Hao Du

Characterization of the β-carotene hydroxylase gene DSM2 conferring drought and oxidative stress resistance by increasing xanthophylls and abscisic acid synthesis in rice.

Drought is a major limiting factor for crop production. To identify critical genes for drought resistance in rice (Oryza sativa), we screened T-DNA mutants and identified a drought-hypersensitive mutant, dsm2. The mutant phenotype was caused by a T-DNA insertion in a gene encoding a putative β-carotene hydroxylase (BCH). BCH is predicted for the biosynthesis of zeaxanthin, a carotenoid precursor of abscisic acid (ABA). The amounts of zeaxanthin and ABA were significantly reduced in two allelic dsm2 mutants after drought stress compared with the wild type. Under drought stress conditions, the mutant leaves lost water faster than the wild type and the photosynthesis rate, biomass, and grain yield were significantly reduced, whereas malondialdehyde level and stomata aperture were increased in the mutant. The mutant is also hypersensitive to oxidative stresses. The mutant had significantly lower maximal efficiency of photosystem II photochemistry and nonphotochemical quenching capacity than the wild type, indicating photoinhibition in photosystem II and decreased capacity for eliminating excess energy by thermal dissipation. Overexpression of DSM2 in rice resulted in significantly increased resistance to drought and oxidative stresses and increases of the xanthophylls and nonphotochemical quenching. Some stress-related ABA-responsive genes were up-regulated in the overexpression line. DSM2 is a chloroplast protein, and the response of DSM2 to environmental stimuli is distinctive from the other two BCH members in rice. We conclude that the DSM2 gene significantly contributes to control of the xanthophyll cycle and ABA synthesis, both of which play critical roles in the establishment of drought resistance in rice.

Identification and expression profiling analysis of TIFY family genes involved in stress and phytohormone responses in rice

The TIFY family is a novel plant-specific gene family involved in the regulation of diverse plant-specific biologic processes, such as development and responses to phytohormones, in Arabidopsis. However, there is limited information about this family in monocot species. This report identifies 20 TIFY genes in rice, the model monocot species. Sequence analysis indicated that rice TIFY proteins have conserved motifs beyond the TIFY domain as was previously shown in Arabidopsis. On the basis of their protein structures, members of the TIFY family can be divided into two groups. Transcript level analysis of OsTIFY genes in tissues and organs revealed different tempo-spatial expression patterns, suggesting that expression and function vary by stage of plant growth and development. Most of the OsTIFY genes were predominantly expressed in leaf. Nine OsTIFY genes were responsive to jasmonic acid and wounding treatments. Interestingly, almost all the OsTIFY genes were responsive to one or more abiotic stresses including drought, salinity, and low temperature. Over-expression of OsTIFY11a, one of the stress-inducible genes, resulted in significantly increased tolerance to salt and dehydration stresses. These results suggest that the OsTIFY family may have important roles in response to abiotic stresses. The data presented in this report provide important clues for further elucidating the functions of the genes in the OsTIFY family.

Endogenous auxin and jasmonic acid levels are differentially modulated by abiotic stresses in rice

Abiotic stresses such as drought, salinity, and adverse temperatures are major limiting factors for plant growth and reproduction. Plant responses to these stresses are coordinated by arrays of regulatory networks including the induction of endogenous abscisic acid (ABA), a well documented phytohormone for stress responses. However, whether or how these abiotic stresses affect the endogenous biosynthesis or metabolism of other phytohormones remains largely unknown. Here, we report the changes of endogenous indole-3-acetic acid (IAA) and jasmonic acid (JA) levels and expression of genes related to the biosynthesis or signaling of these hormones in rice under various abiotic stress conditions. The IAA content was decreased after drought stress, but it was significantly increased under cold and heat stresses. And the auxin-regulated gravitropism of root tip was inhibited by cold stress. Many genes involved in the IAA biosynthesis and signaling were changed in transcript level under these stresses, and the changes were essentially in agreement with the change of endogenous IAA level. Interestingly, the endogenous JA content was increased markedly under drought and cold stresses, but it was reduced by heat stress. Accordingly, many genes involved in JA biosynthesis and signaling were induced by drought and cold treatment but these genes were significantly suppressed by heat stress. We concluded that endogenous levels of IAA and JA were differentially regulated by abiotic stresses in rice, implying diverse roles of these hormones in stress responses.

