Seminar with Weicai Yang and Caixia Gao – University of Copenhagen

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Seminar with Weicai Yang and Caixia Gao

Prof. Weicai Yang: Pollen tube guidance in Arabidopsis: The interplay between the male and female gametohyte

During evolution, novel reproductive structures and mechanisms have evolved to adapt to terrestrial land environment in plants. In angiosperms, such evolutionary development is manifested by the flower, multicellular gametophyte, double fertilization, loss of sperm motility, and siphonogamy in which the immotile sperm was delivered to the egg by a pollen tube produced by the male gametophyte (pollen), a process named pollen tube guidance (PTG).

Previous studies suggested that PTG requires the intimate interactions between the pollen tube and maternal tissue of the pistil and the female gametophyte respectively. Although signaling molecules, such as LURE1 peptide produced by the synergid cell of the female gameptophyte (embryo sac) to attract and guide the pollen tube growth, were identified (for review see Higashiyama and Takeuchi, Annu Rev Plant Biol 2015), how the pollen tube recognizes and responds to the guiding molecule is not yet clear.

Through genetic screen, we isolated a number of Arabidopsis mutants that disrupt PTG processes. CCG, a central cell-specifically expressed gene, is required for the female gametophyte to attract the pollen tube. CCG encodes a nuclear protein that regulates the expression of a number genes important for PTG via CBP1 which interacts with RNA polymerase II, the Mediator complex and AGL transcription factors in the central cells and also LURE1 expression in the synergid cells indirectly (Chen et al., Plant Cell 2007; Li et al., Plant Cell 2015).

POD1, a pollen tube-expressed gene, is required for the male gametophyte to respond to the female signals. POD1 encodes a ER protein that interact specifically with CRT3 which is implicated to control the foldin gof Leucine-Rich Repeat Receptor-Like Kinases (LRR-RLKs). These data suggest that POD1 might play a role in the protein folding of putative receptor proteins. Recently, we identified the male MDIS/MDIK receptor complex that recognizes the female attracting signals (Wang et al., Nature 2016). At the same time, another LRR-RLK PRK6 has also been reported to be LURE1 receptor in Arabidopsis (Takeuchi and Higashiyama, Nature 2016). These findings provide novel insight to mechanims controlling PTG and will be discussed.

Prof. Caixia Gao: Crop genome engineering using genome editing technologies

Crop improvement requires the constant creation and use of new allelic variants. Conventional breeding can be limited in providing the genes and alleles required to meet the agricultural challenges. In the past decade, Genome editing can accelerate plant breeding by allowing the introduction of precise and predictable modifications directly in an elite background. 

The most promising utilization of both the CRISPR/Cas9 system and TALENs can be used to generate targeted genome modifications including mutations, insertions, replacements and chromosome rearrangements. This talk will address recent advances in developing targeted mutagenesis systems for trait improvement in crop plants.

By knocking out all six alleles encoding the Mildew-Resistance Locus (MLO) protein with one pair of TALENs, we generated a mutant line with broad-spectrum resistance to powdery mildew – a devastating fungal disease. Typically, TALEN or CRISPR expression cassettes are delivered to plant cells and expressed, which cleaves chromosomal target sites and produces site-specific DNA double-strand breaks (DSBs), leading to genome modifications during the repair process.

We developed simple and efficient genome editing approaches in which wheat plants are regenerated from callus cells transiently expressed with CRISPR/Cas9 reagents introduced as DNA, RNA or RNP. The effectiveness of the three methods in yielding specifically targeted, transgene-free mutants in T0 generation was validated using six different wheat genes.

This approach may be widely applicable for producing genome edited crop plants and has a good prospect of being commercialized. Very recently, a plant base editing method (nCas9-PBE), using a Cas9 nickase-cytidine deaminase fusion, has been optimized and validated for genome editing in rice, wheat and maize in my group.

This approach will not only technologically advance plant genome engineering, but may also provide better solution for social acceptance of genome-edited crops as they do not require a donor DNA template or chromosomal cleavage.