Role of Maize Expansins in Reproductive Development
Open Access
- Author:
- Valdivia, Elene Ruth
- Graduate Program:
- Plant Physiology
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- September 23, 2005
- Committee Members:
- Daniel J Cosgrove, Committee Chair/Co-Chair
Andrew George Stephenson, Committee Chair/Co-Chair
Teh Hui Kao, Committee Member
Hong Ma, Committee Member
Surinder Chopra, Committee Member - Keywords:
- Expansins
Maize
pollen
pollen allergens - Abstract:
- Groups-I and II/III grass pollen allergens are some of the major elicitors of grass allergies, with approximately 90% of grass pollen-allergic patients sensitive to them. Group-I pollen allergens are recognized as a subfamily of â-expansins, a group of cell wall proteins that loosen plant cell walls in characteristic ways. The proposed role of the pollen â-expansins is to loosen the cell walls of the silks to aid in the penetration and growth of the pollen tube through the silk. Group-II/III pollen allergens (G2A) are not classified as expansins because they lack one of the two domains that characterize the expansin superfamily. Recent results suggest that maize G2As have wall loosening activity on their own. In this work I have looked at the role of expansins and G2As during maize reproductive development. Maize group-I pollen allergens (pollen â-expansins) can be divided into two classes, A and B. I have identified a minimum of 14 class A genes and 6 class B genes in cultivar B73. Class A contains two distinct subclasses that are located in different chromosomes. On the other hand, class B genes are found in a single genomic location. I have also found in maize cultivar B73 at least 11 G2A genes that can be separated into two groups, one coding for high pI proteins and another for low pI proteins. To understand the role of the pollen â-expasnins, I have studied a maize line with a transposon insertion in one of the pollen â-expansin genes, EXPB1. I found that the insertional mutation has no effect on pollen viability, in vitro pollen tube growth or the proportion of progeny sired when small pollen loads are deposited onto stigmas. When fertilization is effected using large pollen loads deposited onto the stigmas, however, the transposon mutation is greatly underrepresented in the progeny. These results suggest that this protein has a large effect on pollen tube growth rates in vivo and plays a major role in male-male competition for access to the ovules. Furthermore, pollen from the insertional lines tends to form large aggregates. In order to corroborate this phenotype, maize calli were transformed with an RNAi construct, driven by a pollen specific promoter, to silence class B pollen beta-expansin genes. Many lines in the first generation were infertile. I have identified several lines that show phenotypes possibly linked to reductions in expression of class B pollen â-expansins. Some lines produced seed when crossed as females, but did not shed any pollen before the anthers started to senesce. In other lines, segregation analysis from crosses of transgenic plants to wild type plants suggests problems in the transmission of the transgene through the pollen. The formation of large pollen aggregates was also observed in some cases. Since a similar phenotype was seen in the transposon lines defective in EXPB1, it is very likely that the silencing of the class B genes is responsible for the observed phenotypes. An RNAi construct for the G2A proteins resulted in the identification of seven lines that show problems with transmission of the transgene when the transgenic parent is male. This transmission problem could be linked to reduced levels of G2A expression. In some lines the transmission problems were seen in multiple generations. These lines do not show any problems with viability or in-vitro growth and they do not produce pollen aggregates. However, upon examination of pollen tube growth of pollen from hemizygous plants in-vivo, I saw fewer pollen tubes growing through the silk. On the basis of these results I propose that both class B pollen â-expansins and G2A proteins loosen the cell walls of the silks to aid in the penetration and growth of the pollen tube. The pollen â-expansins also appear to play a role in pollen separation.