EbARC1, an E3 Ubiquitin Ligase Gene in Erigeron breviscapus, Confers Self-Incompatibility in Transgenic Arabidopsis thaliana

Genome-wide identification and characterization of SRLK gene family reveal their roles in self-incompatibility of Erigeron breviscapus

Self-incompatibility (SI) is a reproductive protection mechanism that plants acquired during evolution to prevent self-recession. As the female determinant of SI specificity, SRK has been shown to be the only recognized gene on the stigma and plays important roles in SI response. Asteraceae is the largest family of dicotyledonous plants, many of which exhibit self-incompatibility. However, systematic studies on SRK gene family in Asteraceae are still limited due to lack of high-quality genomic data. In this study, we performed the first systematic genome-wide identification of S-locus receptor like kinases (SRLKs) in the self-incompatible Asteraceae species, Erigeron breviscapus, which is also a widely used perennial medicinal plant endemic to China.52 SRLK genes were identified in the E. breviscapus genome. Structural analysis revealed that the EbSRLK proteins in E. breviscapus are conserved. SRLK proteins from E. breviscapus and other SI plants are clustered into 7 clades, and the majority of the EbSRLK proteins are distributed in Clade I. Chromosomal and duplication analyses indicate that 65% of the EbSRLK genes belong to tandem repeats and could be divided into six tandem gene clusters. Gene expression patterns obtained in E. breviscapus multiple-tissue RNA-Seq data revealed differential temporal and spatial features of EbSRLK genes. Among these, two EbSRLK genes having high expression levels in tongue flowers were cloned. Subcellular localization assay demonstrated that both of their fused proteins are localized on the plasma membrane. All these results indicated that EbSRLK genes possibly involved in SI response in E. breviscapus. This comprehensive genome-wide study of the SRLK gene family in E. breviscapus provides valuable information for understanding the mechanism of SSI in Asteraceae.

The E3 Ligase GmPUB21 Negatively Regulates Drought and Salinity Stress Response in Soybean

E3-ubiquitin ligases are known to confer abiotic stress responses in plants. In the present study, GmPUB21, a novel U-box E3-ubiquitin ligase-encoding gene, was isolated from soybean and functionally characterized. The expression of GmPUB21, which possesses E3-ubiquitin ligase activity, was found to be significantly up-regulated by drought, salinity, and ABA treatments. The fusion protein GmPUB21-GFP was localized in the cytoplasm, nucleus, and plasma membrane. Transgenic lines of the Nicotiana benthamiana over-expressing GmPUB21 showed more sensitive to osmotic, salinity stress and ABA in seed germination and inhibited mannitol/NaCl-mediated stomatal closure. Moreover, higher reactive oxygen species accumulation was observed in GmPUB21 overexpressing plants after drought and salinity treatment than in wild-type (WT) plants. Contrarily, silencing of GmPUB21 in soybean plants significantly enhanced the tolerance to drought and salinity stresses. Collectively, our results revealed that GmPUB21 negatively regulates the drought and salinity tolerance by increasing the stomatal density and aperture via the ABA signaling pathway. These findings improved our understanding of the role of GmPUB21 under drought and salinity stresses in soybean.

Microscopic and Transcriptomic Analysis of Pollination Processes in Self-Incompatible Taraxacum koksaghyz

The transition of the Russian dandelion Taraxacum koksaghyz (Asteraceae) to a profitable, alternative crop producing natural rubber and inulin requires the optimization of several agronomic traits, cultivation conditions and harvesting procedures to improve the yield. However, efficient breeding is hindered by the obligatory sexual outcrossing of this species. Several other asters have been investigated to determine the mechanism of self-incompatibility, but the underlying molecular basis remains unclear. We therefore investigated the self-pollination and cross-pollination of two compatible T. koksaghyz varieties (TkMS2 and TkMS3) by microscopy and transcriptomic analysis to shed light on the pollination process. Self-pollination showed typical sporophytic self-incompatibility characteristics, with the rare pollen swelling at the pollen tube apex. In contrast, cross-pollination was characterized by pollen germination and penetration of the stigma by the growing pollen tubes. RNA-Seq was used to profile gene expression in the floret tissue during self-pollination and cross-pollination, and the differentially expressed genes were identified. This revealed three candidates for the early regulation of pollination in T. koksaghyz, which can be used to examine self-incompatibility mechanisms in more detail and to facilitate breeding programs.