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Summary: Improving plant biomass yield and/or feedstock quality for highly efficient lignocellulose conversion has been the main research focus in genetic modification of switchgrass (Panicum virgatum L.), a dedicated model plant for biofuel production. Here, we proved that overexpression of miR396 (OE‐miR396) leads to reduced plant height and lignin content mainly by reducing G‐lignin monomer content. We identified nineteen PvGRFs in switchgrass and proved thirteen of them were cleaved by miR396. MiR396‐targeted PvGRF1, PvGRF9 and PvGRF3 showed significantly higher expression in stem. By separately overexpressing rPvGRF1, 3 and 9, in which synonymous mutations abolished the miR396 target sites, and suppression of PvGRF1/3/9 activity via PvGRF1/3/9‐SRDX overexpression in switchgrass, we confirmed PvGRF1 and PvGRF9 played positive roles in improving plant height and G‐lignin content. Overexpression of PvGRF9 was sufficient to complement the defective phenotype of OE‐miR396 plants. MiR396‐PvGRF9 modulates these traits partly by interfering GA and auxin biosynthesis and signalling transduction and cell wall lignin, glucose and xylan biosynthesis pathways. Moreover, by enzymatic hydrolysis analyses, we found that overexpression of rPvGRF9 significantly enhanced per plant sugar yield. Our results suggest that PvGRF9 can be utilized as a candidate molecular tool in modifying plant biomass yield and feedstock quality. [ABSTRACT FROM AUTHOR]

Summary: Salinity‐induced accumulation of certain microRNAs accompanied by gaseous phytohormone ethylene production has been recognized as a mechanism of plant salt tolerance. MicroRNA319 (miR319) has been characterized as an important player in abiotic stress resistance in some C3 plants, such as Arabidopsis thaliana and rice. However, its role in the dedicated biomass plant switchgrass (Panicum virgatum L.), a C4 plant, has not been reported. Here, we show crosstalk between miR319 and ethylene (ET) for increasing salt tolerance. By overexpressing Osa‐MIR319b and a target mimicry form of miR319 (MIM319), we showed that miR319 positively regulated ET synthesis and salt tolerance in switchgrass. By experimental treatments, we demonstrated that ET‐mediated salt tolerance in switchgrass was dose‐dependent, and miR319 regulated the switchgrass salt response by fine‐tuning ET synthesis. Further experiments showed that the repression of a miR319 target, PvPCF5, in switchgrass also led to enhanced ethylene accumulation and salt tolerance in transgenic plants. Genome‐wide transcriptome analysis demonstrated that overexpression of miR319 (OE‐miR319) down‐regulated the expression of key genes in the methionine (Met) cycle but promoted the expression of genes in ethylene synthesis. The results enrich our understanding of the synergistic effects of the miR319‐PvPCF5 module and ethylene synthesis in the salt tolerance of switchgrass, a C4 bioenergy plant. [ABSTRACT FROM AUTHOR]

Highlights • Overexpression of OsPIL1 gene in switchgrass improved biomass yield and saccharification efficiency of OsPIL1 transgenic switchgrass plant. • The higher expression level of OsPIL1 co-related to taller plant height and longer epidermal cell of transgenic plant. • The results suggested that the OsPIL1 can be used as a useful molecular tool in improving plant biomass with the purpose of plant fiber biofuel ethanol production. Abstract Switchgrass (Panicum virgatum L.) is a herbaceous cellulosic biofuel plant with broad adaptability. However, the intrinsic recalcitrance of biomass and limited land for switchgrass planting hinder its utilization as feedstock for biofuel ethanol production. The OsPIL1 (PHYTOCHROME INTERACTING FACTOR 3-LIKE 1) gene encodes a basic helix-loop-helix transcription factor. Its expression is induced by light, which facilitated the expression of cell wall-related genes, promoted cell elongation and resulted in longer internode in rice. Here, we introduced the OsPIL1 gene into switchgrass by Agrobacterium -mediated transformation with the aim of improving biomass yield of transgenic switchgrass plants. The transgenic plants were verified by PCR, Southern-blotting, RT-PCR and qRT-PCR tests, respectively. The transgenic plants overexpression of OsPIL1 showed increased plant height and biomass yield. Microscopy analysis showed that the length of epidermal cells of transgenic plants was longer than that of wild type. OsPIL1 overexpressed transgenic switchgrass plants also released more soluble sugar after enzymatic hydrolysis, indicating improved saccharification efficiency. The results suggest OsPIL1 can be used as a useful molecular tool in improving plant biomass and saccharification efficiency with the purpose of plant fiber biofuel ethanol production. [ABSTRACT FROM AUTHOR]