Increased RNA labelling in boron-deficient root-tip segments

Cotton ovules cultured in an insufficiency of boron (10 micromolar), showed inhibition of fiber growth by the ninth day in culture. Averaging data from eight to eleven days of culture under these conditions, total incorporation of 16-4Clorotic acid into fiber was inhibited by 59%. Inhibition was evident in all radioactively labeled pools, indicating that the effect may be at the membrane transport level or at an early stage of orotic acid metabolism. On a per cent basis, incorporation into RNA under boron deficiency was higher than under sufficiency. The effect is greater on the eighth day of culture, with a decreasing difference from controls up to the eleventh day. Conversely, the per cent incorporation into UDP-glucose was lower under boron deficiency than in controls, having a more or less constant value from 8 to 11 days of culture. Thus, a prinary event of boron deficiency in cotton fiber culture is an alteration in the flow of metabolites through the pyrimidine synthesis pathway.A possible primary event of boron deficiency is the influence of boron on pyrimidine metabolism. A Robertson and Loughman (18,19) for Vicia roots where elongation was being inhibited but had not ceased. In recent studies, Chapman and Jackson (6, 13), working with Phaseolus root tips, showed both early and late effects. There was an early (6-10 h) increased uptake of ["C]-uridine or [32Plinorganic phosphate into RNA at a time when the RNA levels were unchanged. RNA levels fell only after root elongation ceased (100 h) concurrently with manifestation of other boron deficiency symptoms. Increased ribonuclease activity may accompany the decrease in RNA in boron-deficient tissues (6,25).Information about the influence of boron on nucleotide levels is very sparse (4,5,28). Robertson and Loughman (18,19) Working with the in vitro-cultured cotton fiber system, Birnbaum et al. (21) found that the OMP'-decarboxylase inhibitor, 6-azauracil gave symptoms resembling those caused by boron deficiency (3). The correlation was further strengthened by the finding that low uracil concentrations partially overcame both boron deficiency and 6-azauracil effects, presumably by utilization of the uracil salvage pathway.In the present study, we sought to establish more directly whether boron regulates the pyrimidine pathway in some way. Our experimental tissue was fiber from the cotton ovule culture system. We studied the incorporation of [14CJOA into intermediates of the pyrimidine pathway. A factor which had to be taken into account was the possible effect of boron deficiency on OA uptake by the tissue itself.A defmitive study on the interaction of boron with ion transport mechanisms has been done by Loughman's group (14, 16-19), using mainly V. faba seedlings at a stage where root elongation was inhibited by boron deficiency but had not totally ceased. They discovered that the reduced uptake of [32P]phosphate and other inorganic ions caused by boron deficiency, actually involves several components including translocation, metabolism, inter...

Section: Introductionmentioning

confidence: 99%

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confidence: 99%

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confidence: 99%

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Pyrimidine Pathway in Boron-deficient Cotton Fiber

Wainwright

Palmer²,

Dugger³

1980

“…Several workers (1,7,14,16) cursors into RNA provide evidence that RNA synthesis is also impaired during boron deficiency. For example, Sherstnev and Razumova (29) reported decreased incorporation of ["4Cladenine into RNA of boron-deficient sunflower leaves and roots; while other workers have demonstrated increased incorporation of [14Cluridine (7) and [14CJorotic acid (36) into RNA in borondeficient mung bean root apices and cotton ovules, respectively. These reports suggest that the utilization or the level of available purine or pyrimidine nucleotides, or both, is altered by boron deprivation.…”

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confidence: 99%

Synthesis, Salvage, and Catabolism of Uridine Nucleotides in Boron-Deficient Squash Roots

