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AbstractAmino acid transporters (AATs) play an important role in transport process of various amino acids, which are indispensable in plant growth and development, while many putative AATs have been identified and the complete genomic sequences of the important plants have already been completed by splicing and assembling. There is still little knowledge about the expression, regulation and various biological functions of AATs in plants, including the major food crops. This study mainly reviewed the expression, regulation and various biological functions of AATs in plants, and the application of AATs in crop genetic improvement was also prospected. Thus, this review will provide important information for genetic improvement of staple food crops in plants.
Key wordsAATs; Expression; Regulation; Function; Application
Received: June 29, 2018Accepted: September 30, 2018
Supported by National Natural Science Foundation of China (U1604110, U1404319, 31600992, 31801332); Key Project of Science and Technology in Henan Province (182102110442, 152102110036); Nanhu Scholars Program for Young Scholars of XYNU (2016054); Scientific Research Innovation Project for Postgraduate of XYNU (2018KYJJ47); Major Science and Technology Project in Henan Province (121100110200); National Innovation and Entrepreneurship Training Program for Undergraduates (201810477004); Student Research Fund Project of XYNU (2018DXS066); Key Scientific Research Projects of Universities in Henan Province (19A180030); Institute for Conservation and Utilization of Agrobioresources in Dabie Mountains.
Bo PENG (1980-), male, P. R. China, associate professor, devoted to research about genetic breeding of rice.
*Corresponding author. Email: pengbo@xynu.edu.cn; yhongyu92@163.com.
Amino acids play an important role in the growth, development and metabolism of plants, which is because amino acids are the basic components of various enzymes and proteins in plants, and are precursors or nitrogen sources for nucleic acids, chloroplasts, hormones and secondary metabolites in plants, which are essential for the growth and development in plants. Amino acids can be synthesized by plastids, cytoplasm, mitochondria and peroxisomes in roots or leaf cells of plants［1］, and plants can also absorb amino acids directly from the soil or ultimately convert inorganic nitrogen into amino acids［2］. Some of the amino acids synthesized in plants or absorbed from outside are immediately metabolized, and some are temporarily stored or transported through the phloem to growing parts or sink organs of plants［3-4］. In all these processes, amino acid transporters (AATs) are essential. A large number of studies have also shown that AATs are a key regulatory gene family in plant metabolism［3-9］, and amino acid transporters encoded by them play an important role in plant growth and development. There are at least 5 AAT gene families, such as amino acidpolyaminecholine (APC) superfamily, sodiumdicarboxylate symporter (SDS) superfamily, neurotransmitter superfamily (NTS), amino acid transporter superfamily 1 (ATF1) and amino acid transporters within the major facilitator superfamily (MFS)［6］. In plants, APC transporter superfamily mainly include two gene families: Amino acid/auxin permease (AAAP) and APC families, of which AAAP family includes amino acid permeases (AAPs), lysinehistidinelike transporters (LHTs) and proline transporters (ProTs), γaminobutyric acid transporters (GATs), ANT1like aromatic, and neutral amino acid transporters and auxin transporters (AUXs )［1,6,8,11］, and APC family included cationic amino acid transporters (CATs), amino acid/choline transporters (ACTs) and polyamine H+symporters (PHSs)［8,12-13］.
In Arabidopsis thaliana, more than 60 AATs have been identified, and 85 AATs that may exist in rice are distributed on 12 chromosomes of rice. However, Most AATs belong to the AAAP superfamily, and some belong to the APC superfamily. Among them, many different AATs are relatively conservative［8］. In the past ten years, a great progress has been made on AATs in the field of plants, the research object has evolved from A. thaliana to important food crops, and more and more members in AAT gene family have bveen separated, cloned, and anazlyed for function in crops［9-12］. Therefore, this study reviewed recent advances on AATs in plant research, including expression, regulation, function, and application of AATs in genetic crop improvement, aiming at providing reference for the indepth study of AATs in plants, especially important food crops.
