Haploid Plants

One of the main hurdles in genetic analysis of plants is the necessity of inbreeding for several generations to create stable lines for a given mutation. This necessity stems from the plant’s ploidy level, which ranges from diploid, which we are very familiar with, to more complex polyploid individuals. One way to make this process more streamlined would be to produce haploid plants which can then be converted to diploid. We would then have a homozygous individual, bypassing many generations of breeding. Achieving this may seem unfamiliar in plants, but the underlying mechanisms are explained clearly in the article.

There are a few ways to produce haploid plants (Arabidopsis thalania, in this case), but the one discussed in this article is via elimination of one parental genome after fertilization. This can happen when one of the parent's centromeric regions on chromosomes doesn’t attach well to the mitotic spindle, making those chromosomes more likely to be lost, leaving only a single set left for the plant. Researchers took advantage of this observation by making a null version a gene coding for a centromeric histone protein, CENH3. This protein is an ortholog to the human CENPA, which we discussed in class. When a male or female CENH3 null mutant is crossed with a normal plant, the mutant’s genome is eliminated, and a haploid individual can be produced.

The next step for the haploid plant is conversion to diploid, which would give the desired homozygous product. For this to occur, a chance event of non-reductive meiosis is required. At the end of meiosis I, chromosomes segregate equally to either side of the cell during anaphase. In this case, our haploid individual has 5 chromosomes, which segregate randomly in a few possible patterns; 2 -3 or 4-1, for example. Tetrads formed by these segregations are often aneuploid, and thus inviable. If 5-0 segregation is observed, however, the cell is a usual product of meiosis I, and after meiosis II, viable gametes are produced. If the plant is self fertilized using these gametes, we now have an individual homozygous for the exact same set of genes!

This discovery has great potential for applications in more efficient research in plant sciences, but there are social and economic implications as well. It provides an interesting option for seed companies like Monsanto (which has a headquarters in Davis). The CENH3 gene is conserved in all eukaryotes, meaning this procedure could be applied to popular crop plants like maize or soybean. This could make it much easier for companies to produce sterile lines of hybrid seed, which must be re-purchased by farmers each time a new crop is planted. This ability of companies like Monsanto ensures their maximum control of industrial level farming, which is becoming a more and more controversial issue in our time.

This topic sparked my interest when I overheard a conversation at my internship about producing haploid Arabidopsis plants. For the blog, I decided to investigate this concept and, to my surprise, discovered an article in Nature written by a UC Davis professor, Simon Chan, and a postdoc in his lab, Maruthachalam Ravi. It's quite exciting to know that the research responsible for this discovery happened right here on campus.

link to article: http://www.nature.com/nature/journal/v464/n7288/full/nature08842.html#B2

(I think you may have to log in through the library website's VPN, then search for this issue of Nature, which is Vol 464, 25 March 2010)