Pollen-Specific CRISPR/Cas9 System to Increase Heritable Gene Mutations in Maize

The CRISPR/Cas9 system has been widely utilized in plant biotechnology as a gene editing tool. However, a conventional design with ubiquitously expressed CRISPR/Cas9 was observed to cause large numbers of somatic mutations that complicated the identification of heritable mutations. We constructed a pollen-specific CRISPR/Cas9 (PSC) system using pollen-specific promoters of maize Profilin 1 and Profilin 3 (pZmPRO1 and pZmPRO3) to drive Cas9 expression, and the bZIP transcription factor Opaque2 (O2) was employed as the target gene. The maize ubiquitin promoter (pZmUbi)-driven CRISPR/Cas9 (UC) system was employed as a control. We generated transgenic plants for the PSC and UC systems and analyzed three independent events for each system. We found that the pZmPRO1 PSC system generated no target gene mutations in the T0 generation but successfully generated 0–90% target gene mutations in the T1 generation. A total of 31 of 33 mutations in the T1 generation could be inherited in the T2 generation. In addition, 88.9–97.3% of T2 mutations were from the T1 generation. The UC system generated mutations in the T0 generation, and 0%, 50% and 92.9% of T1 mutations were from the T0 generation. Our results demonstrate that the PSC system provided stable, heritable mutants in the next generation, and this approach might also be applied in other crops using germinal cell-specific CRISPR/Cas9 systems to facilitate plant breeding.

Over-expression of the rice OsAMT1-1 gene increases ammonium uptake and content, but impairs growth and development of plants under high ammonium nutrition

A transgenic approach was undertaken to investigate the role of a rice ammonium transporter (OsAMT1-1) in ammonium uptake and consequent ammonium assimilation under different nitrogen regimes. Transgenic lines overexpressing OsAMT1-1 were produced by Agrobacterium-mediated transformation of two rice cultivars, Taipei 309 and Jarrah, with an OsAMT1-1 cDNA gene construct driven by the maize ubiquitin promoter. Transcript levels of OsAMT1-1 in both Taipei 309 and Jarrah transgenic lines correlated positively with transgene copy number. Shoot and root biomass of some transgenic lines decreased during seedling and early vegetative stage compared to the wild type, especially when grown under high (2 mm) ammonium nutrition. Transgenic plants, particularly those of cv. Jarrah recovered in the mid-vegetative stage under high ammonium nutrition. Roots of the transgenic plants showed increased ammonium uptake and ammonium content. We conclude that the decreased biomass of the transgenic lines at early stages of growth might be caused by the accumulation of ammonium in the roots owing to the inability of ammonium assimilation to match the greater ammonium uptake.

A suite of novel promoters and terminators for plant biotechnology. II. The pPLEX series for use in monocots

A suite of plant expression vectors (pPLEX), constructed from the gene regulation signals from subterranean clover stunt virus (SCSV) genome, has previously been used in dicot transformation for a variety of applications in plant biotechnology. To assess their use for the transformation of monocots, a number of modifications were made to the basic vector series and assessed in rice. In their unmodified forms, the SCSV promoters directed low levels of gene expression, however, insertion of an intron between the promoter and the transgene open reading frame (analogous to the rice actin and maize ubiquitin promoter systems) increased transgene expression 50-fold. The expression patterns from the intron-modified SCSV (segments 4 and 7) promoters were very similar to those directed by the actin or ubiquitin promoters. All promoter systems investigated directed expression that appeared to be constitutive within leaf tissue, and localised to the epidermal and vascular tissues of the root. The pPLEX vectors described here are an important counterpart to the dicot pPLEX series and have the potential to be useful in monocot research and biotechnology.