ATP-Binding Cassette G Transporters SGE1 and MtABCG13 Control Stigma Exsertion.

Stigma exsertion is an important agricultural trait that facilitates the application of heterosis in crop breeding. Although several quantitative trait loci associated with stigma exsertion have been fine-mapped or cloned, the underlying genetic basis, particularly in legumes, remains unclear. In this study, we identified and characterized the exserted stigma mutant stigma exsertion1 (sge1) in the model legume Medicago truncatula The exserted stigma phenotype of sge1 is mainly caused by physical interaction between floral organs, in which normal petal and stamen elongation are inhibited due to flower cuticle defects. SGE1 encodes an ATP-binding cassette G (ABCG) transporter that plays a critical role in regulating floral cutin and wax secretion in M. truncatula SGE1 physically interacts with another half-size transporter, MtABCG13, to form a functional heterodimer. Mutation of MtABCG13 results in flower cuticle defects similar to those in sge1 as well as stigma exsertion, indicating that SGE1 and MtABCG13 are indispensable for flower cuticle secretion and collaboratively control stigma exsertion in M. truncatula Our findings reveal novel functions for ABCG transporters in determining stigma exsertion by affecting the physical interactions of floral organs, providing insight into the molecular mechanism underlying stigma exsertion in leguminous plants with complex zygomorphic flowers.

AGAMOUS AND TERMINAL FLOWER controls floral organ identity and inflorescence development in Medicago truncatula.

Angiosperms integrate a multitude of endogenous and environmental signals to control floral development, thereby ensuring reproductive success. Here, we report the identification of AGAMOUS AND TERMINAL FLOWER (AGTFL), a novel regulator of floral development in Medicago truncatula. Mutation of AGTFL led to the transformation of carpels and stamens into numerous sepals and petals and altered primary inflorescence identity. AGTFL encodes a nucleus-localized protein containing a putative Myb/SANT-like DNA-binding domain and a PKc kinase domain. Molecular and genetic analyses revealed that AGTFL regulates the transcription of MtAGs and MtTFL1 to control floral organ identity and inflorescence development.

Cold acclimation-induced freezing tolerance of Medicago truncatula seedlings is negatively regulated by ethylene.

To evaluate the role of ethylene in cold acclimation and cold stress, freezing tolerance and characteristics associated with cold acclimation were investigated using legume model plant Medicago truncatula Gaertn Jemalong A17. There was a rapid suppression of ethylene production during cold acclimation in A17 plants. Ethylene level was negatively correlated with freezing tolerance as inhibition of ethylene biosynthesis by inhibitors of ethylene biosynthesis enhanced freezing tolerance, while exogenous application of ethylene reduced cold acclimation-induced freezing tolerance. The involvement of ethylene signaling in modulation of freezing tolerance and cold acclimation was further studied using ethylene-insensitive mutant sickle skl. Although skl mutant was more tolerant to freezing than its wild-type counterpart A17 plants, cold acclimation enhanced freezing tolerance in 17 plants, but not in skl mutant. Expression of several ethylene response genes including EIN3, EIN3/EIL and ERFs was suppressed in skl mutant compared to A17 plants under non-cold-acclimated conditions. Cold acclimation downregulated expression of EIN3, EIN3/EIL and ERFs in A17 plants, while expression patterns of these genes were relatively constant in skl mutant during cold acclimation. Cold acclimation-induced increases in transcription of MtCBFs and MtCAS15 were suppressed in skl mutant compared with A17 plants. These results suggest that MtSKL1 is required for perception of the change of ethylene level in M. truncatula plants for the full development of the cold acclimation response by suppressing expression of MtEIN3 and MtEIN3/EIL1, which in turn downregulates expression of MtERFs, leading to the enhanced tolerance of M. truncatula to freezing by upregulating MtCBFs and MtCAS15.

Expression of a Medicago falcata small GTPase gene, MfARL1 enhanced tolerance to salt stress in Arabidopsis thaliana.

To understand the role of small GTPases in response to abiotic stress, we isolated a gene encoding a small GTPase, designated MfARL1, from a subtracted cDNA library in Medicago falcata, a native legume species in semi-arid grassland in northern China. The function of MfARL1 in response to salt stress was studied by expressing MfARL1 in Arabidopsis. Wild-type (WT) and transgenic plants constitutively expressing MfARL1 showed comparable phenotype when grown under control conditions. Germination of seeds expressing MfARL1 was less suppressed by salt stress than that of WT seeds. Transgenic seedlings had higher survival rate than WT seedlings under salt stress, suggesting that expression of MfARL1 confers tolerance to salt stress. The physiological and molecular mechanisms underlying these phenomena were elucidated. Salt stress led to a significant decrease in chlorophyll contents in WT plants, but not in transgenic plants. Transgenic plants accumulated less amounts of H(2)O(2) and malondialdehyde than their WT counterparts under salt stress, which can be accounted for by the higher catalase activities, lower activities of superoxide dismutase, and peroxidase in transgenic plants than in WT plants. Transgenic plants displayed lower Na(+)/K(+) ratio due to less accumulation of Na(+) than wild-type under salt stress conditions. The lower Na(+)/K(+) ratio may result from less accumulation of Na(+) due to reduced expression of AtHKT1 that encodes Na(+) transporter in transgenic plants under salt stress. These findings demonstrate that MfARL1 encodes a novel stress-responsive small GTPase that is involved in tolerance to salt stress.