Integrative Analysis of the Wheat PHT1 Gene Family Reveals A Novel Member Involved in Arbuscular Mycorrhizal Phosphate Transport and Immunity

Abstract

Phosphorus (P) deficiency is one of the main growth-limiting factors for plants. However, arbuscular mycorrhizal (AM) symbiosis can significantly promote P uptake. Generally, PHT1 transporters play key roles in plants' P uptake, and thus, <i>PHT1</i> genes have been investigated in some plants, but the regulation and functions of these genes in wheat (<i>TaPHT1</i>) during AM symbiosis have not been studied in depth. Therefore, a comprehensive analysis of <i>TaPHT1</i> genes was performed, including sequence, phylogeny, <i>cis</i>-elements, expression, subcellular localization and functions, to elucidate their roles in AM-associated phosphate transport and immunity. In total, 35 <i>TaPHT1s</i> were identified in the latest high-quality bread wheat genome, 34 of which were unevenly distributed on 13 chromosomes, and divided into five groups. Sequence analysis indicated that there are 11 types of motif architectures and five types of exon-intron structures in the <i>TaPHT1</i> family. Duplication mode analysis indicated that the <i>TaPHT1</i> family has expanded mainly through segmental and tandem duplication events, and that all duplicated gene pairs have been under purifying selection. Transcription analysis of the 35 <i>TaPHT1s</i> revealed that not only known the mycorrhizal-specific genes <i>TaPht-myc</i>, <i>TaPT15-4B</i> (<i>TaPT11</i>) and <i>TaPT19-4D</i> (<i>TaPT10</i>), but also four novel mycorrhizal-specific/inducible genes (<i>TaPT3-2D</i>, <i>TaPT11-4A</i>, <i>TaPT29-6A</i>, and <i>TaPT31-7A</i>) are highly up-regulated in AM wheat roots. Furthermore, the mycorrhizal-specific/inducible genes are significantly induced in wheat roots at different stages of infection by colonizing fungi. Transient <i>Agrobacterium tumefaciens</i>-mediated transformation expression in onion epidermal cells showed that TaPT29-6A is a membrane-localized protein. In contrast to other AM-specific/inducible <i>PHT1</i> genes, <i>TaPT29-6A</i> is apparently required for the symbiotic and direct Pi pathway. <i>TaPT29-6A</i>-silenced lines exhibited reduced levels of AM fungal colonization and arbuscules, but increased susceptibility to biotrophic, hemi-biotrophic and necrotrophic pathogens. In conclusion, <i>TaPT29-6A</i> was not only essential for the AM symbiosis, but also played vital roles in immunity.