Differential heat-response characteristics of two plastid isoforms of triose phosphate isomerase in tomato

Heat stress causes dysfunction of the carbon-assimilation metabolism. As a member of Calvin-Benson-Bassham (CBB) cycle, the chloroplast triose phosphate isomerases (TPI) catalyse the interconversion of glyceraldehyde 3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP). The tomato (Solanum lycopersicum) genome contains two individual SlTPI genes, Solyc10g054870 and Solyc01g111120, which encode the chloroplast-located proteins SlTPI1 and SlTPI2, respectively. The tpi1 and tpi2 single mutants had no visible phenotypes, but the leaves of their double mutant lines tpi1tpi2 had obviously reduced TPI activity and displayed chlorotic variegation, dysplasic chloroplasts and lower carbon-assimilation efficiency. In addition to altering carbon metabolism, proteomic data showed that the loss of both SlTPI1 and SlTPI2 severely affected photosystem proteins, reducing photosynthetic capacity. None of these phenotypes was evident in the tpi1 or tpi2 single mutants, suggesting that SlTPI1 and SlTPI2 are functionally redundant. However, the two proteins differed in their responses to heat stress; the protein encoded by the heat-induced SlTPI2 showed a higher level of thermotolerance than that encoded by the heat-suppressed SlTPI1. Notably, heat-induced transcription factors, SlWRKY21 and SlHSFA2/7, which negatively regulated SlTPI1 expression and positively regulated SlTPI2 expression, respectively. Our findings thus reveal that SlTPI1 and SlTPI2 have different thermostabilities and expression patterns in response to heat stress, which have the potential to be applied in thermotolerance strategies in crops.

SlMYB41 positively regulates tomato thermotolerance by activating the expression of SlHSP90.3

High-temperature stress has become a major abiotic factor that dramatically limits plant growth and crop yield. Plants have evolved complex mechanisms to cope with high-temperature stress, but the factors that regulate plant thermotolerance remain to be discovered. Here, a high temperature-induced MYB transcription factor SlMYB41 was cloned from tomato (Solanum lycopersicum). Two individual SlMYB41-RNA interference (RNAi) lines (MR) and one CRISPR/Cas9 mediated myb41 mutant (MC) were obtained to investigate the function of SlMYB41 in tomato thermotolerance. Under high-temperature stress, we found that the MR and MC lines showed more wilting than the wild type (WT), with more ion leakage, more MDA accumulation, lower contents of osmotic adjustment substances, and more accumulation of reactive oxygen species (ROS) which was resulted from lower antioxidative enzyme activities. In addition, the photosynthetic capacity and complex of MR and MC lines were damaged more seriously than WT plants under high-temperature stress, mainly manifested in lower photosynthetic rate and Fv/Fm. Moreover, heat stress-related genes, such as SlHSP17.6, SlHSP17.7, and SlHSP90.3 were downregulated in MR and MC lines. Importantly, Y1H and LUC analysis indicated that SlMYB41 can directly activate the transcription of SlHSP90.3. Together, our study suggest that SlMYB41 positively regulates tomato thermotolerance by activating the expression of SlHSP90.3.

ETHYLENE-INSENSITIVE 3-LIKE 2 regulates β-carotene and ascorbic acid accumulation in tomatoes during ripening

ETHYLENE-INSENSITIVE 3/ETHYLENE-INSENSITIVE 3-LIKEs (EIN3/EILs) are important ethylene response factors during fruit ripening. Here, we discovered that EIL2 controls carotenoid metabolism and ascorbic acid (AsA) biosynthesis in tomato (Solanum lycopersicum). In contrast to the red fruits presented in the wild type (WT) 45 d after pollination, the fruits of CRISPR/Cas9 eil2 mutants and SlEIL2 RNA interference lines (ERIs) showed yellow or orange fruits. Correlation analysis of transcriptome and metabolome data for the ERI and WT ripe fruits revealed that SlEIL2 is involved in β-carotene and AsA accumulation. ETHYLENE RESPONSE FACTORs (ERFs) are the typical components downstream of EIN3 in the ethylene response pathway. Through a comprehensive screening of ERF family members, we determined that SlEIL2 directly regulates the expression of 4 SlERFs. Two of these, SlERF.H30 and SlERF.G6, encode proteins that participate in the regulation of LYCOPENE-β-CYCLASE 2 (SlLCYB2), encoding an enzyme that mediates the conversion of lycopene to carotene in fruits. In addition, SlEIL2 transcriptionally repressed L-GALACTOSE 1-PHOSPHATE PHOSPHATASE 3 (SlGPP3) and MYO-INOSITOL OXYGENASE 1 (SlMIOX1) expression, which resulted in a 1.62-fold increase of AsA via both the L-galactose and myoinositol pathways. Overall, we demonstrated that SlEIL2 functions in controlling β-carotene and AsA levels, providing a potential strategy for genetic engineering to improve the nutritional value and quality of tomato fruit.

