The OsEIL1-OsWOX11 transcription factor module controls rice crown root development in response to soil compaction.

Optimizing the root architecture of crops is an effective strategy for improving crop yields. Soil compaction is a serious global problem that limits crop productivity by restricting root growth, but the underlying molecular mechanisms are largely unclear. Here, we show that ethylene stimulates rice (Oryza sativa) crown root development in response to soil compaction. First, we demonstrate that compacted soil promotes ethylene production and the accumulation of ETHYLENE INSENSITIVE 3-LIKE 1 (OsEIL1) in rice roots, stimulating crown root primordia initiation and development, thereby increasing crown root number in lower stem nodes. Through transcriptome profiling and molecular analyses, we reveal that OsEIL1 directly activates the expression of WUSCHEL-RELATED HOMEOBOX 11 (OsWOX11), an activator of crown root emergence and growth, and that OsWOX11 mutations delay crown root development, thus impairing the plant's response to ethylene and soil compaction. Genetic analysis demonstrates that OsWOX11 functions downstream of OsEIL1. In summary, our results demonstrate that the OsEIL1-OsWOX11 module regulates ethylene action during crown root development in response to soil compaction, providing a strategy for the genetic modification of crop root architecture and grain agronomic traits.

Orchestration of ethylene and gibberellin signals determines primary root elongation in rice.

Primary root growth in cereal crops is fundamental for early establishment of the seedling and grain yield. In young rice (Oryza sativa) seedlings, the primary root grows rapidly for 7-10 days after germination and then stops; however, the underlying mechanism determining primary root growth is unclear. Here, we report that the interplay of ethylene and gibberellin (GA) controls the orchestrated development of the primary root in young rice seedlings. Our analyses advance the knowledge that primary root growth is maintained by higher ethylene production, which lowers bioactive GA contents. Further investigations unraveled that ethylene signaling transcription factor ETHYLENE INSENSITIVE3-LIKE 1 (OsEIL1) activates the expression of the GA metabolism genes GIBBERELLIN 2-OXIDASE 1 (OsGA2ox1), OsGA2ox2, OsGA2ox3, and OsGA2ox5, thereby deactivating GA activity, inhibiting cell proliferation in the root meristem, and ultimately gradually inhibiting primary root growth. Mutation in OsGA2ox3 weakened ethylene-induced GA inactivation and reduced the ethylene sensitivity of the root. Genetic analysis revealed that OsGA2ox3 functions downstream of OsEIL1. Taken together, we identify a molecular pathway impacted by ethylene during primary root elongation in rice and provide insight into the coordination of ethylene and GA signals during root development and seedling establishment.

A natural variation in OsDSK2a modulates plant growth and salt tolerance through phosphorylation by SnRK1A in rice.

Plants grow rapidly for maximal production under optimal conditions; however, they adopt a slower growth strategy to maintain survival when facing environmental stresses. As salt stress restricts crop architecture and grain yield, identifying genetic variations associated with growth and yield responses to salinity is critical for breeding optimal crop varieties. OsDSK2a is a pivotal modulator of plant growth and salt tolerance via the modulation of gibberellic acid (GA) metabolism; however, its regulation remains unclear. Here, we showed that OsDSK2a can be phosphorylated at the second amino acid (S2) to maintain its stability. The gene-edited mutant osdsk2aS2G showed decreased plant height and enhanced salt tolerance. SnRK1A modulated OsDSK2a-S2 phosphorylation and played a substantial role in GA metabolism. Genetic analysis indicated that SnRK1A functions upstream of OsDSK2a and affects plant growth and salt tolerance. Moreover, SnRK1A activity was suppressed under salt stress, resulting in decreased phosphorylation and abundance of OsDSK2a. Thus, SnRK1A preserves the stability of OsDSK2a to maintain plant growth under normal conditions, and reduces the abundance of OsDSK2a to limit growth under salt stress. Haplotype analysis using 3 K-RG data identified a natural variation in OsDSK2a-S2. The allele of OsDSK2a-G downregulates plant height and improves salt-inhibited grain yield. Thus, our findings revealed a new mechanism for OsDSK2a stability and provided a valuable target for crop breeding to overcome yield limitations under salinity stress.

Abscisic acid promotes auxin biosynthesis to inhibit primary root elongation in rice.

