Overexpression of OsARD1 Improves Submergence, Drought, and Salt Tolerances of Seedling Through the Enhancement of Ethylene Synthesis in Rice.

Acireductone dioxygenase (ARD) is a metal-binding metalloenzyme and involved in the methionine salvage pathway. In rice, OsARD1 binds Fe2+ and catalyzes the formation of 2-keto-4-methylthiobutyrate (KMTB) to produce methionine, which is an initial substrate in ethylene synthesis pathway. Here, we report that overexpression of OsARD1 elevates the endogenous ethylene release rate, enhances the tolerance to submergence stress, and reduces the sensitivity to drought, salt, and osmotic stresses in rice. OsARD1 is strongly induced by submergence, drought, salinity, PEG6000, and mechanical damage stresses and exhibits high expression level in senescent leaves. Transgenic plants overexpressing OsARD1 (OsARD1-OE) display fast elongation growth to escape submergence stress. The ethylene content is significantly maximized in OsARD1-OE plants compared with the wide type. OsARD1-OE plants display increased shoot elongation and inhibition of root elongation under the submergence stress and grow in dark due to increase of ethylene. The elongation of coleoptile under anaerobic germination is also significantly promoted in OsARD1-OE lines due to the increase of ethylene content. The sensitivity to drought and salt stresses is reduced in OsARD1-OE transgenic lines. Water holding capacity is enhanced, and the stomata and trichomes on leaves increase in OsARD1-OE lines. Drought and salt tolerance and ethylene synthesis-related genes are upregulated in OsARD1-OE plants. Subcellular localization shows that OsARD1 displays strong localization signal in cell nucleus, suggesting OsARD1 may interact with the transcription factors. Taken together, the results provide the understanding of the function of OsARD1 in ethylene synthesis and abiotic stress response in rice.

Overexpression of OsHox32 Results in Pleiotropic Effects on Plant Type Architecture and Leaf Development in Rice.

BACKGROUND: The Class III homeodomain Leu zipper (HD-Zip III) gene family plays important roles in plant growth and development. Here, we analyze the function of OsHox32, an HD-Zip III family member, and show that it exhibits pleiotropic effects on regulating plant type architecture and leaf development in rice. RESULTS: Transgenic lines overexpressing OsHox32 (OsHox32-OV) produce narrow leaves that roll towards the adaxial side. Histological analysis revealed a decreased number of bulliform cells in OsHox32-OV lines. In addition, the angle between the leaf and culm was reduced, resulting in an erect plant phenotype. The height of the plants was reduced, resulting in a semi-dwarf phenotype. In addition, the chlorophyll level was reduced, resulting in a decrease in the photosynthetic rate, but water use efficiency was significantly improved, presumably due to the rolled leaf phenotype. OsHox32 exhibited constitutive expression in different organs, with higher mRNA levels in the stem, leaf sheath, shoot apical meristems and young roots, suggesting a role in plant-type and leaf development. Moreover, OsHox32 mRNA levels were higher in light and lower in the dark under both long-day and short-day conditions, indicating that OsHox32 may be associated with light regulation. Photosynthesis-associated and chlorophyll biosynthesis-associated genes were down-regulated to result in the reduction of photosynthetic capacity in OsHox32-OV lines. mRNA level of six rice YABBY genes is up-regulated or down-regulated by OsHox32, suggesting that OsHox32 may regulate the architecture of plant type and leaf development by controlling the expression of YABBY genes in rice. In addition, OsHox32 mRNA level was induced by the phytohormones, indicating that OsHox32 may be involved in phytohormones regulatory pathways. CONCLUSIONS: OsHox32, an HD-Zip III family member, plays pleiotropic effects on plant type architecture and leaf development in rice.

[Ac/Ds tagging system and functional genomics in rice].

With the completion of the rice genome sequence in 2002, the study on functional genomics in rice has become a major task. Establishment of rice mutant library is an essential approach for rice functional genomics study. At present, utilizing maize transposable element Ac/Ds (Activator/Dissociation) is a promising method to construct insertional mutagenesis library of rice. Ac/Ds tagging system has received extensive application in rice during the past several years, but it is still confronted with practical problems. In this paper, constructing rice insertional mutagenesis library using Ac/Ds-tagging system and transpositional behaviors of an Ac/Ds-tagging system, difficulties and advantages are reviewed. The research advances and challenge in rice functional genomics study using Ac/Ds tagging system are also discussed and summarized in the paper.

An efficient field screening procedure for identifying transposants for constructing an Ac/Ds-based insertional-mutant library of rice.

An efficient system was developed, and several variables tested, for generating a large-scale insertional-mutagenesis population of rice. The most important feature in this improved Ac/Ds tagging system is that one can conveniently carry out large-scale screening in the field and select transposants at the seedling stage. Rice was transformed with a plasmid that includes a Basta-resistance gene (bar). After the Ds element is excised during transposition, bar becomes adjacent to the ubiquitin promoter, and the rice plant becomes resistant to the herbicide Basta. In principle, one can plant up to one million plants in the field and select those plants that survive after spraying with Basta. To test the utility of this system, 4 Ds starter lines were crossed with 14 different Ac plants, and many transposants were successfully identified after planting 134,285 F2 plants in the field. Over 2,800 of these transposants were randomly chosen for PCR analysis, and the results fully confirmed the reliability of the field screening procedure.