Chloroplast metalloproteinase SlL2 reduces the thermotolerance of tomato by decreasing the content of SlCDJ1

Chloroplast is one of the most sensitive organelles to heat stress in plants. In chloroplasts, various proteases affect photosynthesis by degrading proteins under stress conditions. Tomato Lutescent2 (SlL2), a chloroplast zinc metalloprotease, was previously reported to alter chloroplast development and delay fruit ripening. However, its enzyme activity and roles in plant response to abiotic stress are still unclear. Here, we confirmed that the SlL2 protein which localized on thylakoid membrane was an ATP-independent hydrolase, and SlL2 gene responded to heat stress. Phenotype analysis showed that SlL2 plays a negative role in the heat-response mechanism. Under heat stress, the transgenic plants overexpressing SlL2 (OE) grew worse than the wild type (WT), as reflected by their decreased membrane stability, osmotic-regulating substance, and antioxidative enzyme activities, as well as increased reactive oxygen species (ROS) accumulation. By contrast, l2 mutant line showed the opposite phenotype and corresponding physiological indices under heat stress. In addition, overexpression of SlL2 decreased the photosynthetic activities, especially photosystem II. Moreover, SlL2 was found to interact with chloroplast-located chaperone protein SlCDJ1, decreasing its content under heat stress. These results indicate that SlL2 reduces the thermotolerance of tomato by reducing the content of SlCDJ1.

Salicylic acid regulates two photosystem II protection pathways in tomato under chilling stress mediated by ETHYLENE INSENSITIVE 3-like proteins

Chilling stress seriously impairs photosynthesis and activates a series of molecular responses in plants. Previous studies have shown that ETHYLENE INSENSITIVE 3 (EIN3) and EIN3-like (SlEIL) proteins mediate ethylene signaling and reduce plant tolerance to freezing in tomato (Solanum lycopersicum). However, the specific molecular mechanisms underlying an EIN3/EILs-mediated photoprotection pathway under chilling stress are unclear. Here, we discovered that salicylic acid (SA) participates in photosystem II (PSII) protection via SlEIL2 and SlEIL7. Under chilling stress, the phenylalanine ammonia-lyase gene SlPAL5 plays an important role in the production of SA, which also induces WHIRLY1 (SlWHY1) transcription. The resulting accumulation of SlWHY1 activates SlEIL7 expression under chilling stress. SlEIL7 then binds to and blocks the repression domain of the heat shock factor SlHSFB-2B, releasing its inhibition of HEAT SHOCK PROTEIN 21 (HSP21) expression to maintain PSII stability. In addition, SlWHY1 indirectly represses SlEIL2 expression, allowing the expression of l-GALACTOSE-1-PHOSPHATE PHOSPHATASE3 (SlGPP3). The ensuing higher SlGPP3 abundance promotes the accumulation of ascorbic acid (AsA), which scavenges reactive oxygen species produced upon chilling stress and thus protects PSII. Our study demonstrates that SlEIL2 and SlEIL7 protect PSII under chilling stress via two different SA response mechanisms: one involving the antioxidant AsA and the other involving the photoprotective chaperone protein HSP21.

