Changes in the hydroxamic acid content of maize leaves with time and after artificial damage; implications for insect attack

The dynamics of hydroxamic acids (Hx) in maize plants were examined, along with the effects on this process of artificial leaf damage. The concentration of Hx in maize (cv. LGH) leaves declined rapidly with age, although young emerging tissue did have relatively high levels. The decline was not entirely due to a growth dilution effect, as the total Hx content in leaves also decreased with age. This effect was less pronounced in isolated leaves. The plant hormone abscisic acid (ABA) accelerated the decline in Hx in isolated maize leaves, while indole acetic acid (IAA) had no apparent effect. Artificial leaf damage on growing plants produced a significant increase in the concentration of Hx relative to undamaged controls. The difference was greatest (about 18%) two days after damage, but seemed to be largely due to a slower rate of decline relative to controls. Isolated maize leaves generally showed a much higher level of increase (20 - 40%). The results are generally consistent with the theory that Hx functions as a defence against insect attack.     

Evaluation of DNA damage and mutagenicity induced by lead in tobacco plants

Tobacco (Nicotiana tabacum L. var. xanthi) seedlings were treated with aqueous solutions of lead nitrate (Pb2+) at concentrations ranging from 0.4 mM to 2.4 mM for 24 h and from 25 microM to 200 microM for 7 days. The DNA damage measured by the comet assay was high in the root nuclei, but in the leaf nuclei a slight but significant increase in DNA damage could be demonstrated only after a 7-day treatment with 200 microM Pb2+. In tobacco plants growing for 6 weeks in soil polluted with Pb2+ severe toxic effects, expressed by the decrease in leaf area, and a slight but significant increase in DNA damage were observed. The tobacco plants with increased levels of DNA damage were severely injured and showed stunted growth, distorted leaves and brown root tips. The frequency of somatic mutations in tobacco plants growing in the Pb2+-polluted soil did not significantly increase. Analytical studies by inductively coupled plasma optical emission spectrometry demonstrate that after a 24-h treatment of tobacco with 2.4 mM Pb2+, the accumulation of the heavy metal is 40-fold higher in the roots than in the above-ground biomass. Low Pb2+ accumulation in the above-ground parts may explain the lower levels or the absence of Pb2+-induced DNA damage in leaves.     

Induction and repair of DNA damage as measured by the Comet assay and the yield of somatic mutations in gamma-irradiated tobacco seedlings

The advantage of using the tobacco (Nicotiana tabacum var. xanthi) mutagenicity assay is the ability to analyze and compare on the same plants under identical treatment conditions both the induced acute DNA damage in somatic cells as measured by the Comet assay and the yield of induced leaf somatic mutations. Gamma-irradiation of tobacco seedlings induced a dose-dependent increase in somatic mutations from 0.5 (control) to 240 per leaf (10Gy). The increased yield of somatic mutations was highly correlated (r = 0.996) with the increased DNA damage measured by the Comet assay immediately after irradiation. With increased dose of gamma-irradiation, the averaged median tail moment values ( +/- S.E.) significantly increased from 1.08 +/- 0.10 (control) to 20.26 +/- 1.61 microm (10Gy). Nuclei isolated from leaves 24h after irradiation expressed tail moment values that were not significantly different from the control (2.08 +/- 0.11). Thus a complete repair of DNA damage induced by gamma-irradiation and measurable by the Comet assay was observed, whereas the yield of somatic mutations increased in relation to the radiation dose. Data on the kinetics of DNA repair and of DNA damage induced by gamma-radiation on isolated tobacco nuclei, and on nuclei isolated from irradiated leaves and roots are presented.     

Evaluation of DNA damage and mutagenicity induced by lead in tobacco plants

Tobacco (Nicotiana tabacum L. var. xanthi) seedlings were treated with aqueous solutions of lead nitrate (Pb²⁺) at concentrations ranging from 0.4 mM to 2.4 mM for 24 h and from 25 μM to 200 μM for 7 days. The DNA damage measured by the comet assay was high in the root nuclei, but in the leaf nuclei a slight but significant increase in DNA damage could be demonstrated only after a 7-day treatment with 200 μM Pb²⁺. In tobacco plants growing for 6 weeks in soil polluted with Pb²⁺ severe toxic effects, expressed by the decrease in leaf area, and a slight but significant increase in DNA damage were observed. The tobacco plants with increased levels of DNA damage were severely injured and showed stunted growth, distorted leaves and brown root tips. The frequency of somatic mutations in tobacco plants growing in the Pb²⁺-polluted soil did not significantly increase. Analytical studies by inductively coupled plasma optical emission spectrometry demonstrate that after a 24-h treatment of tobacco with 2.4 mM Pb²⁺, the accumulation of the heavy metal is 40-fold higher in the roots than in the above-ground biomass. Low Pb²⁺ accumulation in the above-ground parts may explain the lower levels or the absence of Pb²⁺-induced DNA damage in leaves.     

