Ala a branched chain amino acid has been reported to
Ala, a branched chain amino acid, has been reported to play a functional role in intracellular pH regulation, and it typically accumulates in response to various stresses (Limami et al., 2008, Rocha et al., 2010). Our data demonstrated that increasing Cd concentrations at lower Cd doses resulted in a decline in the content of free Ala in both plants, and can be linked to the increasing use of Ala for the biosynthesis of proline/alanine-rich protein kinases or histidine- and alanine-rich proteins (Komatsu et al., 2009, Mori et al., 2013). When stressed with high Cd amounts, a reduction in the rate of protein synthesis and an increased Ala synthesis due to disturbance of alanine aminotransferase reactions resulted in an increase of free Ala (Hjorth et al., 2006). As another branched chain AA, Val is pivotal in balancing the fluxes between different amino acids pathways. The accumulation of Val may serve to promote stress-induced protein synthesis and to maintain amino acids homeostasis (Joshi et al., 2010). A stimulating effect of Cd in soil on the content of free Val in the leaves of both studied species was observed through the experiments, while an opposite effect was shown in the stems, which may be a direct biological response to the stress conditions.
His was found to play an important role in regulation of biosynthesis of other amino acids, in chelation and transport of metal ions, and in plant reproduction and growth (Stepansky and Leustek, 2006). This experiment showed that the content of His significantly increased with increasing Cd concentrations, while His was below the detection limit in the roots and stems of A. conyzoides. Thus, the synthesis of His seems to be associated with Cd stress. Gly and Cys are involved in the biosynthesis of phytochelatines and antioxidant metabolites, and are also found in glycine-rich proteins that affect the growth and function of cell walls (Zemanová et al., 2017). Significantly decreased Gly contents in A. conyzoides must be seen as the activation of OICR-9429 processes to the toxic effect of Cd, while in our hyperaccumulator species Gly accumulation was relatively stable. Cys was a major FAA in A. conyzoides, while its content was barely determined in C. crepidioides.Xu's et al. (2012) previous transcriptome analysis revealed that a cysteine desulfurase gene, NFS, which can catalyze Cys to Ala, showed higher expression in the Cd accumulators S. nigrum. The undeterminable Cys concentration in C. crepidioides may be a result of the conversion of Cys to Ala.
Trp, Tyr and Phe, which are aromatic amino acids, plays a major role in the regulation of plant development and defense responses. Moreover, they represent the precursors for the synthesis of protein and hormone auxin (Liu et al., 2011, Sanjaya et al., 2008). In the present study, biosynthesis of Trp and Tyr was induced by Cd stress in the roots of plants and an increased content of Trp and Tyr have been found in shoots under Cd stress. Accumulation of free Trp and Tyr in A. conyzoides stems under Cd12 treatment is more than 70-fold and 10-fold higher in contrast to Cd6 treatment. These observations indicated that Trp and Tyr may be involved in Cd resistance and accumulation by reducing oxidative damage in the studied plants. Phe is also a substrate for the phenylpropanoid pathway, which produces a wide range of antioxidative metabolites and phenolic compounds (Sanayama et al., 2011). A fluctuating content of free Phe was observed in our study indicating their responsiveness during Cd stress.
Acknowledgements This research was financially supported by the National Natural Science Foundation of China (No. 41425014; 41625006; 31600442), China Postdoctoral Science Foundation Funded Project (No. 2015M582578), the National Key Research and Development Program of China through grant 2016YFA0601000 (H.Y. Xiao), the Development Services of Feature Institute Program of Chinese Academy of Sciences (No.TSYJS01), and the Joint Funds of the Natural Science Foundation of Science and Technology Department of Guizhou Province, China (No. LH  7168).