Fighting on two fronts: Elevated insect resistance in flooded maize

To grow and thrive plants must be able to adapt to both adverse environmental conditions and attack by a variety of pests. Elucidating the sophisticated mechanisms plants have developed to achieve this has been the focus of many studies. What is less well understood is how plants respond when faced with multiple stressors simultaneously. In this study, we assess the response of Zea mays (maize) to the combinatorial stress of flooding and infestation with the insect pest Spodoptera frugiperda (fall armyworm). This combined stress leads to elevated production of the defence hormone salicylic acid, which does not occur in the individual stresses, and the resultant salicylic acid-dependent increase in S. frugiperda resistance. Remodelling of phenylpropanoid pathways also occurs in response to this combinatorial stress leading to increased production of the anti-insect C-glycosyl flavones (maysins) and the herbivore-induced volatile phenolics, benzyl acetate, and phenethyl acetate. Furthermore, changes in cellular redox status also occur, as indicated by reductions in peroxidase and polyphenol oxidase activity. These data suggest that metabolite changes important for flooding tolerance and anti-insect defence may act both additively and synergistically to provide extra protection to the plant.     

UV-B component of sunlight causes measurable damage in field-grown maize (Zea mays L.): developmental and cellular heterogeneity of damage and repair

Ultraviolet radiation has diverse morphogenetic and damaging effects on plants. The end point of damage is reduced plant growth, but in the short term UV radiation damages specific cellular components. We measured cyclobutane pyrimidine dimers in maize DNA from plants grown in natural solar radiation. Green maize tissues had detectable DNA damage, roots had less damage, and anthers had much more damage than green leaves. This heterogeneity in damage levels may reflect differences in dose received or in damage repair. The architecture of green tissues had no measurable effects on DNA damage levels, as leaf sheath and leaf blade were equivalent. We observed a slight increase in damage levels in plants sampled at the end of the day, but there was no accumulation of damage over the growing season. We measured photoreactivation, and found substantial levels of this light-dependent repair in both the epidermis and inner cell layers of leaves, and in all organelles that contain DNA – the nucleus, chloroplasts and mitochondria. We conclude that maize has efficient mechanisms for photorepair of daily UV-induced DNA damage that prevent accumulation.     

Changes in accumulation of seed nitrogen compounds in maize under conditions of sulphur deficiency

Maize ( Zea mays L., hybrid INRA 260) was grown in the greenhouse with mineral nutrition of different sulphate concentrations. Mature seeds from these plants were compared for their free amino acid and protein N forms. For the most S-deficient sample, the Asx (asparagine + aspartic acid) content increased by 30% as compared with control, while methionine and cysteine decreased (by 25 and 30%, respectively), as well as glycine, lysine, histidine, arginine and tryptophan. In seeds lowest in S the non-protein N to total N ratio was 77% higher than in the control. Free asparagine dominated in starved seeds (50 mol % of total free amino acids) and was ten-fold more concentrated than in the control, where proline was the predominant free amino acid. Thus the Asx of non-protein N reached 28% of the total mol Asx of the whole starved seed. Altered S nutrition had virtually no effect on the amino acid composition of the main protein fractions, but it significantly changed their ratios. Zeins, which are poor in S-containing amino acids, showed 25% higher level than in seeds supplied with normal S. As a counterbalance, two glutelin subfractions rich in S-containing amino acids, decreased by 36–71% under limiting S nutrition.
It is concluded that the plant reacts against S deficiency by modifying its N metabolism. Significant accumulation occurred of free asparagine, which is the main form of N transportation. The biosynthesis of seed storage protein occurred through the accumulation of the highest possible protein quantity allowed by the available S-containing amino acids, i.e. proteins low in S-containing amino acids were preferentially synthesized.     