The Expression of AATs in Plants
Strategies for gene knockout or inhibition of gene expression are often used in the study of gene function, but these mutants do not necessarily produce desired effects when studying the function of A. thaliana AATs. For instance, there is no significant difference between the TDNAinserted mutant of AtAAP3 and the wild type under the same growth condition; and the phenotype of RNAi mutant of AtAAP1 is also the same of normal A. thaliana as control［4,13］. AtAAP2, AtAAP5 and AtAAP6 could be coexpressed with AtAAP3 in roots of A. thaliana, and the amino acid permeases encoded by these three genes and AtAAP3 transport similar amino acids in A. thaliana. Therefore, AtAAP2, AtAAP5 and AtAAP6 may complement the function of AtAAP3 to some extent［4］. Similarly, AtAAP5 is mainly expressed in prophyllum, flowers and seeds, which is similar to the expression pattern of AtAAP1, which might be due to that AtAAP5 can complement the function of AtAAP1 and cause no significant change in the phenotype of AtAAP1 mutant. There are also functional studies of AATrelated genes in rice. Thirteen AATs were found in the mutant library of rice, and most of the mutants were found to not affect the yield traits such as tiller number and 1 000grain weight, while osaa49 could reduce rice yield by 37.7%［12］. Some mutants have an important inhibitory effect on the biomass of rice plants (such as mutant osaa5 and osaa7) or a promoting effect (such as mutant osaa24). Further studies have found that these mutants can change the ratio of carbon to nitrogen in rice seed and its relative content［12］. A gene (Bh4) that controls rice husks in black (in Oryza sativa) was also identified in rice and was found to encode an amino acid transporter. This gene has a 22bp deletion mutation in the third exon in cultivated rice, which will cause rice husk color to turn to pale yellow, and it is speculated that Bh4 may be related to domestication［7］. Recently, we cloned from a natural population in rice, a major QTL gene, OsAAP6, which controls the content of grain storage proteins in rice, and belongs to the amino acid permease gene subfamily in the amino acid transporter gene family［9］. OsAAP6 gene is a constitutively expressed gene, which is expressed at a relatively higher level in rice microtubule tissue. OsAAP6 regulates the grain nutritional quality of rice and affects its eating quality by regulating the synthesis and accumulation of grain storage protein and starch in rice, while how does OsAAP6 regulate this process still needs further study.
Application of AATs in Genetic Development of Crops
There are a variety of amino acids in plants that play a very important role in the growth, development and metabolism in plants. This is because amino acids are the basic components for synthesis of various enzymes and proteins, and amino acids are precursors or nitrogen donors of some substances that are important to plant development (such as nucleic acids, chloroplasts, hormones, and secondary metabolites)［59］. However, amino acid transporters play an important role in invivo or intercellular transport of amino acids. In the case of A. thaliana in the dicotyledon, a large number of AAT genes have been successfully isolated and cloned, and the function of these genes have been studied intensively; and in monocotyledonous rice, the members of AAT gene family have been identified in whole genome, their expression patterns and molecular features have been resolved and validated［8,12］, and the related rice mutants have also been tested in the field［12］. Two AAT gene family members have been cloned from rice which is an important food crop, and their functions have been revealed［7,9］. They are found to play an extremely important role in crop genetic improvement. However, the molecular mechanism by which AATencoded amino acid transporters transport amino acids to or from cells is not well understood. How do AATs regulate their functions? And how do they play an important role in the amino acid signaling pathway? It is believed that with the rapid development of biotechnology and functional genomics, more and more AATs are isolated and cloned in model plants, and their biological functions will be gradually analyzed and applied to genetic improvement of major food crops, to accelerate the breeding process of highquality, highyield and multiresistant new crop varieties. References
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Editor: Yingzhi GUANGProofreader: Xinxiu ZHU
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Editor: Chunmei WUProofreader: Xinxiu ZHU
Key wordsAATs; Expression; Regulation; Function; Application
Received: June 29, 2018Accepted: September 30, 2018
Supported by National Natural Science Foundation of China (U1604110, U1404319, 31600992, 31801332); Key Project of Science and Technology in Henan Province (182102110442, 152102110036); Nanhu Scholars Program for Young Scholars of XYNU (2016054); Scientific Research Innovation Project for Postgraduate of XYNU (2018KYJJ47); Major Science and Technology Project in Henan Province (121100110200); National Innovation and Entrepreneurship Training Program for Undergraduates (201810477004); Student Research Fund Project of XYNU (2018DXS066); Key Scientific Research Projects of Universities in Henan Province (19A180030); Institute for Conservation and Utilization of Agrobioresources in Dabie Mountains.
Bo PENG (1980-), male, P. R. China, associate professor, devoted to research about genetic breeding of rice.
*Corresponding author. Email: pengbo@xynu.edu.cn; yhongyu92@163.com.
Amino acids play an important role in the growth, development and metabolism of plants, which is because amino acids are the basic components of various enzymes and proteins in plants, and are precursors or nitrogen sources for nucleic acids, chloroplasts, hormones and secondary metabolites in plants, which are essential for the growth and development in plants. Amino acids can be synthesized by plastids, cytoplasm, mitochondria and peroxisomes in roots or leaf cells of plants［1］, and plants can also absorb amino acids directly from the soil or ultimately convert inorganic nitrogen into amino acids［2］. Some of the amino acids synthesized in plants or absorbed from outside are immediately metabolized, and some are temporarily stored or transported through the phloem to growing parts or sink organs of plants［3-4］. In all these processes, amino acid transporters (AATs) are essential. A large number of studies have also shown that AATs are a key regulatory gene family in plant metabolism［3-9］, and amino acid transporters encoded by them play an important role in plant growth and development. There are at least 5 AAT gene families, such as amino acidpolyaminecholine (APC) superfamily, sodiumdicarboxylate symporter (SDS) superfamily, neurotransmitter superfamily (NTS), amino acid transporter superfamily 1 (ATF1) and amino acid transporters within the major facilitator superfamily (MFS)［6］. In plants, APC transporter superfamily mainly include two gene families: Amino acid/auxin permease (AAAP) and APC families, of which AAAP family includes amino acid permeases (AAPs), lysinehistidinelike transporters (LHTs) and proline transporters (ProTs), γaminobutyric acid transporters (GATs), ANT1like aromatic, and neutral amino acid transporters and auxin transporters (AUXs )［1,6,8,11］, and APC family included cationic amino acid transporters (CATs), amino acid/choline transporters (ACTs) and polyamine H+symporters (PHSs)［8,12-13］.