Contribution of phenolic compounds to the UV-B screening capacity of developing barley primary leaves in relation to DNA damage and repair under elevated UV-B levels

Epidermally located UV-absorbing hydroxycinnamic acid conjugates and flavonoid glycosides are known to be efficient UV-B protectants in higher plants, although important biological molecules are not always fully protected. However, repair mechanisms also exist, such as repair of damaged DNA by photolyases. To distinguish between the relative importance of the phenolic compounds and of DNA repair, developing primary leaves of two barley lines, mutant ant 30-310, deficient in flavonoids, and its parent line Ca 33787, were grown under relatively high visible light (650-700 micromol m(-2) s(-1) max for 6 h in a 13 h photoperiod) and supplemented with (+ UV-B) or without (-UV-B) 12 kJ m(-2) UV-B(BE) for 6 h daily. UV-B screening capacity of the leaf phenolics was determined at 315 nm during leaf development and compared with thymine dimers (TD) accumulation, as an indicator of UV-B-induced DNA damage and potential subsequent repair. The degree of damage was related to the phenolic contents of the leaves. UV-B screening capacity was increased ca. 4-fold in the parent line (+ UV-B), mainly due to UV-induced flavonoid (saponarin, lutonarin) accumulation in epidermal and subepidermal mesophyll tissue, relative to the flavonoid-deficient mutant. Nevertheless, in the parent line an 8-fold increase in TD levels occurred over the growth period of 18 days, whereas the mutant accumulated additional DNA damage, with 6- to 9-fold higher TD amounts. Surprisingly, under the high UV-B irradiation, growth and development of the primary leaves in both lines were only slightly reduced.     

Influence of water stress on endogenous hormone contents and cell damage of maize seedlings

Phytohormones play critical roles In regulating plant responses to stress. We Investigated the effects of water stress Induced by adding 12% (w/v) polyethylene glycol to the root medium on the levels of abscisic acid (ABA), indole-3-acid (IAA), zeatin (ZT), and gibberellin3 (GA3) in maize leaves. The results suggested that water stress had significant effects on the four hormone levels. There was a transient increase in the IAA content during the initial stage of adaptation to water stress in maize leaves, but it dropped sharply thereafter in response to water stress. ABA content increased dramatically in maize leaves after 24 h of exposure to water stress, and then the high levels of ABA were maintained to the end, The contents Of ZT and GA3 rapidly declined in maize leaves subjected to water stress. The effects of water stress on chlorophyll content, electrolyte leakage and malondialdehyde levels in maize leaves were also studied. The variation of cell damage was negatively correlated with ZT and GA3 levels in maize leaves under water stress. Thus, we explored the roles of ZT and GA3 on the growth of maize seedlings under water stress by exogenous application. It is possible that both ZT and GA3 were effective in protecting maize seedlings from water stress, which would be of great importance for the improvement of drought tolerance in maize by genetic manipulation.     

Ultraviolet-B-induced DNA damage and ultraviolet-B tolerance mechanisms in species with different functional groups coexisting in subalpine moorlands

High doses of ultraviolet-B (UV-B; 280–315 nm) radiation can have detrimental effects on plants, and especially damage their DNA. Plants have DNA repair and protection mechanisms to prevent UV-B damage. However, it remains unclear how DNA damage and tolerance mechanisms vary among field species. We studied DNA damage and tolerance mechanisms in 26 species with different functional groups coexisting in two moorlands at two elevations. We collected current-year leaves in July and August, and determined accumulation of cyclobutane pyrimidine dimer (CPD) as UV-B damage and photorepair activity (PRA) and concentrations of UV-absorbing compounds (UACs) and carotenoids (CARs) as UV-B tolerance mechanisms. DNA damage was greater in dicot than in monocot species, and higher in herbaceous than in woody species. Evergreen species accumulated more CPDs than deciduous species. PRA was higher in Poaceae than in species of other families. UACs were significantly higher in woody than in herbaceous species. The CPD level was not explained by the mechanisms across species, but was significantly related to PRA and UACs when we ignored species with low CPD, PRA and UACs, implying the presence of another effective tolerance mechanism. UACs were correlated negatively with PRA and positively with CARs. Our results revealed that UV-induced DNA damage significantly varies among native species, and this variation is related to functional groups. DNA repair, rather than UV-B protection, dominates in UV-B tolerance in the field. Our findings also suggest that UV-B tolerance mechanisms vary among species under evolutionary trade-off and synergism.     

