Timing is everything - obtaining accurate measures of plant uptake of amino acids

Plants are known to have the capacity to take up and utilise amino acids for growth. The significance of this uptake, however, remains elusive, partly due to methodological challenges and biological implications associated with acquiring and interpreting data. This study compared bulk stable isotope analysis and compound-specific liquid chromatography-mass spectrometry, two established methods for determining amino acid uptake. Root amino acid uptake was assayed using U-¹³ C₅ -¹⁵ N₂ -l-glutamine and axenically grown Arabidopsis thaliana. After 15-120 min of exposure, the content of intact glutamine measured in the roots was constant, whilst the ¹⁵ N and ¹³ C content increased over time, resulting in very different estimated uptake rates. The ¹³ C : ¹⁵ N ratio in roots declined with time, suggesting a loss of glutamine carbon of up to 15% within 120 min. The results presented indicate that, regardless of method used, time is a crucial factor when determining plant amino acid uptake. Due to post-uptake metabolism, compound-specific methods should primarily be used in experiments with a time frame of minutes rather than hours or days. Post-uptake metabolism in plants may account for significant loss of carbon, suggesting that it is not just pre-uptake metabolism by microbes that accounts for the ¹⁵ N-¹³ C mismatch reported in ecological studies, but also post-uptake metabolism in the plant.