Miroslava Luxová

Silicon modifies root anatomy, and uptake and subcellular distribution of cadmium in young maize plants

BACKGROUND AND AIMS Silicon (Si) has been shown to ameliorate the negative influence of cadmium (Cd) on plant growth and development. However, the mechanism of this phenomenon is not fully understood. Here we describe the effect of Si on growth, and uptake and subcellular distribution of Cd in maize plants in relation to the development of root tissues. METHODS Young maize plants (Zea mays) were cultivated for 10 d hydroponically with 5 or 50 µm Cd and/or 5 mm Si. Growth parameters and the concentrations of Cd and Si were determined in root and shoot by atomic absorption spectrometry or inductively coupled plasma mass spectroscopy. The development of apoplasmic barriers (Casparian bands and suberin lamellae) and vascular tissues in roots were analysed, and the influence of Si on apoplasmic and symplasmic distribution of (109)Cd applied at 34 nm was investigated between root and shoot. KEY RESULTS Si stimulated the growth of young maize plants exposed to Cd and influenced the development of Casparian bands and suberin lamellae as well as vascular tissues in root. Si did not affect the distribution of apoplasmic and symplasmic Cd in maize roots, but considerably decreased symplasmic and increased apoplasmic concentration of Cd in maize shoots. CONCLUSIONS Differences in Cd uptake of roots and shoots are probably related to the development of apoplasmic barriers and maturation of vascular tissues in roots. Alleviation of Cd toxicity by Si might be attributed to enhanced binding of Cd to the apoplasmic fraction in maize shoots.

Silicification of bamboo (Phyllostachys heterocycla Mitf.) root and leaf

Bamboo is a silicon accumulating plant. In leaves, the major place of silicon (Si) deposition is the epidermis, with the highest concentration of Si in silica cells. In bamboo roots, the deposition of Si is found only in endodermal cell walls. The silicification of leaves and roots was examined in the economically important bamboo plant Phyllostachys heterocycla, using an environmental scanning electron microscope coupled with X-ray microanalysis, as well as gravimetric quantification. The content of Si on a dry weight basis measured by gravimetric quantification was 7.6% in leaves and 2.4% in roots, respectively. Moreover, quantification of EDX data showed high Si impregnation of the inner tangential endodermal walls. Si content in this part of the root endodermal cell walls was even higher than that in the outer leaf epidermal walls, where conspicuous deposition of Si often occurs in grass plants.

Influence of silicon on maize roots exposed to antimony – Growth and antioxidative response

Pollution of antimony (Sb) raises a serious environmental problem. Although this non-essential element can be taken up by roots and accumulated in plant tissues in relatively high concentrations, there is still lack of knowledge about the effect of Sb on biochemical and metabolic processes in plants. It was shown that application of silicon (Si) can decrease the toxicity of other heavy metals and toxic elements in various plants. The aim of this study was to assess how Si influences the growth and antioxidative response of young Zea mays L. roots exposed to elevated concentrations of Sb. Antimony reduced the root growth and induced oxidative stress and activated antioxidant defense mechanisms in maize. Silicon addition to Sb treated roots decreased oxidative stress symptoms documented by lower lipid peroxidation, proline accumulation, and decreased activity of antioxidative enzymes (ascorbate peroxidase, EC 1.11.1.11; catalase, EC 1.11.1.6; and guaiacol peroxidase, EC 1.11.1.7). Although neither positive nor negative effect of Si has been observed on root length and biomass, changes in the oxidative response of plants exposed to Sb indicate a possible mitigation role of Si on Sb toxicity in plants.

Short-and long-term effects of cadmium on transmembrane electric potential (E m) in maize roots