Changes in leaf protein and pigment contents and photosynthetic activities during senescence of detached maize leaves: influence of different ultraviolet radiations

Senescence induced loss in pigments and proteins of detached maize (Zea mays L. cv. Col) leaves was significantly enhanced on the exposure of leaves to different ranges of ultraviolet (UV) radiation. Compared to UV-A (320-400 nm) and UV-B (280-320 nm), the UV-C (200-320 nm) was the most damaging for the pigments and macromolecules. A severe decline in photosystem (PS) 2 mediated photoreduction during senescence of detached leaves exposed to UV irradiation suggested a damage of the system. The PS1 mediated photoreduction of methylviologen with 2,6-dichlorophenol indophenol as electron donor was stimulated by UV-A and UV-B radiations, suggesting a reorganisation of the PS1 complex. These results were fortified by the values of fast and slow kinetics of chlorophyll (Chl) a fluorescence transients. This revised version was published online in June 2006 with corrections to the Cover Date.     

重金属镉（Cd）和增强UV—B辐射复合对大豆生长和生理代谢的影响

研究了大豆的生长、生物量、抗氧化酶活性和吲哚乙酸（IAA）氧化酶活性在Cd^2 、UV－B辐射和二者复合胁迫（Cd UV-B)下的变化。结果表明,Cd^2 和UV－B辐射都抑制大豆生长,并显著抑制根的伸长,二者复合后加强了对根伸长的抑制。UV－B辐射显著增强了POD、SOD活性,Cd^2 对POD活性影响不明显,但却拮抗UV－B对POD活性的诱导,SOD活性在各种胁迫下显著增强。虽然Cd%2 对叶片类黄酮含量影响不明显,但对UV－B诱导的类黄酮合成有一定影响。IAA氧化酶活性在复合作用下下降,可能是复合胁迫影响大豆生长的重要因素之一。     

Variation in aggressiveness among isolates of Cercospora zeae-maydis inlow-, medium- and high-altitude maize agro-ecologies of Tanzania

The objective of this study was to assess the aggressiveness and adaptation of Cercospora zeae-maydis the causal of grey leaf spot of maize in low-, mid- and high-altitude maize agro-ecologies. Field and laboratory studies were carried out to assess components of resistance, pathogen aggressiveness, sporulation, culture variations and morphological features of C. zeae-maydis isolates. Pathogen aggressiveness and morphological features differed among 30 isolates of C. zeae-maydis collected at different altitudes (p ≤ 0.05). Conidiophores per stroma between isolates ranged from 3.3 to 9.2 μ, conidiophore length 22.4–93.2 μ and colony growth rate ranged from 1.2 to 3.9 mm^?1. High-altitude isolates grew 0.25 mm per day faster (9.3%), formed 1.1 cm longer lesions (25.9%), produced 1.1 × 10⁴ more spores per cm² of the lesion (13.6%) and formed symptoms 1 day earlier than low-altitude isolates. Prevalence of high numbers of aggressive isolates in low-altitude agro-ecologies indicates increased adaptation of C. zeae-maydis to warmer conditions.     

An allele of ZmPORB2 encoding a protochlorophyllide oxidoreductase promotes tocopherol accumulation in both leaves and kernels of maize

Phytol is one of the key precursors for tocopherol synthesis in plants, however, the underlying mechanisms concerning the accumulation of tocopherol remain poorly understood. In this study, qVE5, a major QTL affecting tocopherol accumulation in maize kernels was identified via a positional cloning approach. qVE5 encodes a protochlorophyllide oxidoreductase (ZmPORB2), which localizes to the chloroplast. Overexpression of ZmPORB2 increased tocopherol content in both leaves and kernels. Candidate gene association analysis identified a 5/8‐bp insertion/deletion (InDel058) in the 5′ untranslated region (UTR) as the causal polymorphism in affecting ZmPORB2 expression and being highly associated with tocopherol content. We showed that higher expression of ZmPORB2 correlated with more chlorophyll metabolites in the leaf following pollination. RNA‐sequencing and metabolic analysis in near isogenic lines (NILs) support that ZmPORB2 participates in chlorophyll metabolism enabling the production of phytol, an important precursor of tocopherol. We also found that the tocopherol content in the kernel is mainly determined by the maternal genotype, a fact that was further confirmed by in vitro culture experiments. Finally, a PCR‐based marker based on Indel058 was developed in order to facilitate the high tocopherol (vitamin E) maize breeding.     