Lovatt¹,

Albert²,

Tremblay³

1981

Previous work has provided evidence that plants may require boron to maintain adequate levels of pynmidine nucleotides, suggesting that the state of boron deficiency may actually be one of pyrimidine starvation. Since the avaflability of pyrimidine nucleotides is influenced by their rates of synthesis, salvage, and catabolism, we compared these activities in the terminal 3 centimeters of roots excised from boron-deficient and -sufficient squash plants (Cscurbita pepo L. (7) and [14CJorotic acid (36) into RNA in borondeficient mung bean root apices and cotton ovules, respectively. These reports suggest that the utilization or the level of available purine or pyrimidine nucleotides, or both, is altered by boron deprivation.We were particularly interested in the observation (30) that plants growing in the absence of boron could be protected from developing boron-deficiency symptoms by adding a hydrolysate of yeast RNA to the nutrient solution. Work in our laboratory (2, 15, 16), and elsewhere (4, 5) tested the effects of both purine and pyrimidine bases on plant growth to determine which component(s) of the RNA hydrolysate afforded this protection. Intact plants and isolated organs cultured in the absence of boron were protected to varying degrees from developing boron deficiency symptoms when pyrimidine bases were added to the medium.This result was taken as evidence that the state of boron deficiency may, in fact, be a case of pyrimidine starvation. Such an interpretation was supported by the observations that both barbituric acid and 6-azauracil, known inhibitors of pyrimidine biosynthesis (13,25,26), produced symptoms identical with those of boron deprivation (2, 5, 16). Various investigators (5,(19)(20)(21)36) have suggested that boron deficiency results in impaired de novo biosynthesis of pyrimidine nucleotides. Such impairment could occur through either a loss in amount of one or more enzymes or enhanced sensitivity of the de novo pathway to end product inhibition. Starvation for pyrimidine nucleotides could also result from an inability of boron-deficient plants to salvage or reutilize pyrimidine bases or nucleosides, or from an acceleration of pyrimidine catabolism.In this communication, we report the results of a comparison of the capacity of roots from boron-sufficient (+B)3 and borondeficient (-B) squash plants (Cucurbita pepo L., cv. Early Prolific straightneck) to synthesize uridine nucleotides de novo. We also include an examination of the possibility that boron deprivation interferes with the mechanism regulating the de novo pathway through end product inhibition. In addition, since catabolism opposes de novo biosynthesis, we also assessed the capacity of +B and -B roots to catabolize uridine and uracil. Finally, we measured salvage activity for the reutilization of pyrimidines during these two states of boron nutrition.

“…Some workers have reported that RNA levels increase during boron deficiency (8)(9)(10) while others have observed a decrease in nucleic acid levels (5,7). Chapman and Jackson (7) found that an early sign of boron deficiency in Phaseolus aureus root tip segments was increased incorporation of radioactive precursors into RNA. Only later, concomitant with increased ribonuclease activity and cessation of elongation, did a reduction in RNA levels occur.…”

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Interaction of Boron with Components of Nucleic Acid Metabolism in Cotton Ovules Cultured in Vitro

Birnbaum¹,

Dugger²,

Beasley³

1977

Cotton (Gossypium hirsutum L.) ovules grown in a defined nutrient medium undergo normal morphogenesis, induding fiber production. In identical medium lacking boron, ovules callus and accumulate brown substances. Boron deficiency-like symptoms were induced by 6-azauracil and 6-azauridine in ovules growing in boron-sufficient media. Other nudeoside base analogs either reduced or had no effect on over-all growth, but did not cause typical boron-deficient callus growth of cotton ovules. Orotic acid and uracl countered the effects of 6-azauracil. Actinomycin D, fluorodeoxyundine, and ethidium bromide reduced not only fiber production on ovules growing in boron-sufficient media but also callusing of ovules in boron-deficient media.Similarties between symptoms of boron deficiency and 6-azauracil injury, and the ability of uracil to suppress both, suggest that boron defciency symptoms are related to reduced activity in the pyrimidine biosynthetic pathway. Changes in nucleic acid levels have been correlated with boron deficiency. Some workers have reported that RNA levels increase during boron deficiency (8-10) while others have observed a decrease in nucleic acid levels (5, 7). Chapman and Jackson (7) found that an early sign of boron deficiency in Phaseolus aureus root tip segments was increased incorporation of radioactive precursors into RNA. Only later, concomitant with increased ribonuclease activity and cessation of elongation, did a reduction in RNA levels occur. They point out that reports of boron deficiency-related reductions in RNA were in grossly deficient tissues (1,18) and that a much earlier and possibly more direct effect of boron deficiency was increased RNA synthesis (8, 9).