In this study, the effects of Cd on root growth, respiration, and transmembrane electric potential (Em) of the outer cortical cells in maize roots treated with various Cd concentrations (from 1 µM to 1 mM) for several hours to one week were studied. The Em values of root cells ranged between −120 and −140 mV and after addition of Cd they were depolarized immediately. The depolarization was concentration-dependent reaching the value of diffusion potential (ED) when the Cd concentration exceeded 100 µM. The values of ED ranged between −65 to −68 mV (−66 ± 1.42 mV). The maximum depolarization of Em was registered approx. 2.5 h after addition of Cd to the perfusion solution and in some cases, partial (Cd > 100 µM) or complete repolarization (Cd < 100 µM) was observed within 8–10 h of Cd treatment. In the time-dependent experiments (0 to 168 h) shortly after the maximum repolarization of Em a continuous concentration-dependent decrease of Em followed at all Cd concentrations. Depolarization of Em was accompanied by both increased electrolyte leakage and inhibition of respiration, especially in the range of 50 µM to 1 mM Cd, with the exception of root cells treated with 1 and 10 µM Cd for 24 and 48 h. Time course analysis of Cd impact on root respiration revealed that at higher Cd concentrations (> 50 µM) the respiration gradually declined (∼ 6 h) and then remained at this lowest level for up to 24 h.All the Cd concentrations used in this experiment induced significant inhibition of root elongation and concentrations higher than 100 µM stopped the root growth within the first day of Cd treatment. Our results suggest that Cd does not cause irreversible changes in the electrogenic plasma membrane H+ ATPase because fusicoccin, an H+ ATPase activator diminished the depolarizing effect of Cd on the Em. The depolarization of Em in the outer cortical cells of maize roots was the result of a cumulative effect of Cd on ATP supply, plasmalemma permeability, and activity of H+ ATPase.

Accumulation of pathogenesis-related proteins in barley leaf intercellular spaces during leaf senescence

Accumulation of the pathogenesis-related (PR) proteins localised in intercellular spaces of barley primary leaves, chlorophyll content, structure of chloroplasts, and photosynthesis were examined during natural and in vitro induced leaf senescence (cultivation of whole plants in the dark or detached leaves under nutrient deficiency). Some of PR proteins accumulated during natural senescence, but their accumulation pattern was different from those of pathogen-induced as well as during in vitro-induced senescence, which indicate different molecular bases of these processes. Photosynthetic rate and chlorophyll content indicate that natural senescence of barley primary leaves began from 15th day after sowing. In 35-d-old first leaves, the chloroplasts showed typical characteristics of senescence as significant decrease of size, greater grana, and prominent plastoglobuli. The chloroplasts contained more grana under in vitro induced senescence and they had reduced length in the dark. Correspondingly, accumulation of PR proteins was detectable on about the 15th day but the content of some PR proteins increased in later stages of senescence.

Growth and Differentiation of Root Endodermis in Primula acaulis Jacq.

Adventitious roots of Primula acaulis Jacq. are characterized by broad cortex and narrow stele during the primary development. Secondary thickening of roots occurs through limited cambial growth together with secondary dilatation growth of the persisting cortex. Close to the root tip, at a distance of ca. 4 mm from the apex, Casparian bands (state I of endodermal development) within endodermal cells develop synchronously. During late, asynchronous deposition of suberin lamellae (state II of endodermal development), a positional effect is clearly expressed - suberization starts in the cells opposite to the phloem sectors of the vascular cylinder at a distance of 30 – 40 mm from the root tip. The formation of secondary walls in endodermis (state III of endodermal development) correlates with the beginning of secondary growth of the root at a distance of ca. 60 mm. Endodermis is the only cortical layer of primrose, where not only cell enlargement but also renewed cell division participate in the secondary dilatation growth. The original endodermal cells additionally divide anticlinally only once. Newly-formed radial walls acquire a typical endodermal character by forming Casparian bands and deposition of suberin lamellae. A network of endodermal Casparian bands of equal density develops during the root thickening by the tangential expansion of cells and by the formation of new radial walls with characteristic wall modifications. These data are important since little attention has been paid up till now to the density of endodermal network as a generally significant structural and functional trait of the root.

Silicon improves salinity tolerance and affects ammonia assimilation in maize roots

The present study was conducted to evaluate the effect of different salt concentrations (50 and 200 mM NaCl) on growth, permeability properties (electrolyte leakage, cell viability) and activity of glutamine synthetase (GS) and glutamate dehydrogenase (GDH) in roots of maize seedlings. Both salt concentrations significantly affected growth and permeability properties of maize seedling roots and this negative effect increased with concentration of salt and duration of experiments. On the other hand salinity induced only small changes in the activities of GS and GDH, usually small increase in the activity was observed. To characterise the possible protective effect of silicon (Si) on maize roots exposed to saline stress, different concentrations of Si were simultaneously applied to both, low (50 mM) and high (200 mM) salt concentrations. Possible protective effects of Si on studied parameters were analysed in time range of 3 days treatment with the most positive effect on salt-induced root growth inhibition at high salt concentration and electrolyte leakage. The results show significant increase in GDH activity under all the tested conditions, although the mechanisms underlying this increase have not been elucidated. The results indicate that silicon may ameliorate the salt-induced root growth inhibition and increase the plant vigour at stressful conditions.

