Graduate Thesis Or Dissertation
 

A role for maize ROP2 GTPase in the male gametophyte

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  • ROP GTPases are crucial regulators of pollen tube growth. The Rop GTPase family in maize consists of nine known rop genes, ropl-rop9. A subset of these genes (rop2, rop8, and rop9) are expressed in pollen. The rop2 and rop9 genes are a highly conserved duplicate gene pair of ancient origin. The rop2/rop9 duplicate gene pair displays differential expression in mature and germinated pollen, suggesting different roles for the genes in the process of male gametophyte development. To explore ROP2 function in maize, five Mutator transposon insertions in the rop2 gene were isolated (rop2::Mu alleles). I showed that three of the rop2::Mu alleles displayed reduced transmission through the male and were associated with reduced levels of ROP2-mRNA. Interestingly, the rop2::Mu male-specific transmission defect was apparent only when wild-type pollen was also present, an indication that the mutation reduces the competitive ability of the rop2 gametophytes. Dual pollination and pollen mixing experiments indicated that this competitive disadvantage is expressed by the majority of the mutant gametophytes, and that expression of the phenotype is associated with a delay in the ability to accomplish fertilization. Using the waxy phenotypic marker (linked to rop2 via a reciprocal translocation) to distinguish between rop2::Mu and wild-type pollen derived from heterozygous plants, I demonstrated that the delay is associated with a defect in early progamic development (i.e, germination and early pollen tube growth). The defect was detectable in vivo as early as 15 minutes after pollination. However, quantitative measurements provided no indication that the rop2 mutation affects pollen tube growth in the style. Finally, investigations focusing on the final stages of pollen function raise the possibility that a defect in the very last stages (i.e. either pollen tube guidance through the micropyle to the egg sac, or fertilization of the egg sac) may also contribute to the rop2 mutant delay. This work provides direct in vivo evidence confirming a role for Rop in male gametophyte development, and is the first study to demonstrate a role for Rop in the early stages of post-pollination gametophytic function.
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