Pyrimidine Pathway in Boron-deficient Cotton Fiber

Abstract

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...

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

“…Symptoms characteristic of boron deficiency can also be induced by the addition of 6-azauracil or barbituric acid, known inhibitors of pyrimidine biosynthesis (13,25,26). These observations strongly suggested to us (19,21) and others (5,20,36) that boron plays a fundamental role in pyrimidine metabolism. The hypothesis that boron is essential for maintaining adequate levels of pyrimidine nucleotides unifies several of the roles for boron reported in the literature.…”

“…However, stimulation of de novo uridine pyrimidine biosynthesis by sugars must not be taken as evidence that the formation of all species of pyrimidine nucleotide-sugars is normal in -B tissue. Indeed, reports that UDP-glucose formation is reduced under boron-deficient conditions (5,10,36) underscore the need to identify the pyrimidine nucleotide-sugars formed when roots from +B and -B plants are provided with sucrose.…”

“…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.…”

“…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.…”

Metabolic Requirement of Cucurbita pepo for Boron

“…These two possibilities are both supported by several reports in the literature demonstrating that the utilization or the level of available purine or pyrimidine nucleotides, or both, is altered by boron deprivation (3,4,7,8,11,12,18).…”