The genome of a wild Medicago species provides insights into the tolerant mechanisms of legume forage to environmental stress.

BACKGROUND: Medicago ruthenica, a wild and perennial legume forage widely distributed in semi-arid grasslands, is distinguished by its outstanding tolerance to environmental stress. It is a close relative of commonly cultivated forage of alfalfa (Medicago sativa). The high tolerance of M. ruthenica to environmental stress makes this species a valuable genetic resource for understanding and improving traits associated with tolerance to harsh environments. RESULTS: We sequenced and assembled genome of M. ruthenica using an integrated approach, including PacBio, Illumina, 10×Genomics, and Hi-C. The assembled genome was 904.13 Mb with scaffold N50 of 99.39 Mb, and 50,162 protein-coding genes were annotated. Comparative genomics and transcriptomic analyses were used to elucidate mechanisms underlying its tolerance to environmental stress. The expanded FHY3/FAR1 family was identified to be involved in tolerance of M. ruthenica to drought stress. Many genes involved in tolerance to abiotic stress were retained in M. ruthenica compared to other cultivated Medicago species. Hundreds of candidate genes associated with drought tolerance were identified by analyzing variations in single nucleotide polymorphism using accessions of M. ruthenica with varying tolerance to drought. Transcriptomic data demonstrated the involvements of genes related to transcriptional regulation, stress response, and metabolic regulation in tolerance of M. ruthenica. CONCLUSIONS: We present a high-quality genome assembly and identification of drought-related genes in the wild species of M. ruthenica, providing a valuable resource for genomic studies on perennial legume forages.

ROS and calcium oscillations are required for polarized root hair growth.

Root hairs are filamentous extensions from epidermis of plant roots with growth limited to the apical dome. Cell expansion undergoes tightly regulated processes, including the coordination between cell wall loosening and cell wall crosslinking, to form the final shape and size. Tip-focused gradients and oscillations of reactive oxygen species (ROS) together with calcium ions (Ca2+) as indispensable regulated mechanisms control rapid and polarized elongation of root hair cells. ROS homeostasis mediated by plasma membrane-localized NADPH oxidases, known as respiratory burst oxidase homologues (RBOHs), and class III cell wall peroxidases (PRXs), modulates cell wall properties during cell expansion. The expression levels of RBOHC, an NADPH oxidase that produces ROS, and class III PRXs are directly upregulated by ROOT HAIR DEFECTIVE SIX-LIKE 4 (RSL4), encoding a basic-helix-loop-helix (bHLH) transcription factor, to modulate root hair elongation. Cyclic nucleotide-gated channels (CNGCs), as central regulators of Ca2+ oscillations, also regulate root hair extension. Here, we review how the gradients and oscillations of Ca2+ and ROS interact to promote the expansion of root hair cells.

[Evolution of Extracellular Polymeric Substances of the Activated Sludge with Calcium Ion Addition During Set-up Period of Sequencing Batch Reactors].

Great attention has been paid to accelerate the start-up period and enhance floc properties and structural stability in activated sludge reactors with the aid of inorganic chemical agents such as calcium ion. The laboratory-scale sequencing batch reactors (SBRs) were operated continuously for 35 days to investigate the effect of calcium ion (Ca2+) on the physicochemical properties and evolution of extracellular polymeric substances (EPS) of activated sludge during set-up period. When compared to the control (non-calcium ion addition), the addition of 150 mg·L-1Ca2+ to the influent significantly increased the mixed liquid suspended solids (MLSS) and the mixed liquid volatile suspended solids (MLVSS) by 89.6% and 75.6% on 28 d, respectively, and decreased the sludge volume index (SVI) by 47.9% following SBRs set-up. Compared with the control system, the contents of EPS, polysaccharides (PS) and proteins (PN) were increased by 76.4%, 28.8% and 31.6% under the condition of Ca2+ dosage of 150 mg·L-1. The PS/PN ratio was 68.8 for Ca2+ addition, compared to only 36.6 for the control reactor. Analysis using three-dimensional excitation emission matrix fluorescence spectroscopy and Fourier transform infrared spectrum revealed that Ca2+ addition changed the compositional characteristics of EPS. Results from this study provided a fundamental knowledge basis for the improvement of the settling properties of activated sludge with calcium ion addition.

The shift of the microbial community in activated sludge with calcium treatment and its implication to sludge settleability.

The sludge settleability is of prime importance for the activated sludge process. The effect of calcium ion on the biological performance of sludge was investigated in a lab-scale activated sludge system with varying Ca(2+) concentration. Results indicated that addition of 150mg/L Ca(2+) to the influent significantly improved the settling characteristics and metabolic reactivity of activated sludge in the bioreactors. Analyses using denaturing gradient gel electrophoresis (DGGE) and 16S rRNA sequencing showed that a significant difference in the presence of certain bacterial groups between the sludge systems with 150mg/L Ca(2+) and those with 0-100mg/L Ca(2+) addition. Ca(2+) also increased the production of the extracellular polymeric substance (EPS) and facilitated the development of microbial cluster in the bioreactor. Study showed that an addition of 150mg/L Ca(2+) to the influent provides a simple approach to improve the settling properties of activated sludge and maintain high pollutant removal efficiency.