In the chloroplasts, glutamine synthase incorporates this ammonia as the amide group of glutamine, using glutamate as a substrate. In addition, it was earlier reported that the activity of a large number of enzymes involved in nitrogen assimilation decreases under long-term abiotic stress conditions, while short-term stress increases others. In the process, NH 4 + is further assimilated by glutamine synthase and glutamate synthase into amino acids. During these events, nitrate reductase (NR) and nitrite reductase (NiR) convert the exogenous nitrate to ammonium (NH 4 +). However, external fluctuations in the supply of nitrogen to plants can affect its uptake, and lead to activation of various regulatory networks in order to optimize N uptake and utilization. Nitrogen, a key macronutrient is required in the process, among others. Plant growth, development, and productivity require the permanent availability of nutrients, and the plant nutrient needs increase with the growth stage. These results suggest that the inhibitory potential of KClO 3 on the reduction activity of the nitrate reductase (NR), as well as that of the genes encoding the nitrate and ammonium transporters, and glutamate synthase are tissue-specific, which may differentially affect the transport and assimilation of nitrate or ammonium in rice. The changes in the chloroplast pigments and proline content propose these compounds as emerging biomarkers for assessing the overall plant health status.
Moreover, the induction of catalase (CAT) and polyphenol oxidase (PPO) enzyme activities in Saeilmi (P1, KClO 3 resistant), and the decrease in Milyang23 (P2, KClO 3 sensitive), coupled with the malondialdehyde (MDA) content, indicated the extent of the oxidative stress, and the induction of the adaptive response mechanism, tending to maintain a balanced reduction–oxidation state in response to KClO 3.
Furthermore, the transcriptional divergence of OsAMT1.3 and OsAMT2.3, OsGLU1 and OsGLU2, between NR and NRT, coupled with the NR activity pattern in the roots, would indicate the prevalence of nitrate (NO 3¯) transport over ammonium (NH 4 +) transport. Under the same conditions, the activity of NR was inhibited in the roots and differentially regulated in the stem and leaf tissues. In the same way, the expression patterns of OsNIA1 and OsNIA2 in the roots, stem, and leaves indicated a differential transcriptional regulation by KClO 3, with OsNIA2 prevailing over OsNIA1 in the roots. However, in the stem and leaves, OsNR2 was upregulated in the NR introgression lines, but downregulation in the NRT introgression lines. When expressed in the roots, OsNR2 was downregulated in all introgression lines. In addition, the expression of OsNR2 was differentially regulated between the roots, stem, and leaf tissues, and between introgression lines. The results revealed that Saeilmi (P1, japonica) and Milyang23 (P2, indica) showed distinctive phenotypic responses. The phenotypic responses were recorded 7 days after treatment, and samples for gene expression, physiological, and biochemical analyses were collected at 0 h (control) and 3 h after KClO 3 application. A set of two KClO 3 sensitive nitrate reductase (NR) and two nitrate transporter (NRT) introgression rice lines (BC2F7), carrying the indica alleles of NR or NRT, derived from a cross between Saeilmi ( japonica, P1) and Milyang23 ( indica, P2), were exposed to KClO 3 at the seedling stage. This study investigated the transcriptional regulation of major genes involved in the NUE in rice treated with KClO 3, which acts as an inhibitor of the reducing activity of nitrate reductase (NR) in higher plants. Potassium chlorate (KClO 3) has been widely used to evaluate the divergence in nitrogen use efficiency (NUE) between indica and japonica rice subspecies.
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