Soil compaction is a global problem causing inadequate rooting and poor yield in crops. Accumulating evidence indicates that phytohormones coordinately regulate root growth via regulating specific growth processes in distinct tissues. However, how abscisic acid (ABA) signaling translates into auxin production to control root growth during adaptation to different soil environments is still unclear. In this study, we report that ABA has biphasic effects on primary root growth in rice (Oryza sativa) through an auxin biosynthesis-mediated process, causing suppression of root elongation and promotion of root swelling in response to soil compaction. We found that ABA treatment induced the expression of auxin biosynthesis genes and auxin accumulation in roots. Conversely, blocking auxin biosynthesis reduced ABA sensitivity in roots, showing longer and thinner primary roots with larger root meristem size and smaller root diameter. Further investigation revealed that the transcription factor basic region and leucine zipper 46 (OsbZIP46), involved in ABA signaling, can directly bind to the YUCCA8/rice ethylene-insensitive 7 (OsYUC8/REIN7) promoter to activate its expression, and genetic analysis revealed that OsYUC8/REIN7 is located downstream of OsbZIP46. Moreover, roots of mutants defective in ABA or auxin biosynthesis displayed the enhanced ability to penetrate compacted soil. Thus, our results disclose the mechanism in which ABA employs auxin as a downstream signal to modify root elongation and radial expansion, resulting in short and swollen roots impaired in their ability to penetrate compacted soil. These findings provide avenues for breeders to select crops resilient to soil compaction.

SALT AND ABA RESPONSE ERF1 improves seed germination and salt tolerance by repressing ABA signaling in rice.

Rice (Oryza sativa) germination and seedling establishment, particularly in increasingly saline soils, are critical to ensure successful crop yields. Seed vigor, which determines germination and seedling growth, is a complex trait affected by exogenous (environmental) and endogenous (hormonal) factors. Here, we used genetic and biochemical analyses to uncover the role of an APETALA2-type transcription factor, SALT AND ABA RESPONSE ERF1 (OsSAE1), as a positive regulator of seed germination and salt tolerance in rice by repressing the expression of ABSCISIC ACID-INSENSITIVE5 (OsABI5). ossae1 knockout lines exhibited delayed seed germination, enhanced sensitivity to abscisic acid (ABA) during germination and in early seedling growth, and reduced seedling salt tolerance. OsSAE1 overexpression lines exhibited the converse phenotype, with increased seed germination and salt tolerance. In vivo and in vitro assays indicated that OsSAE1 binds directly to the promoter of OsABI5, a major downstream component of the ABA signaling pathway and acts as a major regulator of seed germination and stress response. Genetic analyses revealed that OsABI5-mediated ABA signaling functions downstream of OsSAE1. This study provides important insights into OsSAE1 regulation of seed vigor and salt tolerance and facilitates the practical use of OsSAE1 in breeding salt-tolerant varieties suitable for direct seeding cultivation.

Cellulose synthase-like protein OsCSLD4 plays an important role in the response of rice to salt stress by mediating abscisic acid biosynthesis to regulate osmotic stress tolerance.

Cell wall polysaccharide biosynthesis enzymes play important roles in plant growth, development and stress responses. The functions of cell wall polysaccharide synthesis enzymes in plant growth and development have been well studied. In contrast, their roles in plant responses to environmental stress are poorly understood. Previous studies have demonstrated that the rice cell wall cellulose synthase-like D4 protein (OsCSLD4) is involved in cell wall polysaccharide synthesis and is important for rice growth and development. This study demonstrated that the OsCSLD4 function-disrupted mutant nd1 was sensitive to salt stress, but insensitive to abscisic acid (ABA). The expression of some ABA synthesis and response genes was repressed in nd1 under both normal and salt stress conditions. Exogenous ABA can restore nd1-impaired salt stress tolerance. Moreover, overexpression of OsCSLD4 can enhance rice ABA synthesis gene expression, increase ABA content and improve rice salt tolerance, thus implying that OsCSLD4-regulated rice salt stress tolerance is mediated by ABA synthesis. Additionally, nd1 decreased rice tolerance to osmotic stress, but not ion toxic tolerance. The results from the transcriptome analysis showed that more osmotic stress-responsive genes were impaired in nd1 than salt stress-responsive genes, thus indicating that OsCSLD4 is involved in rice salt stress response through an ABA-induced osmotic response pathway. Intriguingly, the disruption of OsCSLD4 function decreased grain width and weight, while overexpression of OsCSLD4 increased grain width and weight. Taken together, this study demonstrates a novel plant salt stress adaptation mechanism by which crops can coordinate salt stress tolerance and yield.

