SORTING NEXIN 1 Functions in Plant Salt Stress Tolerance Through Changes of NO Accumulation by Regulating NO Synthase-Like Activity

cGMP functions as an important messenger involved in SlSAMS1-regulated salt stress tolerance in tomato

Salt stress adversely affects the growth, development, and yield of tomato (Solanum lycopersicum). SAM Synthetase (SAMS), which is responsible for the biosynthesis of S-adenosylmethionine (SAM, a precursor of polyamine biosynthesis), participates in plant response to abiotic stress. However, the regulatory mechanism of SAMS-mediated salt stress tolerance remains elusive. In this study, we characterized a SAMS homologue SlSAMS1 in tomato. We found that SlSAMS1 is highly expressed in tomato roots, and its expression can be induced by salt stress. Crucially, overexpression of SlSAMS1 in tomato enhances salt stress tolerance. Through metabolomic profiling, we identified some differentially accumulated metabolites, especially, a secondary messenger guanosine 3',5'-cyclic monophosphate (cGMP) which may play a key role in SlSAMS1-regulated salt tolerance. A series of physiological and biochemical data suggest that cGMP alleviates salt stress-induced growth inhibition, and potentially acts downstream of the polyamine-nitric oxide (PA-NO) signaling pathway to trigger H2O2 signaling in response to salt stress. Taken together, the study reveals that SlSAMS1 regulates tomato salt tolerance via the PA-NO-cGMP-H2O2 signal module. Our findings elucidate the regulatory pathway of SlSAMS1-induced plant response to salt stress and indicate a pivotal role of cGMP in salt tolerance.

ELO2 Participates in the Regulation of Osmotic Stress Response by Modulating Nitric Oxide Accumulation in Arabidopsis

The ELO family is involved in synthesizing very-long-chain fatty acids (VLCFAs) and VLCFAs play a crucial role in plant development, protein transport, and disease resistance, but the physiological function of the plant ELO family is largely unknown. Further, while nitric oxide synthase (NOS)-like activity acts in various plant environmental responses by modulating nitric oxide (NO) accumulation, how the NOS-like activity is regulated in such different stress responses remains misty. Here, we report that the yeast mutant Δelo3 is defective in H₂O₂-triggered cell apoptosis with decreased NOS-like activity and NO accumulation, while its Arabidopsis homologous gene ELO2 (ELO HOMOLOG 2) could complement such defects in Δelo3. The expression of this gene is enhanced and required in plant osmotic stress response because the T-DNA insertion mutant elo2 is more sensitive to the stress than wild-type plants, and ELO2 expression could rescue the sensitivity phenotype of elo2. In addition, osmotic stress-promoted NOS-like activity and NO accumulation are significantly repressed in elo2, while exogenous application of NO donors can rescue this sensitivity of elo2 in terms of germination rate, fresh weight, chlorophyll content, and ion leakage. Furthermore, stress-responsive gene expression, proline accumulation, and catalase activity are also repressed in elo2 compared with the wild type under osmotic stress. In conclusion, our study identifies ELO2 as a pivotal factor involved in plant osmotic stress response and reveals its role in regulating NOS-like activity and NO accumulation.

Sorting Nexin1 negatively modulates phosphate uptake by facilitating Phosphate Transporter1;1 degradation in Arabidopsis

High-affinity phosphate (Pi) transporters (PHTs) PHT1;1 and PHT1;4 are necessary for plant root Pi uptake especially under Pi-deficient conditions, but how their protein stability is modulated remains elusive. Here, we identified a Ttransfer DNA insertion mutant of Sorting Nexin1 (SNX1), which had more Pi content and less anthocyanin accumulation than the wild type under deficient Pi. By contrast, the snx1-2 mutant displayed higher sensitivity to exogenous arsenate in terms of seed germination and root elongation, revealing higher Pi uptake rates. Further study showed that SNX1 could co-localize and interact with PHT1;1 and PHT1;4 in vesicles and at the plasma membrane. Genetic analysis showed that increased Pi content in the snx1-2 mutant under low Pi conditions could be extensively compromised by mutating PHT1;1 in the double mutant snx1-2 pht1;1, revealing that SNX1 is epistatic to PHT1;1. In addition, SNX1 negatively controls PHT1;1 protein stability; therefore, PHT1;1 protein abundance in the plasma membrane was increased in the snx1-2 mutant compared with the wild type under either sufficient or deficient Pi. Together, our study (i) identifies SNX1 as a key modulator of the plant response to low Pi and (ii) unravels its role in the modulation of PHT1;1 protein stability, PHT1;1 accumulation at the plasma membrane, and root Pi uptake.