Variation in damage to cotton affecting larval feeding preference of Spodoptera littoralis

Feeding experiments with larvae of Spodoptera littoralis were performed with leaves from cotton plants subjected to damage and from undamaged plants. In the experiments, four different time intervals (1, 3, 7, and 14 days) after damage induction and two different levels (high and low) of herbivore damage were tested. Seven days after damage induction larvae fed less on the young top leaves from damaged plants for both levels of damage. At the high damage level, the larvae fed less on leaves from the damaged plants after just three days, and this effect still remained 14 days after damage infliction. When mature leaves from the middle of the plant were compared, no difference between treatments was observed.Two plant sizes were tested, small plants with 4–5 true leaves and large plants with 8–10 true leaves. In small plants the induced changes affecting larval feeding were found mainly in the youngest leaf at the top of the plant, while in large plants the induced effects were found in both the youngest and the second youngest leaves.In plants subjected to artificial damage, larvae fed less on top leaves of the damaged plants when compared to leaves from undamaged plants. When leaves from plants that had been artificially damaged were directly compared with leaves from plants damaged by herbivores, larvae fed more on the youngest leaves from artificially damaged plants when the plants were large. In small plants no significant difference was found when comparing artificial and herbivore damage.     

Integration and scaling of UV‐B radiation effects on plants: from DNA to leaf

A process‐based model integrating the effects of UV‐B radiation through epidermis, cellular DNA, and its consequences to the leaf expansion was developed from key parameters in the published literature. Enhanced UV‐B radiation‐induced DNA damage significantly delayed cell division, resulting in significant reductions in leaf growth and development. Ambient UV‐B radiation‐induced DNA damage significantly reduced the leaf growth of species with high relative epidermal absorbance at longer wavelengths and average/low pyrimidine cyclobutane dimers (CPD) photorepair rates. Leaf expansion was highly dependent on the number of CPD present in the DNA, as a result of UV‐B radiation dose, quantitative and qualitative absorptive properties of epidermal pigments, and repair mechanisms. Formation of pyrimidine‐pyrimidone (6‐4) photoproducts (6‐4PP) has no effect on the leaf expansion. Repair mechanisms could not solely prevent the UV‐B radiation interference with the cell division. Avoidance or effective shielding by increased or modified qualitative epidermal absorptance was required. Sustained increased UV‐B radiation levels are more detrimental than short, high doses of UV‐B radiation. The combination of low temperature and increased UV‐B radiation was more significant in the level of UV‐B radiation‐induced damage than UV‐B radiation alone. Slow‐growing leaves were more affected by increased UV‐B radiation than fast‐growing leaves.     

紫外线-B辐射对植物DNA及蛋白质的影响

大气平流层中的臭氧衰减,导致太阳辐射中的紫外辐射量有明显的增加,其中UV-B辐射对植物会产生不同程度的影响。分子生态学理论认为,UV-B辐射对植物造成的损伤,首先伤害植物的生物大分子,即进行光化学修饰。本文就臭氧衰减对生态环境和植物的影响途径进行了讨论,重点论述了UV-B辐射对植物蛋白质合成的抑制和DNA的损伤修复途径。并应用分子生物学技术研究植物对UV-B辐射的抗性机理和DNA修复技术的前景进行了展望。     

Interspecific competition affects growth and herbivore damage of Brassica napus in the field

Resource competition can influence plant fitness either directly, or indirectly by influencing the amount of herbivore damage received by plants in the field. We previously found that competition could constrain the constitutive and woundinduced expression of defensive trypsin inhibitors in pot-grown Brassica napus seedlings in the greenhouse, suggesting that the ability of a plant to chemically defend itself could be constrained by competition in the field. Guided by these results, we investigated whether competition would affect growth and the presence of herbivores and herbivore damage on B. napus plants in the field. We established sixteen 1 m 2 plots in the field in a 7 x7 mgrid. Nine two-week-old B. napus seedlings were transplanted from the greenhouse into each 1 m 2 plot. Half of the plots were kept weed-free and half were left to develop interspecific weed competi-tors.After six weeks, three randomly chosen plants in each plot were measured for height, number of leaves, leaf area removed by herbivores, and the presence of aphids, leaf miners, and eggs of ladybird beetles. Consistent with the induction of the shade-avoidance response, plants in plots with weed competitors were significantly taller and had half as many leaves as plants in weed-free plots. Competing plants also had 60% more leaf arearemoved by herbivores, an 80% higher proportion of leaves with aphids, and an equal proportion of leaves with leaf miners. In this study, weed competition had dramatic effects on growth, leaf area removal by herbivores, and the presence of aphids on B. napus plants in the field. Together with our demonstration that competition can constrain the expression of trypsin inhibitor activity, these results suggest that resource competition may limit theability of a plant to defend itself from natural enemies, leading to greater herbivory. In turn, increased herbivory on competing plants could exacerbate the direct effects of competition on plant fitness.     