Salt Stress Promotes Abscisic Acid Accumulation to Affect Cell Proliferation and Expansion of Primary Roots in Rice.

The primary root is the basic component of the root system and plays a key role in early seedling growth in rice. Its growth is easily affected by environmental cues, such as salt stress. Abscisic acid (ABA) plays an essential role in root development, but the molecular mechanism underlying ABA-regulated root growth in response to salt stress remains poorly understood. In this study, we report that salt stress inhibits primary root elongation and promotes primary root swelling. Moreover, salt stress induces the expression of ABA-responsive genes and ABA accumulation in the primary root, revealing that ABA plays an essential role in salt-modulated root growth. Transgenic lines of OsSAPK10-OE and OsABIL2-OE, which constitutively express OsSAPK10 or OsABIL2, with enhanced or attenuated ABA signaling, show increased and decreased sensitivity to salt, correspondingly. Microscopic analysis indicates that salt and ABA inhibits cell proliferation and promotes cell expansion in the root apical meristem. Transcriptome analysis showed that ABA induces the expression of EXPANSIN genes. Further investigations indicate that ABA exerts these effects largely through ABA signaling. Thus, our findings deepen our understanding of the role of ABA in controlling primary root growth in response to salt stress, and this knowledge can be used by breeders to cultivate rice varieties suitable for saline-alkali land.

The Synthesis of Ascorbic Acid in Rice Roots Plays an Important Role in the Salt Tolerance of Rice by Scavenging ROS.

The root plays an important role in the responses of plants to stresses, but the detailed mechanisms of roots in stress responses are still obscure. The GDP-mannose pyrophosphate synthetase (GMPase) OsVTC1-3 is a key factor of ascorbic acid (AsA) synthesis in rice roots. The present study showed that the transcript of OsVTC1-3 was induced by salt stress in roots, but not in leaves. Inhibiting the expression of OsVTC1-3 by RNA interfering (RI) technology significantly impaired the tolerance of rice to salt stress. The roots of OsVTC1-3 RI plants rapidly produced more O2-, and later accumulated amounts of H2O2 under salt stress, indicating the impaired tolerance of OsVTC1-3 RI plants to salt stress due to the decreasing ability of scavenging reactive oxygen species (ROS). Moreover, exogenous AsA restored the salt tolerance of OsVTC1-3 RI plants, indicating that the AsA synthesis in rice roots is an important factor for the response of rice to salt stress. Further studies showed that the salt-induced AsA synthesis was limited in the roots of OsVTC1-3 RI plants. The above results showed that specifically regulating AsA synthesis to scavenge ROS in rice roots was one of important factors in enhancing the tolerance of rice to salt stress.

OsERF2 controls rice root growth and hormone responses through tuning expression of key genes involved in hormone signaling and sucrose metabolism.

Root determines plant distribution, development progresses, stress response, as well as crop qualities and yields, which is under the tight control of genetic programs and environmental stimuli. Ethylene responsive factor proteins (ERFs) play important roles in plant growth and development. Here, the regulatory function of OsERF2 involved in root growth was investigated using the gain-function mutant of OsERF2 (nsf2857) and the artificial microRNA-mediated silenced lines of OsERF2 (Ami-OsERF2). nsf2857 showed short primary roots compared with the wild type (WT), while the primary roots of Ami-OsERF2 lines were longer than those of WT. Consistent with this phenotype, several auxin/cytokinin responsive genes involved in root growth were downregulated in nsf2857, but upregulated in Ami-OsERF2. Then, we found that nsf2857 seedlings exhibited decreased ABA accumulation and sensitivity to ABA and reduced ethylene-mediated root inhibition, while those were the opposite in Ami-ERF2 plants. Moreover, several key genes involved in ABA synthesis were downregulated in nsf2857, but unregulated in Ami-ERF2 lines. In addition, OsERF2 affected the accumulation of sucrose and UDPG by mediating expression of key genes involved in sucrose metabolism. These results indicate that OsERF2 is required for the control of root architecture and ABA- and ethylene-response by tuning expression of series genes involved in sugar metabolism and hormone signaling pathways.