SORTING NEXIN2 proteins mediate stomatal movement and the response to drought stress by modulating trafficking and protein levels of the ABA exporter ABCG25

The phytohormone abscisic acid (ABA) regulates ion channel activity and stomatal movement in response to drought stress. Cellular ABA levels change depending on cellular and environmental conditions via modulation of its biosynthesis, catabolism and transport. Although factors involved in ABA biosynthesis and degradation have been studied extensively, how ABA transporters are modulated to fine-tune ABA levels, especially under drought stress, remains elusive. Here, we show that Arabidopsis thaliana SORTING NEXIN 2 (SNX2) proteins play a critical role in endosomal trafficking of the ABA exporter ATP BINDING CASETTE G25 (ABCG25) via direct interaction at endosomes, leading to its degradation in the vacuole. In agreement, snx2a and snx2b mutant plants showed enhanced recycling of GFP-ABCG25 from early endosomes to the plasma membrane and higher accumulation of GFP-ABCG25. Phenotypically, snx2a and snx2b plants were highly sensitive to exogenous ABA and displayed enhanced ABA-mediated inhibition of inward K⁺ currents and ABA-mediated activation of slow anion currents in guard cells, resulting in an increased tolerance to drought stress. Based on these results, we propose that SNX2 proteins play a crucial role in stomatal movement and tolerance to drought stress by modulating the endosomal trafficking of ABCG25 and thus cellular ABA levels.

CAND2/PMTR1 Is Required for Melatonin-Conferred Osmotic Stress Tolerance in Arabidopsis

Osmotic stress severely inhibits plant growth and development, causing huge loss of crop quality and quantity worldwide. Melatonin is an important signaling molecule that generally confers plant increased tolerance to various environmental stresses, however, whether and how melatonin participates in plant osmotic stress response remain elusive. Here, we report that melatonin enhances plant osmotic stress tolerance through increasing ROS-scavenging ability, and melatonin receptor CAND2 plays a key role in melatonin-mediated plant response to osmotic stress. Upon osmotic stress treatment, the expression of melatonin biosynthetic genes including SNAT1, COMT1, and ASMT1 and the accumulation of melatonin are increased in the wild-type plants. The snat1 mutant is defective in osmotic stress-induced melatonin accumulation and thus sensitive to osmotic stress, while exogenous melatonin enhances the tolerance of the wild-type plant and rescues the sensitivity of the snat1 mutant to osmotic stress by upregulating the expression and activity of catalase and superoxide dismutase to repress H₂O₂ accumulation. Further study showed that the melatonin receptor mutant cand2 exhibits reduced osmotic stress tolerance with increased ROS accumulation, but exogenous melatonin cannot revert its osmotic stress phenotype. Together, our study reveals that CADN2 functions necessarily in melatonin-conferred osmotic stress tolerance by activating ROS-scavenging ability in Arabidopsis.

Phospholipase D-derived phosphatidic acid promotes root hair development under phosphorus deficiency by suppressing vacuolar degradation of PIN-FORMED2

Root hair development is crucial for phosphate absorption, but how phosphorus deficiency affects root hair initiation and elongation remains unclear. We demonstrated the roles of auxin efflux carrier PIN-FORMED2 (PIN2) and phospholipase D (PLD)-derived phosphatidic acid (PA), a key signaling molecule, in promoting root hair development in Arabidopsis thaliana under a low phosphate (LP) condition. Root hair elongation under LP conditions was greatly suppressed in pin2 mutant or under treatment with a PLDζ2-specific inhibitor, revealing that PIN2 and polar auxin transport and PLDζ2-PA are crucial in LP responses. PIN2 was accumulated and degraded in the vacuole under a normal phosphate (NP) condition, whereas its vacuolar accumulation was suppressed under the LP or NP plus PA conditions. Vacuolar accumulation of PIN2 was increased in pldζ2 mutants under LP conditions. Increased or decreased PIN2 vacuolar accumulation is not observed in sorting nexin1 (snx1) mutant, indicating that vacuolar accumulation of PIN2 is mediated by SNX1 and the relevant trafficking process. PA binds to SNX1 and promotes its accumulation at the plasma membrane, especially under LP conditions, and hence promotes root hair development by suppressing the vacuolar degradation of PIN2. We uncovered a link between PLD-derived PA and SNX1-dependent vacuolar degradation of PIN2 in regulating root hair development under phosphorus deficiency.