Introduction: The nitrate stress challenge in tomato cultivation
Tomato (Solanum lycopersicum) is a globally important crop, prized for its nutritional compounds and economic value in protected agriculture. However, excessive calcium nitrate in saline greenhouse soils imposes nitrate stress, reducing germination, growth, photosynthesis, and overall yield. Understanding the genes that confer nitrate tolerance is crucial for sustainable production.
Recent research has turned attention to the molybdate transporter family, which includes MOT1 and MOT2. These transporters are central to molybdenum cofactor biosynthesis, a key cofactor for enzymes involved in nitrogen and sulfur metabolism. In tomato, SlMOT2 emerged as a nitrate-stress-responsive candidate gene, identified through transcriptome screening and subsequent functional analysis.
Key findings: SlMOT2 expression responds to nitrate stress
When tomato seedlings were exposed to calcium nitrate, SlMOT2 expression initially rose and then declined, peaking at day 5 with a 2.56-fold increase over control conditions. This dynamic expression pattern suggests SlMOT2 is actively engaged in the nitrate stress response, potentially shaping the plant’s adaptive program during early to mid-stress phases.
Functional impact: SlMOT2 overexpression improves growth under stress
Transgenic lines overexpressing SlMOT2 (OE2 and OEA) and knockout mutants (T4 and TA) were subjected to calcium nitrate stress alongside wild-type plants. Under nitrate stress, OE2 and OEA plants exhibited superior growth metrics—plant height, fresh weight, and root length—compared with WT, while T4 and TA showed reduced growth. The data indicate that SlMOT2 positively regulates growth under nitrate stress by supporting root and shoot development when nitrate is in excess.
Photosynthesis and stress signaling: preserving function under nitrate stress
Calcium nitrate stress significantly dampened photosynthetic gas exchange (Pn, Gs, Ci, Tr) and chlorophyll fluorescence (Fv’/Fm’, Fv/Fm, Y(II), qP), yet SlMOT2-overexpressing plants fared better than WT, while knockouts suffered more pronounced declines. This suggests SlMOT2 helps stabilize photosynthetic machinery, enabling continued energy production and carbon assimilation during nitrate perturbation.
Oxidative stress and osmoprotection: SlMOT2 enhances antioxidant defenses
Under nitrate stress, antioxidant enzymes SOD, POD, and CAT rose markedly in OE2 and OEA, while declining in T4 and TA. Correspondingly, malondialdehyde (MDA) levels decreased in overexpressors, indicating reduced lipid peroxidation. Histochemical stains confirmed restrained ROS accumulation in OE lines. Proline levels rose more in OE lines, suggesting improved osmotic adjustment and ROS scavenging capacity, contributing to stress resilience.
Sulfur metabolism and hormonal crosstalk: a broader protective network
SlMOT2 overexpression also increased total sulfur and hydrogen sulfide (H2S) under nitrate stress, hinting at enhanced sulfur assimilation supporting antioxidant capacity and signaling. GO and KEGG analyses revealed enrichment of pathways tied to amino acid metabolism, phenylpropanoid biosynthesis, and hormone signaling, notably ABA and brassinosteroids. RT-qPCR validated key pathway genes, reinforcing that SlMOT2 orchestrates a multifaceted defense that combines metabolic reprogramming with stress signaling.
Putting it together: SlMOT2 as a regulatory hub for nitrate tolerance
The study presents a coherent model: SlMOT2 overexpression shifts metabolic and signaling networks toward improved photosynthetic stability, stronger ROS-scavenging systems, and enhanced osmolyte production, all converging to mitigate nitrate stress in tomato seedlings. The observed ABA-related upregulation and sulfur metabolism upshift imply a coordinated hormonal and metabolic response that sustains growth while defending cellular integrity.
Implications for breeding and cultivation
These findings position SlMOT2 as a promising target for breeding tomato varieties with enhanced nitrate stress tolerance, especially in greenhouse systems affected by soil salinity and high-nitrate inputs. By stacking SlMOT2-driven pathways with other stress-tolerance traits, breeders could improve yield stability and fruit quality under challenging nutrient regimes.
Conclusion
Transcriptomic analysis demonstrates that SlMOT2 modulates nitrate stress tolerance in tomato by promoting growth, preserving photosynthesis, boosting antioxidant defenses, and engaging sulfur metabolism and hormone signaling. This integrated response highlights the potential of manipulating molybdate transporters to fortify crops against abiotic stressors linked to intensive fertilization and saline soils.
