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    • Glu Gln Asp and Asn the main

    2023-01-24

    Glu, Gln, Asp, and Asn, the main Tryptone presented in the present study, are involved in nitrogen assimilation and transport processes within the plants. Furthermore, they are used to build up reserves during periods of nitrogen availability for subsequent use in growth, defense, and reproductive processes (Zemanová et al., 2015a). In all higher plants, inorganic N is at first reduced to ammonia prior to its incorporation into organic compound. Ammonia is assimilated into organic compound as Glu and Gln, which serve as the N donors in the biosynthesis of all essential amino acids and other nitrogen-containing compounds (Sánchez-Pardo et al., 2013). Asp is synthetized by transamination of oxaloacetate and also feeds into the synthesis of Asn, Lys, Met, Thr, and Ile as well as the conversion of Thr into Gly (Angelovici et al., 2009). In the present experiment, a decline in Asp and Glu content was observed in both studied plants at lower Cd doses, as well as an increase at higher Cd doses. According to Zemanová et al. (2013) the declines of both amino acids can be caused by intensive syntheses of plant defense elicitors. These two amino acids are quickly transformed into the required products or incorporated into a protein without increased accumulation in plant when exposed to Cd stress. The accumulation of Glu and Asp at higher Cd doses may be a consequence of buffering effects through the modulation of Pro/His/Orn and Asn/Lys/Thr/Ile/Met (Xu et al., 2012). Gln, with high N/C ratio, is known to be highly reactive and to serve as the major nitrogen transport form in plants. Asn, which is also an amide showing a high N/C ratio, is less reactive than Gln and can be used by the plant as a nitrogen storage compound (Chaffei et al., 2004). In addition, a Cd-Gln and Cd-Asn complex may reduce Cd toxicity. Moreover, asparagine shows the same effect by acting as a ligand towards Cd (Sharma and Dietz, 2006). At lower Cd exposure, increasing Cd doses were associated with clearly increasing Gln and Asn contents in C. crepidioides shoots and with decreasing Gln and Asn contents in A. conyzoides. These results showed a different pathway of nitrogen utilization of both plants. Note a higher accumulation of Glu, Glu, Asp and Asn in the root of C. crepidioides when exposed to highest Cd stress, which supports the observed higher Cd tolerance and Cd accumulation in C. crepidioides than in A. conyzoides. Pro and GABA are well known to be the abiotic and biotic stress indicator and protector in plants (Xu et al., 2012, Zemanová et al., 2017). The induction of free Pro in response to heavy metal exposure has been reported (Sharma and Dietz, 2009, Vassilev and Lidon, 2011, Xu et al., 2009). The functions of Pro seem to be manifold. It plays a major role in adjustment to osmotic stresses, maintaining the water balance as it stabilizes the subcellular structures, and also function as a hydroxyl radical scavenger (Sharma andDietz, 2006). However, Pro exhibited opposite abundance trends between the two tested plants under Cd stress, which indicated that two different stress defense pathways exist in the two species. Pro is a precursor in the Hyp biosynthesis pathway. Thus, increased consumption of Pro for Hyp biosynthesis under Cd stress could be the reason for the observed decrease in Pro content in C. crepidioide shoots. The result corresponds with those by Yi and Kao (2003) who reported that no Pro accumulation was observed in Cd-tolerant plant leaves. Higher accummulation of Pro in A. conyzoides shoots was detected when exposed to Cd stresses, which can be attributed to its protective role in reducing membrane and protein damages, and acting as an osmo-protectant in stressed plants (Pavlík et al., 2010). Besides being the major component of the free amino acid pool, different roles have been designated for GABA. Induction of signaling pathway via Ca2+, C: N balance via primary metabolism, developmental role via cell wall modification, participation to nutrient uptake and stimulation of cell death are among these different roles in plants (Michaeli and Fromm, 2015). Excessive GABA levels in response to different abiotic and biotic environmental stresses are commonly found in several plants (Pavlíková et al., 2014, Shelp et al., 2012). The opposite trend observed in our study may be a consequence of increased inhibition of glutamate decarboxylase (GAD) activity results in a decline of GABA synthesis from Glu under Cd stress.