Regulation of pollen tube growth in Nicotiana

Abstract

Pollen-pistil interactions determine successful or unsuccessful fertilization and seed set. The study of the mechanisms regulating pollen tube growth (PTG) provides fundamental information for improving crop productivity and the evolution and maintenance of species. This research is focused on interspecific and intraspecific PTG regulation. Nicotiana tabacum is incompatible with N. obtusifolia and the class III pistil extensin-like protein (PIII) is essential for the inhibition of its PTG. The PIII is very similar in amino acid sequence to transmitting tissue specific protein (TTS), involved in facilitating self-PTG in N. tabacum. To understand how protein structure of PIII and TTS relate to function, a complementation study was performed. Variants of PIII were produced by domain swapping between PIII and TTS and a single amino acid mutation in the PIII N-terminal domain (NTD; cysteine to alanine) and introduced into a transgenic line with reduced PIII mRNA accumulation. The N-terminal domain of PIII with either the PIII or TTS C-terminal domain (CTD), complemented the loss of PIII function as measured by inhibition N. obtusifolia PTG. To measure parental effects on intraspecific pollen-pistil interaction, a pollination diallel with 17 N. tabacum genotypes were performed and resulted in surprisingly large PTG rate variation among genotype interactions. The morphological analyses showed that pollen size was the only floral trait correlated to PTG and it was positively correlated with the male genotype PTG mean. Evaluations of PTG rates over time showed large differences in rates of PTG in a genotype-interaction specific manner. Pollinations of styles with and without a stigma or transmitting tissue showed genotypes grow slower without a stigma or transmitting tissue, however other genotypes had equal PTG in styles with or without a stigma or transmitting tissue. In the Samsun genotype without a stigma or transmitting tissue ten genotypes had increased growth relative to no stigma with a transmitting tissue. The most significant findings were that PIII NTD is necessary for the inhibition of N. obtusifolia PTG in the N. tabacum style, N. tabacum PTG is determined pollen and pistil genotypes that interact differentially through the stigma and transmitting tissue.