Plant DNA-damage repair/toleration 100 protein repairs UV-B-induced DNA damage

We report the characterization of VvDRT100-L, a grape DNA-damage repair/toleration 100 protein. VvDRT100-L has nine leucine-rich repeats and belongs to the plant DRT100 protein family. VvDRT100-L is expressed abundantly in green organs of grapevines, including tendrils, leaves, and green berry skins. The overexpression of VvDRT100-L in Arabidopsis plants decreased the number of abasic sites and the frequency of DNA single-strand breaks in the DNA damaged by UV-B irradiation, whereas UV-B irradiation markedly increased the number of abasic sites and the frequency of DNA single-strand breaks in T-DNA insertion mutant drt100 plants. VvDRT100-L-overexpressing plants remained viable and noticeably healthy under lethal UV doses, suggesting that VvDRT100-L may enhance UV tolerance in plant. Taken together, we concluded that VvDRT100-L might play an important role in the repair and toleration of UV-B-induced DNA damage. These findings would help us better understand how plants acquire UV stress acclimation, tolerance and DNA repair.     

Chloroplast movement provides photoprotection to plants by redistributing PSII damage within leaves

Plants use light to fix carbon through the process of photosynthesis but light also causes photoinhibition, by damaging photosystem II (PSII). Plants can usually adjust their rate of PSII repair to equal the rate of damage, but under stress conditions or supersaturating light-intensities damage may exceed the rate of repair. Light-induced chloroplast movements are one of the many mechanisms plants have evolved to minimize photoinhibition. We found that chloroplast movements achieve a measure of photoprotection to PSII by altering the distribution of photoinhibition through depth in leaves. When chloroplasts are in the low-light accumulation arrangement a greater proportion of PSII damage occurs near the illuminated surface than for leaves where the chloroplasts are in the high-light avoidance arrangement. According to our findings chloroplast movements can increase the overall efficiency of leaf photosynthesis in at least two ways. The movements alter light profiles within leaves to maximize photosynthetic output and at the same time redistribute PSII damage throughout the leaf to reduce the amount of inhibition received by individual chloroplasts and prevent a decrease in photosynthetic potential.     

Infection with Phloem Limited Abutilon Mosaic Virus Causes Localized Carbohydrate Accumulation in Leaves of Abutilon striatum: Relationships to Symptom Development and Effects on Chlorophyll Fluorescence Quenching During Photosynthetic Induction

Abstract: Infection with phloem limited Abutilon Mosaic Virus caused localized carbohydrate accumulation (high levels of starch, sucrose, and hexoses) in leaves of Abutilon striatum during early symptom development. In mature leaves with attenuated symptoms, tissues showing faint vein-clearing had markedly higher carbohydrate contents than uniformly green areas of the same leaf. A similar pattern of carbohydrate accumulation was found in pale-green mosaics in mature leaves with overt symptoms when compared to green-islands of the same leaf, but overnight carbohydrate loses were comparable to controls. Because leaves with attenuated symptoms showed no further symptom development whereas the pale-green mosaics became yellow and eventually necrotic in leaves with overt symptoms, it seems unlikely that carbohydrate accumulation following impaired translocation was responsible for symptom expression. High carbohydrate status in leaves with attenuated symptoms had little effect on nonphotochemical quenching during early stages of photosynthetic induction. In leaves with overt symptoms, areas of high carbohydrate status with pale-green mosaics showed markedly slower nonphotochemical quenching. Early symptom areas of young leaves, and advanced symptom areas of mature leaves had low starch contents but were otherwise similar to controls in carbohydrate status. Impaired nonphotochemical quenching in these tissues tended to reflect the state of symptom development, rather than carbohydrate status. Plants with overt symptoms grew about half as fast as plants with attenuated symptoms.