ObjectivesPhosphorus (P) is one of the essential macronutrients for plants. Plant PHT1 family proteins have vital roles in plant P absorption, mobilization and re-utilization. Numerous members of the PHT1 family have been cloned from various higher plant species. In this paper, researches in plant PHT1 phosphate transporter family were summarized, including gene expression profiles, gene function and possible regulation mechanism.
Major advancesPlant PHT1 family belongs to a major facilitator superfamily (MFS). The protein structure of PHT1 genes is extraordinarily conserved, sharing a common topology with 12 membrane-spanning domains, which are separated into two groups of six domains by a charged hydrophilic loop, with the protein inserted in the plasma membrane. Meanwhile, PHT1 family genes have the same characterized domains as in Pi: H+ co-transporters, sugar transporters, and MFS transporters as well as a conserved amino acid sequence (GGDYPLSATIMSE). Generally, two to four protons enter the plasma membrane simultaneously while plants uptake one phosphate ion via PHT1 family genes, with changes in membrane potential. There are massive members of PHT1 family in plants which are rather different from each other in transport characteristics. The variation of PHT1 gene sequences can give rise to significant changes of their kinetic parameters of Km value. Until now, a huge number of PHT1 family members have been identified in higher plants such as arabidopsis, rice, soybean, solanaceae etc. The expression profiles of PHT1 family genes vary largely. It is generally recognized that most of the PHT1 family members can be regulated by the signal of low P stress and express in plant roots, while the expression of part of the family members can be detected in other organs of plants, and these genes function distinctly. The transcriptional levels of PHT1 family genes can be regulated by many factors such as external inorganic P concentration. Some transcriptional factors from MYB family and WRKY family as well as ZAT6 can interact with PHT1 family genes by binding with some special cis-elements existing in the promoter region of plant PHT1 family genes, such as MYCS element, P1BS element and W-box element, to regulate the expression levels of PHT1 family genes. Besides, the expression levels of PHT1 family genes can also be regulated by arbuscular mycorrhiza fugi (AMF). In addition to transcriptional regulation, post-transcriptional regulation and signal transduction pathway involved in the expression of PHT1 family genes are reported by different research groups. It shows that numerous genes including PHF1, genes containing SPX domain, MicroRNA, and other genes related to protein phosphorylation and chromatin modification are involved in the process of post-transcriptional regulation of PHT1 family members, as well as plant hormones such as auxin, ethylene and cytokinin etc.
Suggestions and ExpectationsThere is an increasingly number of reports on the mechanism of how plants response to low P stress until now, and some genes are cloned involving P absorption and utilization. The results show that complex mechanism exists in the process of P absorption and utilization as well as signal transduction pathway. Thus, it is a long way to breeding genetically modified cultivars with enhanced P efficiency. In terms of researches on PHT1 family genes, it has changed recently from model plants to crops and other higher plants. However, further researches might be conducted on biochemistry and structural biology of proteins of PHT1 family members. In addition, more researches about PHT1 family genes might also be performed towards polyploid species such as Brassica napus, wheat, barley, cotton etc.
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