The participation of the primary maize root on the assimilation of NH4+ ions

It has been suggested recently that in plants ammonia obtained by reduction of nitrate or absorbed directly from the culture medium is incorporated primarily into organic compounds via glutamine synthetase (GS) and glutamate synthase (GOGAT) in the GS/GOGAT pathway (Arima and Kumazawa, 1977; Lewis et al., 1983; McNally and Hirel, 1983; Miflin and Lea, 1976). This is in contradiction to the previous assumption about ammonia incorporation via reductive amination of 2-oxoglutarate catalyzed by glutamate dehydrogenase (GDH).

The Effect of Silicon on the Activity and Isozymes Pattern of Antioxidative Enzymes of Young Maize Roots under Zinc Stress

The aim of this work was to study the effect of silicon (Si) on the activities of some antioxidative enzymes of young maize plants treated by zinc (Zn). Zinc is an essential element for higher plants but its higher concentrations are toxic for plants. Silicon is not considered as a plant‘s essential element but research has suggested that Si can mitigate the negative effect of various plant stresses. Two maize (Zea mays L.) hybrids, Almansa and Novania, differing in their tolerance to Zn toxicity, were grown in hydroponics and the effect of exogenous Si on the activities of superoxide dismutase (SOD), peroxidase (POX) and ascorbate peroxidase (APX) in the roots of Zn treated plants was studied. The enzymes activities were recorded by spectrophotometry and their isozymes pattern was determined by native PAGE. Silicon mitigated the activity of antioxidative enzymes (SOD, APX and POX) increased by Zn stress in hybrid Almansa but the same effect was not fully observed in hybrid Novania. Our results point out that a positive effect of Si on antioxidative response of Zn treated maize plants is probably hybrid-specific.

Effect of Si on the Antioxidative Defense of Young Maize Roots Under NaCl Stress

Salinity stress usually causes a serious yield reduction in crop production. Silicon (Si) has been reported to be able to alleviate stress damage, but the mechanism is still unclear. The aim of this work was to study the effect of exogenous silicon (Si) on the young maize plants welfare. For that the activities of major antioxidant enzymes (superoxide dismutase, SOD; peroxidases, POX; ascorbate peroxidase, APX) and on the lipid peroxidation (content of malondialdehyde, MDA) were investigated in the roots of salt-stressed maize (Zea mays L.). Two maize hybrids (Novania and Almansa) contrasting in their response to NaCl were tested. Four treatments with three replicates were monitored: control (without Si and NaCl), 2.5 mM Si, 150 mM NaCl, 150 mM NaCl and 2.5 mM Si. Maize roots were harvested 10 days after the treatment. MDA, ascorbate content and enzyme activities were assayed spectrophotometrically. The isoenzyme pattern of the antioxidant enzymes was detected by native PAGE. Compared with the plants treated with salt alone, added Si significantly increased MDA content in hybrid Almansa, while in Novania no increase was observed. The addition of Si significantly enhanced the activities of SOD in salt–stressed plants in both hybrids. The activities of POX and APX in salt stressed roots were significantly increased in Almansa hybrid in the presence of Si. The isoenzyme pattern followed changes in the activity of SOD; however in POX and APX such a tendency was not registered. The changes of the intensity and in the case of POX also the number of bands were detected. From these results we conclude, that Si addition fortified enzymatic antioxidative response operates in roots of young maize plants by a still unknown mechanism. The approval effect of Si is evident in the case of the SOD detoxifying system regardless of the hybrid, and for the POX family in the more sensitive maize hybrid Almansa. The presented outputs point out that silicon may protect plant tissues from oxidative damage under salt stress. Since the alleviative effect seems to be hybrid-specific, silicon alleviation of salt stress is probably not a universal feature in general and should be carefully considered between plants and/or hybrids.