The final step of pollen tube growth is pollen tube-synergid attraction and finally fertilization. This is associated with the transition from autotrophic (nutrients obtained from the pollen grain) to heterotrophic growth (nutrients obtained from transmitting tract ). The initial rate of pollen tube growth through the style is slower but increases as it grows. Pollen tubes grow rapidly and the fastest growing pollen tubes reach the ovules first giving rise to progeny, making pollen tube growth a key step where natural selection may act. The highly differentiated TT evolved with enclosed ovules of angiosperms and is a pathway for pollen tube growth from the stigma to the ovules. Self-incompatibility (SI) is a barrier that helps maintain species genetic variation and interspecific incompatibility (II) prevents gene flow among species, preserving species genetic integrity. Plants evolved multiple prezygotic mechanisms to control fertilization. The stigma, style and TT have a role in genetic isolation of plant populations and consequently species evolution. The transmitting tract regulates pollen tube growth, promoting fertilization (plant compatibility) or rejecting pollen tubes (incompatibility). The pollen tube delivers the male gamete to the female gametophyte, beginning with pollen grain hydration and germination on the stigma. Pollen-pistil interactions are dynamic, complex and spatially differentiated. We hypothesize that the stylar AGPs and NtPRP share a common origin from a single gene that duplicated and diversified into four distinct genes involved in pollen-style interactions. In contrast to TTS, both PELPIII and 120 K genes are more diverse indicating a possible role in speciation or mating preference of Nicotiana spp. Due to its high similarity to the TTS gene, NtPRP may be involved in pollen tube guidance and growth. Conclusionsįurther studies of the NtPRP gene are necessary to elucidate its biological role. A newly discovered AGP gene, Nicotiana tabacum Proline Rich Protein (NtPRP), was found with a similar intron-exon configuration and protein structure resembling other stylar AGPs, particularly TTS. These data indicate that PELPIII and TTS genes are under different selective pressures. The d n/d s ratio for individual species ranged from 0.4 to 0.9 and from 0.1 to 0.8, for PELPIII and TTS genes, respectively. The PELPIII, TTS and 120 K genes undergo negative selection, with d n/d s ratios of 0.59, 0.29 and 0.38 respectively. The TTS protein had the greatest amino acid and predicted O-glycosylation conservation among Nicotiana spp. The C-terminal domain (CTD) contains an Ole e 1-like domain, that was predicted to form beta-sheets that are similar in position and length among Nicotiana spp. The NTD is also the location for the majority of the predicted O-glycosylation sites that were variable among Nicotiana spp. The NTD was predicted to be mainly an intrinsically disordered region (IDR), making it a candidate for protein-protein interactions. The N-terminal domain (NTD) of the stylar AGPs is proline rich and polymorphic among Nicotiana spp. The transmitting tract regulates pollen tube growth, promoting fertilization or rejecting pollen tubes. The three arabinogalactan proteins (AGP) are referred to as stylar AGPs and are the focus of this research. The transmitting tract (TT) of Nicotiana tabacum controls pollen tube growth in part by secreting pistil extensin-like protein III (PELPIII), transmitting-tract-specific (TTS) protein and 120 kDa glycoprotein (120 K) into the stylar extracellular matrix. Pollen tube growth and fertilization are key processes in angiosperm sexual reproduction.
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