A GH3 family member, OsGH3-2, modulates auxin and abscisic acid levels and differentially affects drought and cold tolerance in rice

Plant responses to abiotic stresses are coordinated by arrays of growth and developmental processes. Indole-3-acetic acid (IAA) and abscisic acid (ABA) play critical roles in developmental programmes and environmental responses, respectively, through complex signalling and metabolism networks. However, crosstalk between the two phytohormones in the stress responses remains largely unknown. Here, it is reported that a GH3 family gene, OsGH3-2, encoding an enzyme catalysing IAA conjugation to amino acids, is involved in the modulation of ABA level and stress tolerance. Expression of OsGH3-2 was induced by drought but was suppressed by cold. Overexpression of OsGH3-2 in rice caused significant morphological aberrations related to IAA deficiency, such as dwarfism, smaller leaves, and fewer crown roots and root hairs. The overexpressing line showed significantly reduced carotene, ABA, and free IAA levels, greater stomata aperture, and faster water loss, and was hypersensitive to drought stress. However, the overexpressing line showed increased cold tolerance, which was due to the combined effects of reduced free IAA content, alleviated oxidative damage, and decreased membrane penetrability. Furthermore, expression levels of some ABA synthesis- and stress-related genes were significantly changed in the overexpression line. It was conclude that OsGH3-2 modulates both endogenous free IAA and ABA homeostasis and differentially affects drought and cold tolerance in rice.

A stress-responsive NAC transcription factor SNAC3 confers heat and drought tolerance through modulation of reactive oxygen species in rice

Highlight A novel NAC transcription factor regulates tolerance of rice to multiple abiotic stresses through directly targeting the genes related to reactive oxygen species homeostasis.

Grain Number, Plant Height, and Heading Date7 Is a Central Regulator of Growth, Development, and Stress Response

A protein associated with light signaling regulates a range of functions in growth and development in response to environmental cues to maximize the reproductive success of the rice plant. Grain number, plant height, and heading date7 (Ghd7) has been regarded as an important regulator of heading date and yield potential in rice (Oryza sativa). In this study, we investigated functions of Ghd7 in rice growth, development, and environmental response. As a long-day dependent negative regulator of heading date, the degree of phenotypic effect of Ghd7 on heading date and yield traits is quantitatively related to the transcript level and is also influenced by both environmental conditions and genetic backgrounds. Ghd7 regulates yield traits through modulating panicle branching independent of heading date. Ghd7 also regulates plasticity of tiller branching by mediating the PHYTOCHROME B-TEOSINTE BRANCHED1 pathway. Drought, abscisic acid, jasmonic acid, and high-temperature stress strongly repressed Ghd7 expression, whereas low temperature enhanced Ghd7 expression. Overexpression of Ghd7 increased drought sensitivity, whereas knock-down of Ghd7 enhanced drought tolerance. Gene chip analysis of expression profiles revealed that Ghd7 was involved in the regulation of multiple processes, including flowering time, hormone metabolism, and biotic and abiotic stresses. This study suggests that Ghd7 functions to integrate the dynamic environmental inputs with phase transition, architecture regulation, and stress response to maximize the reproductive success of the rice plant.

Carotenoid deficiency impairs ABA and IAA biosynthesis and differentially affects drought and cold tolerance in rice

Plant responses to abiotic stresses are coordinated by arrays of growth and developmental programs. Phytohormones such as abscisic acid (ABA) and indole-3-acetic acid (IAA) play critical roles in developmental progresses and environmental responses through complex signalling networks. However, crosstalk between the two hormones at the biosynthesis level remains largely unknown. Here, we report that carotenoid-deficient mutants (phs1, phs2, phs3-1, phs4, and PDS-RNAi transgenic rice) were impaired in the biosynthesis of ABA and IAA. Under drought conditions, phs3-1 and PDS-RNAi transgenic rice showed larger stomata aperture and earlier wilting compared to the wild type at both seedling and panicle developmental stage. Interestingly, these carotenoid-deficient lines showed increased cold resistance, which was likely due to the combined effects of reduced IAA content, alleviated oxidative damage and decreased membrane penetrability. Furthermore, we found that IAA content was significantly declined in rice treated with fluridone (a carotenoid and ABA biosynthesis inhibitor), and expression of auxin synthesis and metabolism-related genes were altered in the fluridone-treated rice similar to that in the carotenoid-deficient mutants. In addition, exogenous IAA, but not ABA, could restore the dwarf phenotype of phs3-1 and PDS-RNAi transgenic rice. These results support a crosstalk between ABA and IAA at the biosynthesis level, and this crosstalk is involved in development and differentially affects drought and cold tolerance in rice.

