting the physiological relevance of the function of this enzyme in leaf senescence. Our current results thus indicate that XDH functions in various aspects of plant growth and development.     

The chloroplast division mutant caa33 of Arabidopsis thaliana reveals the crucial impact of chloroplast homeostasis on stress acclimation and retrograde plastid-to-nucleus signaling

Retrograde plastid-to-nucleus signaling tightly controls and coordinates the nuclear and plastid gene expression that is required for plastid biogenesis and chloroplast activity. As chloroplasts act as sensors of environmental changes, plastid-derived signaling also modulates stress responses of plants by transferring stress-related signals and altering nuclear gene expression. Various mutant screens have been undertaken to identify constituents of plastid signaling pathways. Almost all mutations identified in these screens target plastid-specific but not extraplastidic functions. They have been suggested to define either genuine constituents of retrograde signaling pathways or components required for the synthesis of plastid signals. Here we report the characterization of the constitutive activator of AAA-ATPase (caa33) mutant, which reveals another way of how mutations that affect plastid functions may modulate retrograde plastid signaling. caa33 disturbs a plastid-specific function by impeding plastid division, and thereby perturbing plastid homeostasis. This results in preconditioning plants by activating the expression of stress genes, enhancing pathogen resistance and attenuating the capacity of the plant to respond to plastid signals. Our study reveals an intimate link between chloroplast activity and the susceptibility of the plant to stress, and emphasizes the need to consider the possible impact of preconditioning on retrograde plastid-to-nucleus signaling.     

Indirect consequences of artificial selection on plasticity to light quality in Arabidopsis thaliana

Covariation between light quality- and photoperiod-mediated phenotypic plasticity was investigated using Arabidopsis thaliana. Three episodes of artificial selection were imposed on an index that quantified the plastic response to reduced red to far-red ratios (R:FR), with higher index values indicating greater plasticity. Relative to control lines, two high plasticity (HP) lines showed 1.6- and 2.4-fold increases in the index; low plasticity (LP) lines showed 1.4- and 1.1-fold decreases. A factorial experiment combining high and low R:FR conditions with long and short photoperiods assessed indirect consequences of selection on plasticity. Despite divergent R:FR-mediated plasticities in HP vs. LP lines, all four lines showed increases in photoperiod-mediated responses and decreases in mean leaf number. Complex relationships among trait means, plasticities and underlying mechanisms caution against generalizing about the genetic architecture of plastic traits. Partially independent developmental and evolutionary responses to R:FR and photoperiod are somewhat unsurprising, given this species' cosmopolitan nature.     

温度升高对高光强环境下蛋白核小球藻(Chlolorella pyrenoidosa)光能利用和生长的阻抑效应

以蛋白核小球藻（Cholorella pyrenoidosa）为实验材料,研究了温度变化对不同光照水平下蛋白核小球藻的光能利用和生长的影响,以明确光照强度对微藻的光能利用和生长的影响是否因温度不同而发生变化。实验中共设置了3个光照强度水平（50,150,300μmol·m^-2s^-1）和2个温度水平（15℃,25℃）。实验结果表明,不同光照水平下小球藻叶绿素荧光的非光化学淬灭（NPQ）大小与温度有关,光照强度为150,300μmol·m^-2s^-1时,温度升高使小球藻叶绿素荧光NPQ提高,并且光照强度越高小球藻叶绿素荧光NPQ增大越多,50μmol·m^-2s^-1光照强度下温度升高对叶绿素荧光NPQ没有影响。实验发现,25℃培养温度下小球藻的光合电子传递速率（ETR）随光照强度增高而上升的速率要低于15℃时小球藻ETR上升的速率;随着光照强度增高,温度升高使小球藻ETR降低程度增大。实验结果还表明,15℃时小球藻培养液叶绿素a浓度随光照强度升高而增高,300μmol·m^-2s^-1培养光强下具有最高的叶绿素a浓度。但在25℃时,光照强度升高叶绿素a浓度并不一定增高,300μmol·m^-2s^-1光照强度下的叶绿素a浓度比150μmol·m^-2s^-1光照强度下要低。本研究表明,温度升高增大了高光照水平下蛋白核小球藻对光能的热耗散,使光照增强对小球藻生长的促进作用减弱。由于温度升高对小球藻光能利用和生长的阻抑作用,小球藻生长的适宜光照水平因温度升高而降低。