Comparison of exportation and metabolism of xylem-delivered ABA in maize leaves at different water status and xylem sap pH

³H-ABA was introduced into the xylem stream of maize ( Zea mays}) leaves on intact plants by incubation of a semi-attached flap of the sheath in solutions. The relative contribution of exportation and metabolism to the fate of xylem-delivered ABA was assessed in leaves which were either kept at different water potentials through soil drying treatments or subjected to different xylem pHs (pH 7.4 vs. pH 5.5) through a phosphate buffer in the feeding solutions. Xylem-delivered ABA was rapidly metabolised in well-watered leaves with a half-life of 2.19 h in the relatively mature leaves used in this study. Re-exportation of xylem-delivered ABA from leaves was much slower than metabolism. It took 24 h for half of the fed radioactivity to disappear from the well-watered leaves, and very possibly this radioactivity was in the form of metabolites of fed ³H-ABA. Although soil drying usually increases the output of ABA through phloem as reported in previous studies, it greatly reduced the re-exportation of xylem-fed ABA and/or its metabolites. Metabolism was also significantly reduced by the treatment of soil drying (half-life extended from 2.19 to 3.63 h), although the magnitude of change was much less than that of exportation. Manipulation of the pH in the feeding solution also had its effect on the re-exportation. A shift of pH from 5.5 to 7.4 reduced the rate of disappearance of the total radioactivity fed into the attached leaves, but showed no significant effect on the rate of ABA metabolism. It was concluded that it was the ABA metabolism, rather than a re-exportation from leaves, which was mainly responsible for the disposal of the ABA signal from the xylem and therefore preventing an accumulation in leaves. Water stress and pH increase of xylem sap would increase the time of such ABA's presence in the leaves. Since xylem-imported ABA is unlikely to be re-exported from leaves in its intact form, we believe a recycling of ABA from xylem to phloem through leaves plays only a minor role.     

Localization of nitrite and sulfite reductase in bundle sheath and mesophyll cells of maize leaves

The distribution of nitrite reductase (EC 1.7.7.1) and sulfite reductase (EC 1.8.7.1) between mesophyll ceils and bundle sheath cells of maize ( Zea mays L. cv. Seneca 60) leaves was examined. This examination was complicated by the fact that both of these enzymes can reduce both NO-₂ and SO^2-₃ In crude extracts from whole leaves, nitrite reductase activity was 6 to 10 times higher than sulfite reductase activity. Heat treatment (10 min at 55°C) caused a 55% decrease in salfite reductase activity in extracts from bundle sheath cells and mesophyll cells, whereas the loss in nitrite reductase activity was 58 and 82% in bundle sheath cells and mesophyll cell extracts, respectively. This result was explained, together with results from the literature, by the hypothesis that sulfite reductase is present in both bundle sheath cells and mesophyll cells, and that nitrite reductase is restricted to the mesophyll cells. This hypothesis was tested i) by comparing the distribution of nitrite reductase activity and sulfite reductase activity between bundle sheath and mesophyll cells with the presence of the marker enzymes ribulose-l, 5-bisphosphate carboxylase (EC 4.1.1.39) and phosphoe-nolpyruvate carboxylase (EC 4.1.1.32), ii) by examining the effect of cultivation of maize plants in the dark without a nitrogen source on nitrite reductase activity and sulfite reductase activity in the two types of cells, and iii) by studying the action of S^2-on the two enzyme activities in extracts from bundle sheath and mesophyll cells. The results from these experiments are consistent with the above hypothesis.     

Chilling syndrome in light-exposed maize leaves and its easing by low doses of DCMU

When maize plants ( Zea mays L. line ps-lye) were subjected to chilling (at 8 ± 2°C, 80% relative humidity for 24 h under illumination by 80 W m² between 400–700 nm), the leaves were wilted and photosynthetic membranes were permanently damaged. This was shown by the swelling of grana thylakoids and a deerease in the charging capacity of the electron transport chain. Water loss and photosynthetic dysfunction were connected in the process of a chilling-induced increase of stomatal aperture. Chilling injury could be eased to a considerable extent by a mild treatment with DCMU preventing stomatal opening, wilting, and the irreversible loss of CO₂ fixation capacity.     

