Aaron J. Louie
3.13.97
BI 399
Plant Development
Meeks-Wagner

Self-Incompatibility in Arabidopsis: A review of a study

The adaptive importance of maintaining genetic diversity in a population may be the cause for the evolution of self-incompatibility in flowering plants. Self-sterility may have evolved to avoid the presence of negative homozygotic recessive alleles in a population. Self-incompatibility usually involves the interaction between the pollen grain, the pollen tube, and the stigma (Dickinson & Roberts, 1986; Dzelzkalns, Nasrallah, & Nasrallah, 1992; Haring, et al., 1990; McClure, et al., 1990). The mechanisms for self-incompatibility in other plants, such as maize, tobacco, petunia, and Brassica, can be either gametophytic or sporophytically dependent, involving either an interruption of pollen tube germination (early onset) or of pollen tube penetration and subsequent guidance to the ovule (late onset) (Dickinson & Roberts, 1986; Dzelzkalns, Nasrallah, & Nasrallah, 1992; Haring, et al., 1990; McClure, et al., 1990). Wilhelmi & Preuss (1996) have now found mutants of Arabidopsis thaliana that exhibit self-sterility. In flowering plants such as Arabidopsis, a sequence of events involving interactions between cells are responsible for the delivery of sperm to the ovules through precise guidance of the pollen tubes. Recent research has shown that several signaling pathways may be involved in the germination, penetration and guidance of the pollen tube on its way to the ovule. Wilhelmi & Preuss (1996) found that two redundant genes, POP2 and POP3, mediate pollen tube guidance in Arabidopsis. pop2 and pop3 mutants are unable to self-fertilize, and Wilhelmi and Preuss (1996) found a complex interaction between these two genes , resulting in a defect in the molecules present on the surface of the pollen tube (sporophytic inheritance). They propose the idea that other self-incompatibility mechanisms in other flowering plants may work through a similar mechanism. In this paper, I will discuss the findings of Wilhelmi & Preuss (1996) in light of past and current research, especially the genes involved in pollen tube germination, penetration, and guidance. I will also explore the known characteristics of self-incompatibility and self-sterility in Arabidopsis and other plants.

Wilhelmi & Preuss (1996) screened over 80,000 Arabidopsis plants to find a sterile mutant that was defective in two genes that severely affected targeting of pollen tubes to structurally normal ovules. They then crossed this mutant with the wild-type strain, creating heterozygous individuals that were fully fertile, indicating that the mutation in pollen tube guidance was recessive. The resulting F2 generation showed linkage to two genes, POP2 and POP3, which were on different chromosomes. All the sterile individuals were homozygous for the pop2 mutation, yet only one-third were homozygous for pop3, indicating that two genotypes result in sterility, pop2/pop2, pop3/pop3 and pop2/pop2, pop3/POP3+. This also means that, although pop2 is recessive, the pop3 allele is dominant in a pop2/pop2 background. Wilhelmi & Preuss then determined whether combinations of the pop2 and pop3 alleles adversely affected gamete fertility or zygote viability by examining the transmission of the sterile phenotype. They found that the pop alleles are transmitted normally and that inheritance of sterility follows the ratio expected for two independently assorting genes. They then analyzed the genotype of fertile progeny from a pop2/POP2+, pop3/POP3+ plant and found that POP2 and POP3 are indeed two different genotypes. Wilhelmi & Preuss proceeded to examine the results of a cross between pop2 and pop3 single mutants, which yielded F1 plants that always produced sterile progeny. By crossing other variations of POP mutants, they found that no combination of all possible fertile genotypes among F2 plants could be lethal. They also found that pop2 and pop3 could be transmitted through both male and female gametes and that the function of these gametes depends only on the parental genotype, indicating sporophytic inheritance, where function of the haploid gametes is governed by the diploid tissues from which they were derived. Wilhelmi and Preuss then hypothesized that the interactions between the haploid pollen grains occurred via a diffusible component. However, they found that the pop mutation occurred in a cell autonomous manner, leading to the possibility that POP2 and POP3 encode molecules present on the surface of both pollen and pistil.

These results suggest that the POP genes exhibit both sporophytic and gametophytic effects and can result in a sort of self-sterility. I assume that the genetic tests done on the characteristics of the pop mutants found that single mutants were self-sterile. However, it was difficult to extrapolate any relation to self-sterility given Wilhelmi & Preuss's lack of clarity in the discussion of the results. They gave their results in a highly condensed format similar to that in my summary of their experiments and results. Furthermore, Wilhelmi & Preuss do not discuss results at any length in regards to any other research done in self-incompatibility. They also do not mention whether or not any other research has been conducted on Arabidopsis concerning self-incompatibility. Regardless of my opinions of this study, the results may prove to be significant by illuminating the mechanisms behind self-sterility in other plants and by providing an example of how self-incompatibility evolved. Self-incompatibility is important in limiting inbreeding depression and presence of deleterious recessive alleles. I found it interesting that Arabidopsis does not already exhibit any sort of self-incompatibility already. Further research may show that Arabidopsis does have mechanisms for self-sterility. As Wilhelmi & Preuss mention, study of the POP gene may allow us to more closely examine the mechanisms in pollen tube guidance and redundant signaling pathways active in male and female tissues.

References

Dickinson, H. G., & Roberts, I. N. (1986). Cell-surface receptors in the pollen-stigma interaction of Brassica oleracea. In Hormones, Receptors and Cellular Interactions in Plants (C. M. Chadwick, D. R. Garrod, & B. Cinader, Eds.), pp. 225-280. Cambridge University Press, Cambridge.

Dzelzkalns, V. A., Nasrallah, J. B., & Nasrallah, M. E. (1992). Cell-cell communication in plants: Self incompatibility in flower development. Dev. Bio., 153, 70-82.

Haring, V., Gray, J. E., McClure, B. A., Anderson, M. A., & Clarke, A. E. (1990). Self-incompatibility: A self-recognition system in plants. Science, 250, 937-941.

McClure, B. A., Gray, J. E., Anderson, M. A., and Clarke, A. E. (1990). Self-incompatibility in Nicotinia alata involves degradation of pollen rRNA. Nature, 347, 757-760.

Wilhelmi, L. K. & Preuss, D. (1996). Self-sterility in Arabidopsis due to defective pollen tube guidance. Science, 274, 1535-1537.