In Arabidopsis thaliana, more than 60 AATs have been identified, and 85 AATs that may exist in rice are distributed on 12 chromosomes of rice. However, Most AATs belong to the AAAP superfamily, and some belong to the APC superfamily. Among them, many different AATs are relatively conservative［8］. In the past ten years, a great progress has been made on AATs in the field of plants, the research object has evolved from A. thaliana to important food crops, and more and more members in AAT gene family have bveen separated, cloned, and anazlyed for function in crops［9-12］. Therefore, this study reviewed recent advances on AATs in plant research, including expression, regulation, function, and application of AATs in genetic crop improvement, aiming at providing reference for the indepth study of AATs in plants, especially important food crops.
The Expression of AATs in Plants
Strategies for gene knockout or inhibition of gene expression are often used in the study of gene function, but these mutants do not necessarily produce desired effects when studying the function of A. thaliana AATs. For instance, there is no significant difference between the TDNAinserted mutant of AtAAP3 and the wild type under the same growth condition; and the phenotype of RNAi mutant of AtAAP1 is also the same of normal A. thaliana as control［4,13］. AtAAP2, AtAAP5 and AtAAP6 could be coexpressed with AtAAP3 in roots of A. thaliana, and the amino acid permeases encoded by these three genes and AtAAP3 transport similar amino acids in A. thaliana. Therefore, AtAAP2, AtAAP5 and AtAAP6 may complement the function of AtAAP3 to some extent［4］. Similarly, AtAAP5 is mainly expressed in prophyllum, flowers and seeds, which is similar to the expression pattern of AtAAP1, which might be due to that AtAAP5 can complement the function of AtAAP1 and cause no significant change in the phenotype of AtAAP1 mutant. There are also functional studies of AATrelated genes in rice. Thirteen AATs were found in the mutant library of rice, and most of the mutants were found to not affect the yield traits such as tiller number and 1 000grain weight, while osaa49 could reduce rice yield by 37.7%［12］. Some mutants have an important inhibitory effect on the biomass of rice plants (such as mutant osaa5 and osaa7) or a promoting effect (such as mutant osaa24). Further studies have found that these mutants can change the ratio of carbon to nitrogen in rice seed and its relative content［12］. A gene (Bh4) that controls rice husks in black (in Oryza sativa) was also identified in rice and was found to encode an amino acid transporter. This gene has a 22bp deletion mutation in the third exon in cultivated rice, which will cause rice husk color to turn to pale yellow, and it is speculated that Bh4 may be related to domestication［7］. Recently, we cloned from a natural population in rice, a major QTL gene, OsAAP6, which controls the content of grain storage proteins in rice, and belongs to the amino acid permease gene subfamily in the amino acid transporter gene family［9］. OsAAP6 gene is a constitutively expressed gene, which is expressed at a relatively higher level in rice microtubule tissue. OsAAP6 regulates the grain nutritional quality of rice and affects its eating quality by regulating the synthesis and accumulation of grain storage protein and starch in rice, while how does OsAAP6 regulate this process still needs further study.
Application of AATs in Genetic Development of Crops
There are a variety of amino acids in plants that play a very important role in the growth, development and metabolism in plants. This is because amino acids are the basic components for synthesis of various enzymes and proteins, and amino acids are precursors or nitrogen donors of some substances that are important to plant development (such as nucleic acids, chloroplasts, hormones, and secondary metabolites)［59］. However, amino acid transporters play an important role in invivo or intercellular transport of amino acids. In the case of A. thaliana in the dicotyledon, a large number of AAT genes have been successfully isolated and cloned, and the function of these genes have been studied intensively; and in monocotyledonous rice, the members of AAT gene family have been identified in whole genome, their expression patterns and molecular features have been resolved and validated［8,12］, and the related rice mutants have also been tested in the field［12］. Two AAT gene family members have been cloned from rice which is an important food crop, and their functions have been revealed［7,9］. They are found to play an extremely important role in crop genetic improvement. However, the molecular mechanism by which AATencoded amino acid transporters transport amino acids to or from cells is not well understood. How do AATs regulate their functions? And how do they play an important role in the amino acid signaling pathway? It is believed that with the rapid development of biotechnology and functional genomics, more and more AATs are isolated and cloned in model plants, and their biological functions will be gradually analyzed and applied to genetic improvement of major food crops, to accelerate the breeding process of highquality, highyield and multiresistant new crop varieties. References
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