Abstract
PIN-FORMED 1 (PIN1) is an important secondary transporter and determines the direction of intercellular auxin flow. As PIN1 performs the conserved function of auxin transport, it is expected that the sequence and structure of PIN1 is conserved. Therefore, we hypothesized that PIN1 evolve under pervasive purifying selection in the protein-coding sequences in angiosperm. To test this hypothesis, we performed detailed evolutionary analyses of 67 PIN1 sequences from 35 angiosperm species. We found that the PIN1 sequences are highly conserved within their transmembrane regions, part of their hydrophilic regions. We also found that there are two or more PIN1 copies in some of these angiosperm species. PIN1 sequences from Poaceae and Brassicaceae are representative of the modern clade. We identified 12 highly conserved motifs and a significant number of family-specific sites within these motifs. One family-specific site within Motif 11 shows a different residue between monocots and dicots, and is functionally critical for the polarity of PIN1. Likewise, the function of PIN1 appears to be different between monocots and dicots since the phenotype associated with PIN1 overexpression is opposite between Arabidopsis and rice. The evolution of angiosperm PIN1 protein-coding sequences appears to have been primarily driven by purifying selection, but traces of positive selection associated with sequences from certain families also seem to be present. We verified this observation by calculating the numbers of non-synonymous and synonymous changes on each branch of a phylogenetic tree. Our results indicate that the evolution of angiosperm PIN1 sequences involve strong purifying selection. In addition, our results suggest that the conserved sequences of PIN1 derive from a combination of the family-specific site variations and conserved motifs during their unique evolutionary processes, which is critical for the functional integrity and stability of these auxin transporters, especially in new species. Finally, functional difference of PIN1 is likely to be present in angiosperm because the positive selection is occurred in one branch of Poaceae.
Highlights
The plant hormone auxin is involved in many aspects of plant growth and development, including embryogenesis, organogenesis, tissue differentiation and gravitropism[1,2]
The identified sequences were from the plant species Brachypodium distachyon, Hordeum vulgare, Oryza sativa, Panicum virgatum, Sorghum bicolor, Setaria italica, Triticum aestivum, Zea mays, Arabidopsis lyrata, Arabidopsis thaliana, Brassica rapa, Capsella bursa-pastoris, Cardamine hirsuta, Capsella rubella, Thellungiella halophila, Cicer arietinum, Glycine max, Lupinus albus, Medicago truncatula, Pisum sativum, Phaseolus vulgaris, Fragaria vesca, M. domestica, Prunus persica, Cucumis sativus, Momordica charantia, Gossypium raimondii, Theobroma cacao, Manihot esculenta, Populus trichocarpa, Citrus clementina, Citrus sinensis, Nicotiana tabacum, Solanum lycopersicum, Solanum tuberosum, Vitis vinifera, Carica papaya, and Amborella trichopoda (Table S1)
Our analysis shows that angiosperm PIN1 orthologs contain highly conserved stretches of residues associated with the transmembrane and hydrophilic regions, which is consistent with the function of PIN1
Summary
The plant hormone auxin is involved in many aspects of plant growth and development, including embryogenesis, organogenesis, tissue differentiation and gravitropism[1,2]. Auxin as signal molecule between cells, tissues and organs contributes to the coordination and integration of growth and development in the whole plant and to physiological responses of plants to environmental signal[3,4]. For PIN1 transport auxin, they regulate a number of developmental processes, including morphogenesis, organogenesis, and stress responses[5,8,9]. They are oriented in the plasma membrane such that they mediate the directional flux of auxin within tissues and generate auxin gradients that influence development[10,11]
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