Arabidopsis CSN5B interacts with VTC1 and modulates ascorbic acid synthesis.

Light regulates ascorbic acid (AsA) synthesis, which increases in the light, presumably reflecting a need for antioxidants to detoxify reactive molecules produced during photosynthesis. Here, we examine this regulation in Arabidopsis thaliana and find that alterations in the protein levels of the AsA biosynthetic enzyme GDP-Man pyrophosphorylase (VTC1) are associated with changes in AsA contents in light and darkness. To find regulatory factors involved in AsA synthesis, we identified VTC1-interacting proteins by yeast two-hybrid screening of a cDNA library from etiolated seedlings. This screen identified the photomorphogenic factor COP9 signalosome subunit 5B (CSN5B), which interacted with the N terminus of VTC1 in yeast and plants. Gel filtration profiling showed that VTC1-CSN5B also associated with the COP9 signalosome complex, and this interaction promotes ubiquitination-dependent VTC1 degradation through the 26S proteasome pathway. Consistent with this, csn5b mutants showed very high AsA levels in both light and darkness. Also, a double mutant of csn5b with the partial loss-of-function mutant vtc1-1 contained AsA levels between those of vtc1-1 and csn5b, showing that CSN5B modulates AsA synthesis by affecting VTC1. In addition, the csn5b mutant showed higher tolerance to salt, indicating that CSN5B regulation of AsA synthesis affects the response to salt stress. Together, our data reveal a regulatory role of CSN5B in light-dark regulation of AsA synthesis.

Ethylene promotes hypocotyl growth and HY5 degradation by enhancing the movement of COP1 to the nucleus in the light.

In the dark, etiolated seedlings display a long hypocotyl, the growth of which is rapidly inhibited when the seedlings are exposed to light. In contrast, the phytohormone ethylene prevents hypocotyl elongation in the dark but enhances its growth in the light. However, the mechanism by which light and ethylene signalling oppositely affect this process at the protein level is unclear. Here, we report that ethylene enhances the movement of constitutive photomorphogenesis 1 (COP1) to the nucleus where it mediates the degradation of long hypocotyl 5 (HY5), contributing to hypocotyl growth in the light. Our results indicate that HY5 is required for ethylene-promoted hypocotyl growth in the light, but not in the dark. Using genetic and biochemical analyses, we found that HY5 functions downstream of ethylene insensitive 3 (EIN3) for ethylene-promoted hypocotyl growth. Furthermore, the upstream regulation of HY5 stability by ethylene is COP1-dependent, and COP1 is genetically located downstream of EIN3, indicating that the COP1-HY5 complex integrates light and ethylene signalling downstream of EIN3. Importantly, the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) enriched the nuclear localisation of COP1; however, this effect was dependent on EIN3 only in the presence of light, strongly suggesting that ethylene promotes the effects of light on the movement of COP1 from the cytoplasm to the nucleus. Thus, our investigation demonstrates that the COP1-HY5 complex is a novel integrator that plays an essential role in ethylene-promoted hypocotyl growth in the light.

OsEIL1 and OsEIL2, two master regulators of rice ethylene signaling, promote the expression of ROS scavenging genes to facilitate coleoptile elongation and seedling emergence from soil.

Successful emergence from the soil is a prerequisite for survival of germinating seeds in their natural environment. In rice, coleoptile elongation facilitates seedling emergence and establishment, and ethylene plays an important role in this process. However, the underlying regulatory mechanism remains largely unclear. Here, we report that ethylene promotes cell elongation and inhibits cell expansion in rice coleoptiles, resulting in longer and thinner coleoptiles that facilitate seedlings emergence from the soil. Transcriptome analysis showed that genes related to reactive oxygen species (ROS) generation are upregulated and genes involved in ROS scavenging are downregulated in the coleoptiles of ethylene-signaling mutants. Further investigations showed that soil coverage promotes accumulation of ETHYLENE INSENSITIVE 3-LIKE 1 (OsEIL1) and OsEIL2 in the upper region of the coleoptile, and both OsEIL1 and OsEIL2 can bind directly to the promoters of the GDP-mannose pyrophosphorylase (VTC1) gene OsVTC1-3 and the peroxidase (PRX) genes OsPRX37, OsPRX81, OsPRX82, and OsPRX88 to activate their expression. This leads to increased ascorbic acid content, greater peroxidase activity, and decreased ROS accumulation in the upper region of the coleoptile. Disruption of ROS accumulation promotes coleoptile growth and seedling emergence from soil. These findings deepen our understanding of the roles of ethylene and ROS in controlling coleoptile growth, and this information can be used by breeders to produce rice varieties suitable for direct seeding.

