Oat roots

R. Pinton, Z. Varanini, G. Vizzotto, and A. Maggioni, Soil humic molecules affect transport properties of tonoplast vesicles i.solated from oat roots. Plant Soil 142 203 (1992). 

J. A. Trofymow, D. C. Coleman, and C. Cambardella, Rates of rhizodeposition and ammonium depletion in the rhizosphere of axenic oat roots. Plant Soil 97 333 (1987). 

Salicylic acid depolarized PD In epidermal cells of oat roots also. At pH 4.5, 500 pM salicylic acid caused a transient hyperpolarization followed by a dramatic depolarization to about -45 mV (Figure 2). Removal of salicylic acid produced a transient, partial repolarization. At pH 6.5, salicylic acid did not affect PD. These results with different pH s are consistent with the Influence of salicylic acid on K+ absorption in oat roots (32). 

Figure 2. Influence of 500 flM salicylic acid on the electrical potential (PD) of epidermal cells of excised oat roots at pH 4.5. Microelectrode was inserted in a cell at 1 min salicylic acid was added at 2.5 min and removed by flushing at 25 min.

Effects of Allelochemlcals on ATPases. Several flavonoid compounds inhibit ATPase activity that is associated with mineral absorption. Phloretin and quercetin (100 pM) inhibited the plasma membrane ATPase Isolated from oat roots (33). The naphthoquinone juglone was inhibitory also. However, neither ferulic acid nor salicylic acid inhibited the ATPase. Additional research has shown that even at 10 mM salicylic acid inhibits ATPase activity only 10-15% (49). This lack of activity by salicylic acid was substantiated with the plasma membrane ATPase Isolated from Neurospora crassa (50) however, the flavonols fisetln, morin, myricetin, quercetin, and rutin were inhibitory to the Neurospora ATPase. Flavonoids inhibited the transport ATPases of several animal systems also (51-53). Thus, it appears that flavonoids but not phenolic acids might affect mineral transport by inhibiting ATPase enzymes. 

Effects of Allelochemlcals on ATP Supply. Allelochemlcals might decrease the ATP content of tissue by either increasing ATP utilization or decreasing ATP production. Some allelochemlcals that inhibit mineral absorption decrease ATP content of plant tissues. Salicylic acid decreased the ATP content of oat roots in a pH dependent manner (Figure 3). This result suggested that mitochondrial production of ATP was decreased in the tissue. On the other hand, Tillberg (38) found that salicylic acid and cinnamic acid increased the ATP content of Scenedesmus. Various flavonoids inhibited ATP production by mitochondria Isolated from cucumber (Cucumls satlvus L.) hypocotyls (54). Flavones such as kaempferol were more inhibitory than the corresponding flavanones. Substituted cinnamic acids such as caffeic acid were not inhibitory. 

Figure 3. Influence at 500 (lM salicylic acid on ATP content of excised oat roots at four pH values. ATP determined by luciferin/ luciferase assay.

At pH 6.5 salicylic acid produced only a slight, short-lived increase in K efflux from oat roots, but at pH 4.5, the compound caused dramatic and extensive loss of K" " from the tissue (Figure 4). Salicylic acid caused K+ to leak from yeast cells also (63,... 

In addition to minerals, organic metabolites that absorb light at 260 nm leaked from oat roots in the presence of salicylic acid at pH 4.5 but not pH 6.5 (32). 

It remains to be determined if one or both of these hypotheses are correct for plant roots. One feature of the inhibition of absorption by salicylic acid (and probably other phenolic acids) that may be relevant to this point is whether the neutral acid or the anion is responsible for the inhibition. In oat roots, the amount of neutral acid present when salicylic acid caused 50% Inhibition of K+ absorption was constant regardless of pH (Figure 6). However, the concentration of anion present changed several orders of magnitude. This result suggests the neutral acid is the species... 

Figure 4. Retention of K+ ( Rb+) in excised oat roots in the presence and absence of 0.5 mM salicylic acid at pH 6.5 and pH 4.5. The control contained 1% ethanol (ETOH).

Figure 6. Relationship between the concentrations of neutral ssligglic acid (SA°) and salicylate anion (SA ) present when K ( Rb+) absorption by excised oat roots was inhibited 50% at six pH values.

CROMBIE, W.M.L., CROMBIE, L., Distribution of avenacins A-l, A-2, B-l and B-2 in oat roots Their fungicidal activity towards take-all fungus, Phytochemistry, 1986, 25, 2069-2073. 

I. M. Moller, T. Lundborg, and A. Berczi, The negative surface charge density of plasmalemma vesicles from wheat and oat roots, FEBS Lett. 167, 181-185 (1984). 

