S‐deficiency responsive accumulation of amino acids is mainly due to hydrolysis of the previously synthesized proteins – not to de novo synthesis in Brassica napus

Abstract

To characterize the mechanisms of amino acid accumulation under sulphur (S)-deficiency and its physiological significance in Brassica napus, stable isotopes (15) N and (34) S were employed. The plants were exposed for 9 days to S-deficient conditions (0.05 mM vs 1.5 mM sulphate). After 9 days of S-deficiency, leaf-osmotic potential and total chlorophyll content significantly decreased. S uptake decreased by 94%, whereas N uptake and biomass were not significantly changed. Using (15) N and (34) S labelling, de novo synthesis of amino acids and proteins derived from newly absorbed NO(3) (-) and SO(4) (2) (-) and the content of N and S in the previously synthesized amino acids and proteins were quantified. At the whole plant level, S-deficiency increased the pool of amino acids but resulted in strong decrease of incorporation of newly absorbed NO(3) (-) and SO(4) (2) (-) into amino acids by 22.2 and 76.6%, respectively, compared to the controls. Total amount of N and S incorporated into proteins also decreased by 28.8 and 62.1%, respectively. The levels of (14) N- and (32) S-proteins (previously synthesized proteins) strongly decreased, mainly in mature leaves. The data thus indicate that amino acid accumulation under short-term S-deficiency results from the degradation of previously synthesized proteins rather than from de novo synthesis.

Sulfate resupply accentuates protein synthesis in coordination with nitrogen metabolism in sulfur deprived Brassica napus

“…A decrease in APR activity in N-deficient plants correlated with a decrease of mRNA abundance level and enzyme activity involved in sulfate assimilation in Arabidopsis (Koprivova et al, 2000). Under S-deficiency conditions, an increase in soluble nitrogen content including nitrate and amides, and a reduction in the internal S pool have been observed (Lee et al, 2013). In addition, the mRNA of the mitochondrial isoform of OASTL was increased in N-deprived spinach plants (Takahashi and Saito, 1996).…”

“…Seedlings at the trifoliate stage were transplanted to 3 L plastic pot and grown with hydroponic nutrient solution (Lee et al, 2013). The nutrient solution was continuously aerated and renewed every 5 days.…”

“…The resultant supernatant was passed through an H þ column (Dowex 50WX8-400) to get NO 3 À and SO 4 2À fraction. Amino acids were eluted from the column with 25 mL of 0.5 M HCl as previously described (Lee et al, 2013). For the fractionated liquid samples, an appropriate sample volume (usually 0.1 mL) containing more than the minimum quantity (25 mg N) was transferred into a tin capsule that was cooled by liquid nitrogen.…”

“…1C, D). It has been established that S-limitation results in a reduction of NR activity (Reuveny et al, 1980;Prosser et al, 2001) and transient accumulation of amino acids (Lee et al, 2013). Such results have been often explained in terms of the accumulation of some repressor metabolites which either interact with an enzyme (Koralewska et al, 2009;Lee et al, 2013) or down-regulate the transcription of its gene (Koprivova et al, 2000;Prosser et al, 2001).…”

“…The relationship between sulfate content and expression of the sulfate transporters is ambiguous (Koralewska et al, 2009). For example, the increase in the overall capacity of sulfate uptake by root is limited (Hawkesford, 2003;Buchner et al, 2004;Lee et al, 2013Lee et al, , 2014, although the expression of sulfate transporters is strongly enhanced under S-limitation Kataoka et al, 2004;Honsel et al, 2012).…”