SbbHLH85, a bHLH member, modulates resilience to salt stress by regulating root hair growth in sorghum

bHLH family proteins play an important role in plant stress response. However, the molecular mechanism regulating the salt response of bHLH is largely unknown. This study aimed to investigate the function and regulating mechanism of the sweet sorghum SbbHLH85 during salt stress. The results showed that SbbHLH85 was different from its homologs in other species. Also, it was a new atypical bHLH transcription factor and a key gene for root development in sweet sorghum. The overexpression of SbbHLH85 resulted in significantly increased number and length of root hairs via ABA and auxin signaling pathways, increasing the absorption of Na⁺. Thus, SbbHLH85 plays a negative regulatory role in the salt tolerance of sorghum. We identified a potential interaction partner of SbbHLH85, which was phosphate transporter chaperone PHF1 and modulated the distribution of phosphate, through screening a yeast two-hybrid library. Both yeast two-hybrid and BiFC experiments confirmed the interaction between SbbHLH85 and PHF1. The overexpression of SbbHLH85 led to a decrease in the expression of PHF1 as well as the content of Pi. Based on these results, we suggested that the increase in the Na⁺ content and the decrease in the Pi content resulted in the salt sensitivity of transgenic sorghum.

SlWHY2 interacts with SlRECA2 to maintain mitochondrial function under drought stress in tomato

Drought stress in plants leads to inhibition of photosynthesis and respiration, accumulation of reactive oxygen species (ROS), and reprogramming of gene expression. Here, we established that the tomato (Solanum lycopersicum) WHIRLY2 (SlWHY2) gene, which encodes a mitochondrial single-stranded DNA-binding protein, was significantly induced by drought stress. Under drought conditions, SlWHY2 RNAi plants showed more wilting and lower fresh weight, chlorophyll content, quantum yield of photosystem I (PSI; YI), and maximal photochemical efficiency of PSII (Fv/Fm) than the wild type (WT). Drought treatment also caused the SlWHY2 RNAi lines to accumulate more ROS than the WT, and the silenced lines had lower AOX (alternative oxidase) activity. As expected, the mitochondrial membrane potential (MMP) was less stable in the SlWHY2 RNAi lines. The expression levels of seven genes in the mitochondrial genome (SYCF15, NAD7, NAD4, COS2, COX1, COX2, and COX3) were decreased even more in the SlWHY2 RNAi lines than they were in the WT under drought stress. SlWHY2 interacted directly in vivo and in vitro with SlRECA2, a mitochondrial recombinase A that is important for mitochondrial DNA recombination and repair. These results suggest that SlWHY2 plays an essential role in maintaining mitochondrial function and enhancing drought tolerance in tomato.

WHIRLY1 maintains leaf photosynthetic capacity in tomato by regulating the expression of RbcS1 under chilling stress

Rubisco, which consists of eight large subunits (RBCLs) and eight small subunits (RBCSs), is a major photosynthetic enzyme that is sensitive to chilling stress. However, it is largely unclear how plants maintain high Rubisco content under low temperature conditions. Here, we report that tomato WHIRLY1 (SlWHY1) positively regulates the Rubisco level under chilling stress by directly binding to the promoter region of SlRbcS1, resulting in the activation of SlRbcS1 expression. SlRbcS1-overexpressing lines had higher Rubisco contents and were more resistant to chilling stress compared with the wild type. Quantitative real-time PCR analyses showed that, among the five RbcS genes, only SlRbcS1 expression is up-regulated by chilling treatment. These results indicate that SlWHIRLY1 specifically enhances the levels of SlRbcS1 and confers tolerance to chilling stress. The amino acid sequence of SlRBCS1 shows 92.67% identity with those of another two RBCS proteins and three residues are specifically found in SlRBCS1. However, mutation of these residues to alanine in SlRBCS1 does not influence its function during cold adaptation. Thus, we conclude that high levels of Rubisco, but not the specific residues in SlRBCS1, play important roles in tolerance to chilling stress in tomato.

