Characterization of nitrate use efficiency in tea plant (<i>Camellia sinensis</i>) based on leaf chlorate sensitivity

Nitrate (NO₃ ^-), a key form of inorganic nitrogen (N) in soils, is typically lost in tea gardens through leaching. However, NO₃ ^- utilization efficiency (NiUE) and its characteristic mechanism in tea plants remain unclear. This study screened contrastive genotypes of NiUE using leaf chlorate sensitivity and explored the potential genes that regulate this process. Fresh branches of 10 cultivars were hydroponically cultivated and subjected to potassium nitrate (KNO₃) and potassium chlorate (KClO₃) treatments, with the former as the control group. The sensitive cultivar, Zhenong 117 (ZN117), showed a decrease in SPAD and F_v/F_m values following KClO₃ treatment, while the tolerant cultivar, Teiguanyin (TGY), exhibited minimal significant changes. After 5 days of cultivation, the ¹⁵N concentration and proportion in new shoots of ZN117 were significantly higher than those in TGY. Transcriptome analysis revealed that the expression of genes responsible for NO₃ ^- transport, including the nitrate transporters <i>NRT2.4</i>, <i>NPF4.6</i>, <i>NPF6.1</i>, <i>NPF1.10</i>, and <i>NPF1.11</i>, significantly increased in ZN117 after NO₃ ^- supply. Genes involved in NO₃ ^- reduction, chlorophyll synthesis, and photosynthesis were progressively induced. Coexpression network analysis indicated that the squamosa promoter-binding protein activated the onset of NO₃ ^- signaling, while basic helix-loop-helix transcripts were triggered to higher levels during NO₃ ^- supply. This study proposes a rapid characterization method of NiUE in woody plants and a speculative molecular regulatory mechanism for the NO₃ ^- transfer and remobilization of tea plants. A set of specific genes involved in NO₃ ^- transport, reduction, and mobilization were identified and proposed as marker genes for NiUE in tea plants.