The effect of nitrogen status on the regulation of plant‐mediated proteolysis in ingested forage; an assessment using non‐nodulating white clover

Abstract

Production of nitrogenous waste by livestock agriculture is a significant environmental concern in terms of pollution of land and water. In the rumens of cattle and sheep, the excessive proteolysis which contributes to inefficiency of nutrient use involves both the rumen microbial population and the intrinsic plant proteases that can mediate protein degradation in ingested fresh forage on exposure to the environmental stresses of the rumen. Here, white clover (Trifolium repens) plants that do not form root nodules, and so are dependent on nitrate supplied to the roots, have been used to determine how nitrogen status of the plant affects the rate of plant‐mediated proteolysis in forage under conditions that simulate ingestion by grazing ruminants. Plants were grown from seed and supplied with nutrient solution containing 2.5, 5.0, 7.5 or 10 mM nitrate. Protein, free amino acid and protease activity were determined in leaves which had been placed in an in vitro system designed to simulate conditions experienced in the rumen (anaerobic phosphate buffer maintained at 39°C in the dark). Foliar protein content increased with increasing nitrate supply, while in vitro incubation of leaves resulted in time‐dependent decreases in protein concentration and increases in amino acid concentration. Regardless of nitrate supply, 50% of the protein was degraded in 6 h and 80% after 24 h. As the extent of protein decrease was determined by initial protein content, more protein degradation occurred in those plants grown with the highest nitrate supply: after 6 h, 130.7 mg g−1 dry matter (DM) was degraded in leaves grown at 10 mM nitrate but only 52.3 mg g−1 DM in leaves grown at 2.5 mM nitrate. Hence, although the percentage of proteolysis is independent of foliar protein concentration, the latter is critical to the quantity of protein degraded. Heat‐stable serine and cysteine proteases were active throughout the term of the in vitro incubation. Although proteolysis in ingested forage can continue for many hours, mediated by heat‐stable proteases, maximum amino acid accumulation accounted for less than 40% of initial protein. Therefore, it is proposed that continued and extensive proteolysis occurs following leaf excision and exposure to rumen conditions because amino acid accumulation is insufficient to initiate those feedback systems which sense cytoplasmic amino acid concentration and prevent excessive proteolysis during normal source–sink relations.

The potential of novel Festulolium (2n = 4x = 28) hybrids as productive, nutrient‐use‐efficient fodder for ruminants

“…A similar relationship between initial low protein content and slow rates of protein degradation was seen previously with white clover (Kingston‐Smith et al. ). While the outcome in terms of protein content in both Lolium and the Festulolium hybrid genotypes at 24 h was similar, the rate of protein decline was significantly greater in the Lolium genotypes.…”

Nutritional Quality of Sorghum Seeds: Storage Proteins and Amino Acids

“…Vendemiatti et al isoleucine are derived from aspartic acid in a strongly regulated metabolic pathway (Ferreira et al, 2005(Ferreira et al, , 2006. Both, amino acids and nitrogen metabolism have been the focus of intense research (Ayongwa et al, 2006;Kingston-Smith et al, 2006;Tcherkez and Farquhar, 2006) and are important when nutritional benefits are considered (Helm et al, 2004;Pompeu et al, 2006). Moreover, amino acids such as methionine, cysteine, taurine, and carnosine are also important as nonenzymatic antioxidants (Gratão et al, 2005).…”

Nitrogen Application Rate and Genotype Effects on Switchgrass Production and Chemical Characteristics

“…5 [17] when no N was applied. These differences may result from inherent soil physicochemical properties, the plants ability to sequester N deeply in the soil profile through its extensive root system, and time of harvest.…”

“…It is one of the most challenging nutrients to manage effectively. In many agroecosystems, a very substantial portion of applied N is lost from soil to ground water, rivers, and oceans [3] [4], because crop plants only convert 30% -40% of this applied N to useful products [5]. In lignocellulosic crops, it is important to minimize feedstock nutrient concentration that may contribute to ash at combustion causing slagging and fouling of processing equipment as well as reduced efficiency of ethanol conversion processes.…”