WHIRLY1 Regulates HSP21.5A Expression to Promote Thermotolerance in Tomato

Heat stress poses a major threat to plant productivity and crop yields. The induction of heat shock proteins (HSPs) by heat shock factors is a principal defense response of plants exposed to heat stress. In this study, we identified and analyzed the heat stress-induced Whirly1 (SlWHY1) gene in tomato (Solanum lycopersicum). We generated various SlWHY1-overexpressing (OE) and SlWHY1-RNA interference (RNAi) lines to investigate the role of WHIRLY1 in thermotolerance. Compared with the wild type (WT), the OE lines showed less wilting, as reflected by their increased membrane stability and soluble sugar content and reduced reactive oxygen species (ROS) accumulation under heat stress. By contrast, RNAi lines with inhibited SlWHY1 expression showed the opposite phenotype and corresponding physiological indices under heat stress. The heat-induced gene SlHSP21.5A, encoding an endoplasmic reticulum-localized HSP, was upregulated in the OE lines and downregulated in the RNAi lines compared with the WT. RNAi-mediated inhibition of SlHSP21.5A expression also resulted in reduced membrane stability and soluble sugar content and increased ROS accumulation under heat stress compared with the WT. SlWHY1 binds to the elicitor response element-like element in the promoter of SlHSP21.5A to activate its transcription. These findings suggest that SlWHY1 promotes thermotolerance in tomato by regulating SlHSP21.5A expression.

Whirly1 enhances tolerance to chilling stress in tomato via protection of photosystem II and regulation of starch degradation

In plants, the chilling response involves decreased photosynthetic capacity and increased starch accumulation in chloroplasts. However, the mechanisms that modulate these processes remain unclear. We found that the SlWHY1 gene is significantly induced by chilling stress (4°C) in tomato. Three SlWHY1 overexpression (OE) lines grew better than the wild type (WT) under chilling stress; the OE plants retained intact photosynthetic grana lamellae and showed enhanced hydrolysis of starch. By contrast, RNAi lines that inhibited SlWHY1 were more affected than the corresponding WT cultivar. Their grana lamellae were damaged and starch content increased. The psbA gene encodes the key photosystem II (PSII) protein D1. We show that SlWHY1 binds to the upstream region (A/GTTACCCT/A) of SlpsbA and enhances the de novo synthesis of D1 in chloroplasts. Additionally, SlWHY1 regulates the expression of the starch-degrading enzyme α-amylase (SlAMY3-L) and the starch synthesis-related enzyme isoamylase gene (SlISA2) in the nucleus, thus modulating the starch content in chloroplasts. We demonstrate that SlWHY1 enhances the resistance of tomato to chilling stress by maintaining the function of PSII and degrading starch. Thus, overexpression of WHY1 may be an effective strategy for enhancing resistance to chilling stress of chilling-sensitive crops in agricultural production.

Overexpression of a tomato carotenoid ε-hydroxylase gene (SlLUT1) improved the drought tolerance of transgenic tobacco

Drought stress is a considerable environmental factor that restrains photosynthesis. Lutein, the most prolific carotenoid in plant photosynthetic tissues, plays vital roles in the light-harvesting complexes. However, its biological functions under abiotic stresses remain unclear. In our research, transgenic tobacco plants were utilized to investigate the function of the tomato chloroplast-targeted carotenoid epsilon-ring hydroxylase (SlLUT1) in drought stress tolerance. The analysis of SlLUT1-pro-LUC and qRT-PCR showed that drought stress induced SlLUT1 expression. Transgenic tobacco plants exhibit higher lutein content than wild-type (WT) tobacco. Under drought stress, transgenic plants overexpressing SlLUT1 showed better growth performance, higher chlorophyll and relative water contents and more intact chloroplast and PSII supercomplex structures than WT tobacco. The Fv/Fm, Pn, NPQ, and content of D1 protein in transgenic plants were higher than those in WT plants under drought stress. The accumulation of H2O2 and O2- decreased in transgenic tobacco plants. Moreover, transgenic plants exhibited lower MDA accumulation and REL. These results indicate that overexpression of SlLUT1 enhances tolerance to drought stress by maintaining photosynthesis and scavenging ROS in transgenic tobacco.