EAR motif mutation of rice OsERF3 alters the regulation of ethylene biosynthesis and drought tolerance.

OsERF3 is a transcriptional repressor with an ethylene-responsive element-binding factor-associated amphiphilic repression (EAR) motif (F/LDLNxxP), which transcriptionally represses the ethylene emission and drought tolerance in rice. However, its molecular mechanism to explore repression function remains unknown. Here, we first revealed that the expression of OsERF3 was induced by drought, salt, ACC and ABA treatment. In addition, it showed a higher expression level in the root and sheath than that in the leaf. Then, we generated transgenic rice overexpressing full-length OsERF3 (OE) and its mutation of EAR motif with the A 680/C substitution (mEAR), respectively. The physiological analyses showed that mEAR lines showed better drought tolerance and more ethylene emission compared with those of OE lines and wild type plants. Consistent with our previous research, the expression of ethylene synthesis genes, including ACO2, ACS2, and ACS6 was down-regulated in OE lines. However, the repression of OsERF3 was eliminated in mEAR lines. Specifically, ACS2 was up-regulated in mEAR lines compared with that in OE lines and WT plants, suggesting that the Leu/Ala substitution within the EAR motif resulted in loss of repression of OsERF3. Thus, our data reveal that the EAR motif is required for OsERF3 to transcriptionally regulate the ethylene synthesis and drought tolerance in rice, providing new insight to the roles of ethylene-response factor proteins in regulating ethylene biosynthesis and stress response.

The ethylene response factor AtERF11 that is transcriptionally modulated by the bZIP transcription factor HY5 is a crucial repressor for ethylene biosynthesis in Arabidopsis.

The phytohormones abscisic acid (ABA) and ethylene are known to play multiple roles in plant development and stress responses. Ethylene biosynthesis is affected by several factors including drought, cold and the phytohormone auxin, although the role of ABA is unclear. In this work ABA-responsive mutants were screened and a bZIP transcription factor HY5 was identified as a negative regulator of ethylene biosynthesis via modulation of the expression of the ethylene biosynthesis genes ACS2 and ACS5. Members of the ethylene response factor (ERF) family of transcriptional repressors in Arabidopsis have been shown to modulate ABA responses and three ERF members were found to carry putative HY5-binding cis-acting elements. Analyses with biochemical and molecular approaches revealed that HY5 specifically binds to the G-box region of the AtERF11 promoter to activate its transcription. We further demonstrate that AtERF11, which contains a repressor motif at its C-terminal, interacts with the dehydration-responsive element in the ACS2/5 promoters, to repress its expression, resulting in decreased ethylene biosynthesis. Moreover, an AtERF11 knockout mutant showed increased levels of ACS2/5 expression and ethylene emission, while treatment with ABA greatly suppressed ACS5 transcripts but not ACS2 expression and the ethylene content, indicating that AtERF11 is a key negative regulator for ABA-mediated control of ethylene synthesis. In addition, in ethylene over-producer mutants, ABA treatment was shown to suppress ACS5 transcripts and ethylene content, thereby affecting growth and development. Based on these data, in this research we present a model suggesting that the HY5-AtERF11 regulon is a key factor modulating ABA-regulated ethylene biosynthesis.

An ethylene response factor OsWR1 responsive to drought stress transcriptionally activates wax synthesis related genes and increases wax production in rice.