Plant Lindane appeared to be metabolized by several grasses to hexachlorobenzene and a-BHC, the latter isomerizing to p BHC (Steinwandter, 1978 Steinwandter and Schluter, 1978). Oat plants were grown in two soils treated with [ CJlindane. 2,4,5-Trichlorophenol and possibly y-PCCH were identified in soils but no other compounds other than lindane were identified in the oat roots or tops (Fuhremann and Lichtenstein, 1980). The half-life of lindane in alfalfa was 3.3 d (Treece and Ware, 1965). 

The purposes of this paper are to review one of the common mechanisms of detoxication in plants, namely conjugation, and to present data showing that salicylic acid, an allelopathlc phenolic acid, is enzymatically conjugated by oat roots. 

Because selective detoxication by a crop plant would be important if allelochemlcals are to be useful as natural herbicides, the first objective of these studies was to determine if the cereal oats can metabolize exogenously applied salicylic acid, a known allelopathio agent. The second objective was to identify and partially purify any enzyme responsible for such metabolism of salicylic acid in oat roots. 

Absorption of Salicylic Acid. Excised oat roots absorbed salicylic acid in two distinct phases (Figure 1). Upon exposure to salicylic acid the root segments rapidly absorbed the eompound to attain a concentration of about 0.5 pmole/g of tissue. On the assumption that 1 g of tissue equals 1 mL of tissue, this translates to 0.5 mM salicylic acid inside the tissue, the same concentration as the external solution. This coneentration of salicylic acid was present in the tissue after 1 h and was maintained for over 3 h. By 4 h a second phase of absorption was evident (Figure 1). During the second phase, salicylic acid was absorbed at a greater rate that lasted for at least 24 h. At that time, enough salicylic acid had been absorbed that the eoncentration in the tissue was 8.0 mM. Thus, the tissue accumulated salicylic acid to concentrations greater than that in the external solution. An additional experiment (not shown) showed that the tissue would continue to absorb salicylic acid until the compound was depleted from the external solution. 

Distribution of C between Salicylic Acid and Metabolite in Oat Roots Following Exposure for Various Times —... 

Table III. Salicylic Acid Metabolite Production by a 50-65% Ammonium Sulfate Fraction Extracted from Oat Roots Exposed to...

Partial Purification of SA-GTase. Proteins extracted from oat roots Incubated for 20 h in salicylic acid were separated by salt precipitation and gel exclusion and anion exchange chromatography (Table IV). A 5 -fold purification of the SA-GTase was achieved with this... 

Figure 1. Absorption of salicylic acid for 1 to 24 hr by excised oat roots.

Figure 2. Reaction scheme for UDPG salioylio acid glucosyltrans-ferase extracted from oat roots incubated for 20 hr in solution containing salicylic acid.

The plasma membranes of plant cells possess several redox activities that can be related to both plant nutrition and cell wall formation and lignification (Liithje et al., 1997 Berczi and Mpller, 2000). In this context, it has been shown that in oat roots, HMS humic fractions inhibited NADH oxidation in either the presence or absence of an artificial electron acceptor (ferricyanide), whereas LMS fractions inhibited this oxidase only if the electron donor (NADH) and acceptor (ferricyanide) were added at the same time (Pinton et al., 1995). While the first effect could be related to the activity of surface peroxidases that can be involved in cell wall formation and thickening (Vianello and Macri, 1991), the second seems to be exerted on a different redox system with an unknown function (Nardi et al., 2002). 

Pinton, R., Cesco, S., Santi, S., and Varanini, Z. (1995). Effect of soil humic substances on surface redox activity of oat roots. J. Plant Nutr. 18, 2111-2120. 

Assayed on oat root plasma membrane vesicles incubated with 0.075 mg C org liter-1 of a low-molecular-weight (<5kDa) humic fraction isolated with Na O, from the Ao horizon of a lythic rendoll (Varanini et al., 1993). 

Osbourn, A.E., Wubben, J.P., Daniels, M J. Saponin detoxification by phytopathogenic fungi. In Plant-Microbe Interactions Volume 2. Stacey, G, Keen, N.T. ed., New York Chapman and Hall, 1995a, pp. 99-124. Osbourn, A. Bowyer, P., Lunness, P., Clarke, B., Daniels, M. Fungal pathogens of oat roots and tomato leaves employ closely related enzymes to detoxify different host plant saponins. Mol Plant Microb Interact 1995b 8 971-978. 

Earlier, we analyzed oat roots after BOA incubation of seedlings for at least 24 h and up to 72 h. Two new products were present in the extracts, which were identified as BOA-6-OH and BOA-6-O-glucoside.83 In addition, a third product was found that increased with incubation time. It was slightly more hydrophobic than BOA-6-OH with an UV scan very similar to BOA. The compound was purified, subjected to chemical identification and characterized as BOA-A/-glucoside. This product was not found in roots of Vida faba var. Alfred, but this seems to depend on variety e.g., Vida faba var. Dreifach Weisse, was able to produce the compound. Both varieties contained BOA-6-OH and the corresponding glucoside. 

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