A homolog of ETHYLENE OVERPRODUCER, OsETOL1, differentially modulates drought and submergence tolerance in rice

Submergence and drought are major limiting factors for crop production. However, very limited studies have been reported on the distinct or overlapping mechanisms of plants in response to the two water extremes. Here we report an ETHYLENE OVERPRODUCER 1-like gene (OsETOL1) that modulates differentially drought and submergence tolerance in rice (Oryza sativa L.). Two allelic mutants of OsETOL1 showed increased resistance to drought stress at the panicle development stage. Interestingly, the mutants exhibited a significantly slower growth rate under submergence stress at both the seedling and panicle development stages. Over-expression (OE) of OsETOL1 in rice resulted in reverse phenotypes when compared with the mutants. The OsETOL1 transcript was differentially responsive to abiotic stresses. OsETOL1 was found to interact with OsACS2, a homolog of 1-amino-cyclopropane-1-carboxylate (ACC) synthase (ACS), which acts as a rate-limiting enzyme for ethylene biosynthesis. In the osacs2 mutant and OsETOL1-OE plants, ACC and ethylene content were decreased significantly, and exogenous ACC restored the phenotype of osetol1 and OsETOL1-OE to wild-type under submergence stress, implying a negative role for OsETOL1 in ethylene biosynthesis. The expression of several genes related to carbohydrate catabolism and fermentation showed significant changes in the osetol1 and OsETOL1-OE plants, implying that OsETOL1 may affect energy metabolism. These results together suggest that OsETOL1 plays distinct roles in drought and submergence tolerance by modulating ethylene production and energy metabolism. Findings from the expression and functional comparison of three ethylene overproducer (ETOL) family members in rice further supported the specific role of OsETOL1 in the responses to the two water stresses.

Characterization of an inositol 1,3,4-trisphosphate 5/6-kinase gene that is essential for drought and salt stress responses in rice.

Drought and salt stresses are major limiting factors for crop production. To identify critical genes for stress resistance in rice (Oryza sativa L.), we screened T-DNA mutants and identified a drought- and salt-hypersensitive mutant dsm3. The mutant phenotype was caused by a T-DNA insertion in a gene encoding a putative inositol 1,3,4-trisphosphate 5/6-kinase previously named OsITPK2 with unknown function. Under drought stress conditions, the mutant had significantly less accumulation of osmolytes such as proline and soluble sugar and showed significantly reduced root volume, spikelet fertility, biomass, and grain yield; however, malondialdehyde level was increased in the mutant. Interestingly, overexpression of DSM3 (OsITPK2) in rice resulted in drought- and salt-hypersensitive phenotypes and physiological changes similar to those in the mutant. Inositol trisphosphate (IP3) level was decreased in the overexpressors under normal condition and drought stress. A few genes related to osmotic adjustment and reactive oxygen species scavenging were down-regulated in the mutant and overexpression lines. The expression level of DSM3 promoter-driven β-glucuronidase (GUS) reporter gene in rice was induced by drought, salt and abscisic acid. Protoplast transient expression assay indicated that DSM3 is an endoplasmic reticulum protein. Sequence analysis revealed six putative ITPKs in rice. Transcript level analysis of OsITPK genes revealed that they had different tempo-spatial expression patterns, and the responses of DSM3 to abiotic stresses, including drought, salinity, cold, and high temperature, were distinct from the other five members in rice. These results together suggest that DSM3/OsITPK2 is an important member of the OsITPK family for stress responses, and an optimal expression level is essential for drought and salt tolerance in rice.

A special member of the rice SRO family, OsSRO1c, mediates responses to multiple abiotic stresses through interaction with various transcription factors

SIMILAR TO RCD ONE (SRO) is a plant-specific gene family involved in development and abiotic stress responses. SRO proteins are characterized by containing poly (ADP-ribose) polymerase catalytic (PARP) and C-terminal RCD1-SRO-TAF4 domains, and can be classified into two groups and five subgroups on the basis of their PARP domain. Expression analysis of rice SRO genes in response to various abiotic stresses showed that OsSRO1c, a rice SRO gene which functions downstream of the stress-responsive transcription factor SNAC1, is the major stress-responsive gene in the rice SRO family. The ossro1c-1 mutant showed resistance not only to chloroplastic oxidative stress, but also to apoplastic oxidative stress. However, the ossro1c-1 mutant and artificial microRNA-OsSRO1c transgenic rice were significantly impaired in cold tolerance. When compared with the well-characterized Arabidopsis SRO protein radical-induced cell death 1 (RCD1), OsSRO1c has considerable variation in the protein sequence, and the two genes exhibit different expression profiles under abiotic stresses. Furthermore, ossro1c-1 and rcd1 showed different responses to multiple abiotic stresses. By screening an Arabidopsis transcription factor library, 29 transcription factors interacted with OsSRO1c in yeast, but only two of these transcription factors were reported to interact with RCD1, which may partly explain the different responses of the two mutants under various stresses. The data presented in this report provide important clues for further elucidating the molecular and biochemical mechanisms of OsSRO1c in mediating responses to multiple abiotic stresses.