Increasing evidence has revealed the major enzymes-involved in Arabidopsis and maize wax/cutin synthesis; however, there is limited information about the genes-associated with wax/cutin synthesis in rice. Here we report the characterization of an ethylene response factor gene in rice. This rice wax synthesis regulatory gene 1 (OsWR1) is a homolog of Arabidopsis wax/cutin synthesis regulatory gene WIN1/SHN1. Transcript analysis showed that OsWR1 is induced by drought, abscisic acid and salt, and is predominantly expressed in leaves. Functional analyses indicated that overexpressing OsWR1 (Ox-WR1) improved while RNA interference OsWR1 rice (RI-WR1) decreased drought tolerance, consistent with water loss and cuticular permeability, suggesting that OsWR1-triggered drought response might be associated with cuticular characteristics. In addition, OsWR1 activated the expression of the genes-related to oxidative stress response and membrane stability. Gas chromatograph-mass spectrometry analysis further showed that OsWR1 modulated the wax synthesis through alteration of long chain fatty acids and alkanes, evidencing the regulation of OsWR1 in wax synthesis. Detection with real-time PCR amplification indicated that Ox-WR1 enhanced while RI-WR1 decreased the expression of wax/cutin synthesis related genes. Furthermore, OsWR1 physically interacted with the DRE and GCC box in the promoters of wax related genes OsLACS2 and OsFAE1'-L, indicating that OsWR1 at least directly modulates the expression of these genes. Thus our results indicate that OsWR1 is a positive regulator of wax synthesis related genes in rice, and this regulation, distinct from its homology regulator of WIN1/SHN1 in cutin synthesis, subsequently contributes to reduced water loss and enhanced drought tolerance.

An AP2 domain-containing gene, ESE1, targeted by the ethylene signaling component EIN3 is important for the salt response in Arabidopsis.

Accumulating investigations reveal that ethylene signaling is involved in the salt response in Arabidopsis (Arabidopsis thaliana), and it has been reported that overexpression of a number of ethylene response factor (ERF) genes enhances salt tolerance; however, transcriptional regulation of the ethylene signal component ETHYLENE INSENSITIVE3 (EIN3) in the salt response has not been clearly defined. Consulting microarray data and transcriptional confirmations showed that three of the ERF genes were ethylene and salt inducible, named ESE1 to ESE3. Additionally, the expression of one of the ESE genes (ESE1) was suppressed in ein2, ein3-1, eil1-3, and ein3 eil1 but enhanced in EIN3-overexpressing (EIN3ox) lines. Inhibitors of ethylene biosynthesis, aminoethoxyvinylglycine, and ethylene action, AgNO3, reduced the expression of ESE1, while ethylene overproduction eto mutants enhanced the expression of ESE1, indicating that ESE1 is an ethylene-modulated gene downstream of EIN3/EIL1. Further analyses with biochemical and molecular approaches revealed that EIN3 physically binds to the ESE1 promoter, demonstrating that ESE1 was one target of EIN3. ESE1 in turn binds to promoters of salt-related genes, such as RD29A and COR15A. Moreover, either EIN3ox or ESE1ox was sufficient to enhance transcript levels of salt-related genes and salt tolerance. In addition, ESE1ox in ein3 enhanced the salt response during seed germination and seedling development, demonstrating that ESE1 is genetically downstream of EIN3. Thus, the evidence in this report reveals that the transcriptional complex of EIN3-ESE1 is a crucial event in the salt response, thereby connecting the transcriptional regulation of EIN3 and the downstream ERF protein ESE1 in the salt response.

OsQHB Improves Salt Tolerance by Scavenging Reactive Oxygen Species in Rice.

Soil salinity is a major environmental stress that restricts the growth and yield of crops. Mining the key genes involved in the balance of rice salt tolerance and yield will be extremely important for us to cultivate salt-tolerance rice varieties. In this study, we report a WUSCHEL-related homeobox (WOX) gene, quiescent-center-specific homeobox (OsQHB), positively regulates yield-related traits and negatively regulates salt tolerance in rice. Mutation in OsQHB led to a decrease in plant height, tiller number, panicle length, grain length and grain width, and an increase in salt tolerance. Transcriptome and qPCR analysis showed that reactive oxygen species (ROS) scavenging-related genes were regulated by OsQHB. Moreover, the osqhb mutants have higher ROS-scavenging enzymes activities and lower accumulation of ROS and malondialdehyde (MDA) under salt stress. Thus, our findings provide new insights into the role of rice WOX gene family in rice development and salt tolerance, and suggest that OsQHB is a valuable target for improving rice production in environments characterized by salt stress.

Overexpression of an ERF transcription factor TSRF1 improves rice drought tolerance.