Different stomatal responses of maize leaves after blue or red illumination under anoxia

Abstract In normal air, illumination with a low level of blue or red light (40 μmol m^?2 s^?1) did not induce stomatal opening in maize plantlets. In CO₂-free air, 40 μmol m^?2 s^?1 of blue or red light promoted an enhancement in stomatal opening. At the same quantum flux, blue light was more efficient than red light and stomatal closure occurred more rapidly with a significantly shorter lag phase after blue light. Anoxia inhibited light-dependent stomatal opening, even under 320 μmol m^?2 s^?1 illumination. However, after 60 min of illumination with 40 μmol m^?2 s^?1 of blue light in anoxia, transient stomatal opening was observed when the plant was returned to darkness and normal air. This transient stomatal opening was weaker after pretreatment with red light. We conclude that a blue-light-dependent process induced under anoxia leads to stomatal opening provided oxygen is present. Possible mechanisms associated with blue-light-effect and the nature of the oxygen-consuming processes are discussed.     

玉米及马齿苋叶片SOD活性诱导研究

以玉米幼苗及马齿苋作材料,通过甘露醇、H2O2、臭氧和强光等胁迫后,用NBT光化还原抑制法测定叶中SOD活性的变化。臭氧和强光能诱导玉米叶片SOD活性增加。0.5mol／L甘露醇处理玉米叶片12h,SOD活性上升,至48h后下降;在该甘露醇溶液中另外加入10^-2mol/L H2O2;处理12h后SOD活性基本不变。对玉米叶片单独用10^-2mol/L H2O2诱导,30min内SOD活性上升到最高值,随着处理时间的延长又逐渐下降。用耐强光、耐干旱的野生马齿苋作为材料,与玉米相比,其叶片SOD基础活性低于后者;若予以正午强光结合渗透胁迫2h,其叶中SOD活性增幅超过玉米,从种间比较的角度旁证了SOD在抗逆性中的作用。推测植物中存在比活性氧更为直接的物理诱导机制。     

增强UV-B辐射对大豆胚轴DNA损伤、修复和蛋白质含量的影响

大气平流层臭氧层减薄引起到达地表的 UV- B辐射增强。为探讨在增强 UV- B辐射下植物细胞 DNA的损伤修复和蛋白质含量的关系 ,利用 3H- Td R掺入法 ,研究了在 8.2 2 k J/(m2 d)和 12 .4 2 k J/(m2 d) U V- B辐射 (相当于兰州地区大气平流层臭氧减薄约 12 %和 2 0 % )胁迫下 ,大豆胚轴细胞 DNA合成和非按期合成 (UDS)变化 ,并测定了胚轴蛋白质含量变化 ,结果显示 ,UV- B辐射导致 DNA损伤 ,并诱导了 DNA损伤的修复 ,胚轴细胞 UDS效应增强 ,U DS指数增大。低 UV- B辐射强度下 ,胚轴蛋白质含量增加 ,可能是 U V- B诱导了一些与抗性有关的基因表达 ,导致一些新的与抗性有关的蛋白质合成 ;在高强度 UV- B辐射下 ,U DS指数与低强度辐射下无显著差异 (P=0 .0 5 ) ,但蛋白含量较低强度辐射下显著下降 (P=0 .0 5 ) ,说明高强度 UV- B辐射加重了 DNA损伤 ,而修复并未加强 ,并且高强度辐射抑制基因的正常表达和蛋白质合成。这些蛋白质的合成可能与大豆对 UV- B辐射的抗性有关。     

Stress induction of abscisic acid in maize roots

Moderate water stresses in the range 0 to −0.6 MPa applied with PEG 6000 to excised roots of Zea mays L. var. LG 11 induced increases of up to four-fold in the amount of abscisic acid (ABA) determined in the tissue after a 12 h period of xylem exudation. The ABA concentration of xylem exudate collected after a 2 h water stress also increased by up to four-fold. Salt stresses, induced with NaCl solutions, resulted in similar increases in the ABA concentrations. ABA concentrations in both root tissue and xylem exudate were highest 4 h after removal of the stress and then declined over a subsequent 8 h period. These results are interpreted in support of the concept that root-produced ABA may have a role in the fine control of the plant's water balance.