Introduction: Tackling nitrate stress in tomato cultivation
Tomato (Solanum lycopersicum) is a globally important crop, valued for its nutritional compounds and economic significance in protected agriculture. In greenhouse systems, secondary soil salinization and excessive nitrate exposure pose challenges to yield and quality. High nitrate stress, often accompanied by Ca2+ in salinized soils, disrupts growth, photosynthesis, and redox balance. Identifying key regulatory genes that confer nitrate stress tolerance is essential for sustaining tomato production under adverse soil conditions.
The focus on molybdate transporters and SlMOT2
The sulfate transporters and molybdate transporters form a family crucial for transmembrane movement of molybdate and related anions. Molybdate is a precursor for the molybdenum cofactor (Moco), a cofactor for enzymes involved in nitrate assimilation and redox biology. In tomato, SlMOT2 emerged from transcriptome screening as a potential regulator of nitrate stress tolerance, prompting a comprehensive analysis of its role in mediating responses to calcium nitrate stress.
Expression dynamics of SlMOT2 under nitrate stress
Under calcium nitrate treatment, SlMOT2 transcripts in tomato seedlings rose initially and then declined, with a pronounced peak around day 5 where expression in stressed plants was 2.56-fold higher than controls. This temporal pattern suggests SlMOT2 is part of the early-to-mid-stage nitrate stress response, potentially coordinating molybdate-dependent processes during stress adaptation.
Physiological and growth outcomes of SlMOT2 modulation
To assess function, WT plants were compared with SlMOT2-overexpressing lines (OE2, OEA) and knockout mutants (T4, TA) under calcium nitrate stress. Across growth metrics, SlMOT2 overexpression alleviated nitrate-induced growth inhibition, with taller plants, greater fresh weight, and longer roots relative to WT. Conversely, SlMOT2 mutants showed more severe growth suppression, highlighting the protective role of SlMOT2 in nitrate stress tolerance.
Photosynthesis and chlorophyll fluorescence under nitrate stress
Calcium nitrate stress reduced photosynthetic rates (Pn) and related gas-exchange parameters in all genotypes, but OE lines maintained higher Pn, stomatal conductance, and intercellular CO2, while mutants exhibited sharper declines. Chlorophyll fluorescence metrics indicated that SlMOT2 overexpression mitigated PSII impairment (smaller reductions in Fv’/Fm’, Fv/Fm, and Y(II)) and restrained excessive non-photochemical quenching (NPQ). This points to SlMOT2’s role in stabilizing photosynthetic machinery under nitrate stress.
Oxidative stress and osmotic balance
Nitrate stress elevates reactive oxygen species (ROS). In SlMOT2-overexpressing plants, antioxidant enzyme activities (SOD, POD, CAT) increased under stress, while MDA, a lipid peroxidation marker, decreased compared with WT. Mutants showed the opposite trend, with higher ROS indicators. Additionally, proline content rose in OE lines, supporting improved osmotic adjustment and ROS scavenging under calcium nitrate stress.
Molybdate, sulfur metabolism, and hormone signaling
SlMOT2 overexpression led to higher total sulfur and hydrogen sulfide (H2S) levels under stress, with a concurrent enrichment of sulfur metabolism pathways. This suggests SlMOT2 may bolster antioxidant capacity and ABA-related stress signaling through enhanced sulfur assimilation. Transcriptomic data corroborated these findings, with upregulation of pathways linked to phenylpropanoid biosynthesis, amino sugar and nucleotide sugar metabolism, and plant hormone signal transduction in SlMOT2-overexpressing lines under nitrate stress.
GO and KEGG insights into SlMOT2-regulated networks
Gene ontology analyses highlighted enrichment for responses to oxygen-containing compounds, stress, ABA, and chloroplast-related processes in overexpression lines, while WT plants under nitrate stress favored chloroplast and photosynthetic membrane components. KEGG analyses revealed upregulation of MAPK signaling and phenylpropanoid biosynthesis in SlMOT2-overexpressing lines during stress, suggesting coordinated modulation of primary and secondary metabolism to enhance resilience. Downregulated pathways in overexpression lines under stress included photosynthesis-related processes, consistent with a resource reallocation toward defense and redox balance.
Validation and hormonal context
RT-qPCR validated key findings, aligning with transcriptomic trends. Notably, ABA-related genes and ABA content increased in SlMOT2-overexpressing plants under nitrate stress, aligning with the role of ABA in stress adaptation. The data imply that SlMOT2 may promote ABA-mediated signaling and sulfur metabolism to mitigate nitrate-induced damage.
Implications for tomato breeding and cultivation
The SlMOT2 gene represents a promising target for breeding tomato cultivars with enhanced tolerance to nitrate-rich, saline soils common in protected agriculture. By supporting molybdate transport, sulfur metabolism, antioxidant defense, and stress signaling, SlMOT2 overexpression may help maintain photosynthetic efficiency and growth under calcium nitrate stress, contributing to yield stability and fruit quality.
Conclusions
Transcriptomic analyses demonstrate that SlMOT2 synergizes molybdate transport with broader metabolic and signaling networks to improve nitrate stress tolerance in tomato seedlings. Through upregulation of defense pathways and maintenance of redox homeostasis, SlMOT2-overexpressing plants better preserve growth and photosynthesis under challenging nitrate conditions, offering a molecular entry point for crop improvement strategies.