A novel tomato SUMO E3 ligase, SlSIZ1, confers drought tolerance in transgenic tobacco

SUMOylation is an important post-translational modification process that regulates different cellular functions in eukaryotes. SIZ/PIAS-type SAP and Miz1 (SIZ1) proteins exhibit SUMO E3 ligase activity, which modulates SUMOylation. However, SIZ1 in tomato has been rarely investigated. In this study, a tomato SIZ1 gene (SlSIZ1) was isolated and its molecular characteristics and role in tolerance to drought stress are described. SlSIZ1 was up-regulated by cold, sodium chloride (NaCl), polyethylene glycol (PEG), hydrogen peroxide (H2 O2 ) and abscisic acid (ABA), and the corresponding proteins were localized in the nucleus. The expression of SlSIZ1 in Arabidopsis thaliana (Arabidopsis) siz1-2 mutants partially complemented the phenotypes of dwarf, cold sensitivity and ABA hypersensitivity. SlSIZ1 also exhibited the activity of SUMO E3 ligase to promote the accumulation of SUMO conjugates. Under drought stress, the ectopic expression of SlSIZ1 in transgenic tobacco lines enhanced seed germination and reduced the accumulation of reactive oxygen species. SlSIZ1 overexpression conferred the plants with improved growth, high free proline content, minimal malondialdehyde accumulation and increased accumulation of SUMO conjugates. SlSIZ1 is a functional homolog of Arabidopsis SIZ1 with SUMO E3 ligase activity. Therefore, overexpression of SlSIZ1 enhanced the tolerance of transgenic tobacco to drought stress.

The design of long-term effective uranium bioremediation strategy using a community metabolic model

Acetate amendment at uranium contaminated sites in Rifle, CO. leads to an initial bloom of Geobacter accompanied by the removal of U(VI) from the groundwater, followed by an increase of sulfate-reducing bacteria (SRBs) which are poor reducers of U(VI). One of the challenges associated with bioremediation is the decay in Geobacter abundance, which has been attributed to the depletion of bio-accessible Fe(III), motivating the investigation of simultaneous amendments of acetate and Fe(III) as an alternative bioremediation strategy. In order to understand the community metabolism of Geobacter and SRBs during artificial substrate amendment, we have created a genome-scale dynamic community model of Geobacter and SRBs using the previously described Dynamic Multi-species Metabolic Modeling framework. Optimization techniques are used to determine the optimal acetate and Fe(III) addition profile. Field-scale simulation of acetate addition accurately predicted the in situ data. The simulations suggest that batch amendment of Fe(III) along with continuous acetate addition is insufficient to promote long-term bioremediation, while continuous amendment of Fe(III) along with continuous acetate addition is sufficient to promote long-term bioremediation. By computationally minimizing the acetate and Fe(III) addition rates as well as the difference between the predicted and target uranium concentration, we showed that it is possible to maintain the uranium concentration below the environmental safety standard while minimizing the cost of chemical additions. These simulations show that simultaneous addition of acetate and Fe(III) has the potential to be an effective uranium bioremediation strategy. They also show that computational modeling of microbial community is an important tool to design effective strategies for practical applications in environmental biotechnology.

Negative regulation of the rat Na-K-ATPase alpha 3-subunit gene promoter by thyroid hormone

Na-K-ATPase alpha 3-subunit mRNA levels are both positively and negatively controlled by thyroid hormone [3,5,3'triiodothyronine (T3)] in primary cultures of neonatal rat cardiac myocytes. In this study, transient transfection analysis indicated that two regions of the rat alpha 3 gene between nucleotides -116 and -6 and -6 and +80 conferred T3-mediated inhibition of reporter gene expression. Electrophoretic mobility shift assays showed specific binding of T3 receptor monomers and T3 receptor-retinoid X receptor heterodimers at each alpha 3 gene negative T3-response region. The alpha 3 gene region from -116 to -6 base pairs also mediates repression in response to retinoic acid (RA) and binds RA receptor. In the absence of ligand, reporter gene expression driven by the -116 to -6-base pair region is repressed with cotransfection of T3 receptor, whereas it is unaffected by overexpression of RA receptor. These data demonstrate that the proximal promoter of the rat Na-K-ATPase alpha 3 gene contains sequence motifs that mediate repression of alpha 3 gene transcription in response to either T3 or RA in neonatal rat cardiac myocytes.