Inhibition of lettuce seed germination and seedling growth by antimetabolites of nucleic acids, and reversal by nucleic acid precursors and gibberellic acid

Evaluation of the toxic effects of four anti-cancer drugs in plant bioassays and its potency for screening in the context of waste water reuse for irrigation

Anti-cancer drugs are compounds that are of high environmental relevance because of their lack of specific mode of action. They can be extremely harmful to living organisms even at low concentrations. The present study evaluated the toxic effects of four frequently used anti-cancer drugs against plant seedlings, namely Cyclophosphamide (CP), Methotrexate (MTX), 5-Fluorouracil (5-FU) and Imatinib (IM). The phytotoxicity experiments were performed with Lactuca sativa seedlings whereas cytotoxicity, genotoxicity and mutagenicity investigations were performed with the well-established Allium cepa assays. MTX was the most phytotoxic compound, followed by 5-FU, CP and IM. Significant differences in the Mitotic Indexes (MI) were observed in three of the studied compounds (MTX, 5-FU and CP), indicating potential cytotoxic activity of these substances. Chromosome aberrations were registered in cells that were exposed to 5-FU, CP and IM. All the four compounds caused the formation of micronucleated cells indicating mutagenic potential. Besides, the assays performed with MTX samples presented a high number of cell apoptosis (cell death). Although it is unlikely that the pharmaceuticals concentrations measured in the environment could cause lethal effects in plants, the obtained results indicate that these compounds may affect the growth and normal development of these plants. So, both tests can constitute important tools for a fast screening of environmental contamination e.g. in the context of the reuse of treated wastewater and biosolids of agricultural purpose.

[Action of 5-bromouracil and its nucleosides on the morphogenesis of the cuttings of etiolated tomato seedlings]

1. 5-Bromouracil inhibits the growth of the hypocotyl and the rhizogenesis in the cuttings of etiolated tomato seedlings only at high concentrations. 5-Bromouridine is inactive, but 5-bromodeoxyuridine strongly inhibits both phenomena. 2. The inhibition induced by 5-bromouracil cannot be reversed by thymine, deoxythymidine, uracil, uidine or β-alanine, a catabolic product of uracil. 3. Deoxythymidine is the one pyrimidine nucleoside which reverses the inhibition of growth and rhizogenesis brought about by 5-bromodeoxyuridine. 4. Deoxycytidine enhances the inhibition of rhizogenesis caused by 5-bromodeoxyuridine. Such an effect on the growth of the hypocotyl is not observed. 5. Taking into consideration the data in the literature, it is concluded: a)5-bromouracil can act in several ways which possibly differ according to the plant species: in some instances (as in the case described here), it seems to act by a mechanism unconnected with the metabolism of pyrimidine nucleic bases: b) 5-bromodeoxyuridine is a specific antimetabolite of deoxythymidine, but other metabolic interactions exist between 5-bromodeoxyuridine and deoxycytidine, each of the latter substances probably acting as a modifier of the metabolism of the other.

RNA synthesis and the germination of light-sensitive lettuce seeds

The germination of lettuce seeds is inhibited by the nucleotide base analogue 6-methylpurine. RNA synthesis has been measured during imbibition and germination as (32)P-phosphate incorporation into RNA species as fractionated by polyacrylamide gel electrophoresis. Seeds were surface sterilized and imbibed in the presence of various antibiotics. RNA preparations from lettuce seeds were coelectrophoresed with (3)H-RNA prepared from bacteria to check for bacterial contamination of the seeds. There is a much higher rate of RNA synthesis in illuminated, germinating seeds as compared to dark, non-germinating seeds. This difference does not develop until after 12 hours of imbibition at 27°, which is the time of onset of germination and radicle growth.

[The inhibitory effect of puromycin and 2-thiouracil on the phytochrome-mediated synthesis of anthocyanin]

The phytochrome-mediated anthocyanin synthesis of the mustard seelding, Sinapis alba L., can be stopped by the application of relatively low concentrations of puromycin and 2-thiouracil. While actinomycin D, which has been investigated in a previous paper (LANGE and MOHR, 1965), will block phytochrome-induced anthocyanin synthesis only if it is applied before or at the onset of light, puromycin and 2-thiouracil will stop anthocyanin synthesis even if they are applied at a later stage, e.g., 12 hours after the onset of light. - The effect of 2-thiouracil can be reversed if the seedlings are transferred to a medium containing uracil or to blank water. The simultaneous application of 2-thiouracil and uracil also leads to at least a partial reversal of the 2-thiouracil effect. - The kinetics of the puromycin inhibition (Fig. 2) indicate that the enzyme with the shortest "life-time" among those enzymes which are involved in anthocyanin production has a "life-time" in the order of 6 hours. One may reasonably conclude on the basis of Fig. 1 and 2 that synthesis of this particular enzyme is controlled by phytochrome 730. - The effect of 2-thiouracil (Fig. 3) cannot - in our case at least - be understood by only assuming that 2-thiouracil will be incorporated into RNA and thus will lead to the formation of "wrong" RNA. Inhibition by 2-thiouracil is much faster than inhibition by puromycin (Fig. 2,3). We have rather to conclude that 2-thiouracil may exert its effect mainly through a direct competitive enzyme inhibition.

Dependence of gibberellic acid-induced dark germination of lettuce seed on RNA synthesis

Gibberellicacid-induced dark germination of Grand Rapids lettude seed was completely inhibited by 6-azauracil and partly by 2-thiouracil. Other inhibitors of nucleic acid and protein synthesis used were without effect. Inhibition of gibberellic acid-induced dark germination was reversed by uracil but not by thymine, deoxycytidine and orotic acid. The results suggest that gibberellic acid-induced dark germination is dependent on RNA synthesis and not on DNA synthesis.Gibberellic acid-induced lettuce hypocotyl growth was inhibited by all the inhibiters of nucleic acid and protein synthesis used, including actinomycin D, puromycin, chloramphenicol and p-fluorophenylalanine.

[Inhibition of the auxin-induced root-formation in etiolated pea epicotyls by histone and antimetabolites of the RNA- and protein synthesis]

Differentiation of tracheids and root formation in etiolated pea epicotyls is inhibited by actinomycin D, 5-bromouracil, calf thymus histone and chloramphenicol 48 hours after culture initiation. The formation of spots with embryonic cells is thereby not suppressed. The bromouracil-inhibition is competitively reversed by thymine. 2-Thiouracil and 8-azaguanine also inhibit root formation 48 hours after culture initiation, but this effect is not reversed by uracil or guanine. The temporal spectrum of root-inhibition induced by 5-bromouracil, calf thymus histone, chloramphenicol or actinomycin D differs from that induced by 2-TU. Like cystein and dinitrophenol, 2-TU inhibits root-formation entirely during the first five days after culture initiation.Increasing amounts of indole-3-acetic acid cause the formation of an increasing number of roots and the earlier appearance of adventive roots and bring about a prolonged phase of activity, which is inhibited by 5-bromouracil or histone. Moreover, the phases of inhibition start earlier.The temporal position of the root-inhibition phases depends on the season.