One of the major limitations in rice production is a shortage of water. Conventional breeding as well as emerging genetic engineering methods may be used to improve plant stress tolerance. Some transcription factors regulating stress responsive genes have become important target genes for improving plant drought tolerance. Previously, we have shown that a tomato ethylene response factor (ERF) protein TSRF1 that binds to GCC box in the promoters of pathogenesis-related genes positively regulates pathogen resistance in tomato and tobacco, but negatively regulates osmotic response in tobacco. Here, we further report the ability of TSRF1 to regulate osmotic and drought responses in monocot rice. TSRF1 improves the osmotic and drought tolerance of rice seedlings without growth retardation, as determined by physiological analyses of root and leaf growth, leaf water loss and survival rate under stress. In addition, the amounts of proline and soluble sugars in transgenic rice lines increase by 30%-60% compared to those in wild-type plants. Moreover, TSRF1 activates the expression of a putative rice abscisic acid (ABA) synthesis gene SDR, resulting in enhanced ABA sensitivity in transgenic rice. TSRF1 also increases the expression of MYB, MYC and proline synthesis and photosynthesis-related genes, probably by binding to dehydration responsive element and GCC boxes in promoters of the target genes. These results demonstrate that TSRF1 enhances the osmotic and drought tolerance of rice by modulating the increase in stress responsive gene expression.

Mutation in Mg-Protoporphyrin IX Monomethyl Ester Cyclase Decreases Photosynthesis Capacity in Rice.

In photosynthesis, the pigments chlorophyll a/b absorb light energy to convert to chemical energy in chloroplasts. Though most enzymes of chlorophyll biosynthesis from glutamyl-tRNA to chlorophyll a/b have been identified, the exact composition and regulation of the multimeric enzyme Mg-protoporphyrin IX monomethyl ester cyclase (MPEC) is largely unknown. In this study, we isolated a rice pale-green leaf mutant m167 with yellow-green leaf phenotype across the whole lifespan. Chlorophyll content decreases 43-51% and the granal stacks of chloroplasts becomes thinner in m167. Chlorophyll fluorescence parameters, including Fv/Fm (the maximum quantum efficiency of PSII) and quantum yield of PSII (Y(II)), were lower in m167 than those in wild type plants (WT), and photosynthesis rate decreases 40% in leaves of m167 mutant compared with WT plants, which lead to yield reduction in m167. Genetic analysis revealed that yellow-green leaf phenotype of m167 is controlled by a single recessive genetic locus. By positional cloning, a single mutated locus, G286A (Alanine 96 to Threonine in protein), was found in the coding sequence of LOC_Os01g17170 (Rice Copper Response Defect 1, OsCRD1), encoding a putative subunit of MPEC. Expression profile analysis demonstrated that OsCRD1 is mainly expressed in green tissues of rice. Sequence alignment analysis of CRD1 indicated that Alanine 96 is very conserved in all green plants and photosynthetic bacteria. OsCRD1 protein mainly locates in chloroplast and the point mutation A96T in OsCRD1 does not change its location. Therefore, Alanine96 of OsCRD1 might be fundamental for MPEC activity, mutation of which leads to deficiency in chlorophyll biosynthesis and chloroplast development and decreases photosynthetic capacity in rice.

EIN3 and SOS2 synergistically modulate plant salt tolerance.

Ethylene biosynthesis and the ethylene signaling pathway regulate plant salt tolerance by activating the expression of downstream target genes such as those related to ROS and Na+/K+ homeostasis. The Salt Overly Sensitive (SOS) pathway regulates Na+/K+ homeostasis in Arabidopsis under salt stress. However, the connection between these two pathways is unclear. Through genetic screening, we identified two sos2 alleles as salt sensitive mutants in the ein3-1 background. Neither Ethylene-Insensitive 2 (EIN2) nor EIN3 changed the expression patterns of SOS genes including SOS1, SOS2, SOS3 and SOS3-like Calcium Binding Protein 8 (SCaBP8), but SOS2 activated the expression of one target gene of EIN3, Ethylene and Salt-inducible ERF 1 (ESE1). Moreover, Ser/Thr protein kinase SOS2 phosphorylated EIN3 in vitro mainly at the S325 site and weakly at the S35, T42 and S606 sites. EIN3 S325A mutation reduced its transcriptional activating activity on ESE1 promoter:GUS in a transient GUS assay, and impaired its ability to rescue ein3-1 salt hypersensitivity. Furthermore, SOS2 activated salt-responsive ESE1 target gene expression under salt stress. Therefore, EIN3-SOS2 might link the ethylene signaling pathway and the SOS pathway in Arabidopsis salt responses.