Indole-3-acetic acid (IAA) is a primary phytohormone that regulates multiple aspects of plant dev... more Indole-3-acetic acid (IAA) is a primary phytohormone that regulates multiple aspects of plant development. Because polar transport of IAA is an essential determinant of organogenesis and dynamic tropic growth, methods to monitor IAA movement in vivo are in demand. A self-referencing electrochemical microsensor was optimized to non-invasively measure endogenous IAA flux near the surface of Zea mays roots without the addition of exogenous IAA. Enhanced sensor surface modification, decoupling of acquired signals, and integrated flux analyses were combined to provide direct, real time quantification of endogenous IAA movement in B73 maize inbred and brachytic2 (br2) auxin transport mutant roots. BR2 is localized in epidermal and hypodermal tissues at the root apex. br2 roots exhibit reduced shootward IAA transport at the root apex in radiotracer experiments and reduced gravitropic growth. IAA flux data indicates that maximal transport occurs in the distal elongation zone of maize roots, and net transport in/out of br2 roots was decreased compared to B73. Integration of short term real time flux data in this zone revealed oscillatory patterns, with B73 exhibiting shorter oscillatory periods and greater amplitude than br2. IAA efflux and influx were inhibited using 1-N-naphthylphthalamic acid (NPA), and 2-naphthoxyacetic acid (NOA), respectively. A simple harmonic oscillation model of these data produced a correlation between modeled and measured values of 0.70 for B73 and 0.69 for br2. These results indicate that this technique is useful for real-time IAA transport monitoring in surface tissues and that this approach can be performed simultaneously with current live imaging techniques.
Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll ... more Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll synthesis and induction of expression of the mRNAs encoding the major polypeptides of the light-harvesting complex of photosystem I1 (Lhcb genes). I n contrast, many conifers, including pines, firs, and spruces, can accumulate chlorophyll and the light-harvesting chlorophyll a/b-binding proteins of photosystem II in complete darkness. To understand the factors responsible for the regulation of expression of individual Lhcb mRNAs in the pine Pinus palustris, we have prepared sequence-specific cDNA probes for each of three family members, Lhcbf'Ppl, Lhcb2*Ppl, and Lhcb2*Pp2, and have studied the expression of two of these, Lhcbl'Ppl and Lhcb2*Pp2, in detail. The levels of expression of each sequence were disparate, and Lhcbl'Ppl-encoded transcripts were the most abundant i n the light. Both Lhcbl*Ppl and Lhcb2'PpZ mRNAs were expressed i n stems and cotyledons, but Lhcbl'Ppl mRNA was present at about 10-fold lower levels in stems than in cotyledons, i n contrast to Lhcb2*Pp2 mRNA, which was expressed at higher levels in stems than i n cotyledons. Both Lhcbl*Ppl and Lhcb2*Pp2 mRNAs were absent in embryos but were expressed during seedling development. The levels increased with age in both the light and the dark and in both cases were about 2-fold higher i n the light than in the dark. Despite the expression of Lhcbl'Ppl and Lhcb2*Pp2 mRNAs during development in darkness, the levels of both mRNAs increased in dark-grown seedlings given red light i n the low fluence range within 2 h of treatment.
Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll ... more Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll synthesis and induction of expression of the mRNAs encoding the major polypeptides of the light-harvesting complex of photosystem I1 (Lhcb genes). I n contrast, many conifers, including pines, firs, and spruces, can accumulate chlorophyll and the light-harvesting chlorophyll a/b-binding proteins of photosystem II in complete darkness. To understand the factors responsible for the regulation of expression of individual Lhcb mRNAs in the pine Pinus palustris, we have prepared sequence-specific cDNA probes for each of three family members, Lhcbf'Ppl, Lhcb2*Ppl, and Lhcb2*Pp2, and have studied the expression of two of these, Lhcbl'Ppl and Lhcb2*Pp2, in detail. The levels of expression of each sequence were disparate, and Lhcbl'Ppl-encoded transcripts were the most abundant i n the light. Both Lhcbl*Ppl and Lhcb2'PpZ mRNAs were expressed i n stems and cotyledons, but Lhcbl'Ppl mRNA was present at about 10-fold lower levels in stems than in cotyledons, i n contrast to Lhcb2*Pp2 mRNA, which was expressed at higher levels in stems than i n cotyledons. Both Lhcbl*Ppl and Lhcb2*Pp2 mRNAs were absent in embryos but were expressed during seedling development. The levels increased with age in both the light and the dark and in both cases were about 2-fold higher i n the light than in the dark. Despite the expression of Lhcbl'Ppl and Lhcb2*Pp2 mRNAs during development in darkness, the levels of both mRNAs increased in dark-grown seedlings given red light i n the low fluence range within 2 h of treatment.
Metallopeptidases of the M1 family are found in all phyla (except viruses) and are important in t... more Metallopeptidases of the M1 family are found in all phyla (except viruses) and are important in the cell cycle and normal growth and development. M1s often have spatiotemporal expression patterns which allow for strict regulation of activity. Mutations in the genes encoding M1s result in disease and are often lethal. This family of zinc metallopeptidases all share the catalytic region containing a signature amino acid exopeptidase (GXMXN) and a zinc binding (HEXXH[18X]E) motif. In addition, M1 aminopeptidases often also contain additional membrane association and/or protein interaction motifs. These protein interaction domains may function independently of M1 enzymatic activity and can contribute to multifunctionality of the proteins. A brief review of M1 metalloproteases in plants and animals and their roles in the cell cycle is presented. In animals, human puromycin-sensitive aminopeptidase (PSA) acts during mitosis and perhaps meiosis, while the insect homologue puromycin-sensiti...
The plasma membrane is the interface between the cytosol and the external environment. The protei... more The plasma membrane is the interface between the cytosol and the external environment. The proteins that reside and function on the plasma membrane regulate the cellular entrance and exit of bioactive molecules, actuate signaling cascades in response to external stimuli, and potentiate interactions between cells. The presence and abundance of proteins on the plasma membrane is regulated by anterograde and retrograde intracellular vesicular trafficking, exocytosis, and endocytosis. The cytoskeleton is an integral component of cellular trafficking mechanisms, as the vesicles and endosomes move on actin filaments or microtubules. Selection and movement of the protein cargo to be trafficked to and from the plasma membrane depends to a great extent on signature organellar targeting motifs within the proteins themselves as well as interactions with various adaptor proteins. Endocytosis is essential not only to the recycling/turnover of plasma membrane proteins, but it also functions in dynamic processes that recycle proteins back to the plasma membrane. Some evidence suggests that transcytotic trafficking mechanisms function in plants, although these are distinct from basolateral -apical redirection mechanisms characterized in animal cells.
Members of the ABC (for ATP binding cassette) superfamily of integral membrane transporters funct... more Members of the ABC (for ATP binding cassette) superfamily of integral membrane transporters function in cellular detoxification, cell-to-cell signaling, and channel regulation. More recently, members of the multidrug resistance P-glycoprotein (MDR/PGP) subfamily of ABC transporters have been shown to function in the transport of the phytohormone auxin in both monocots and dicots. Here, we report that the Arabidopsis thaliana MDR/PGP PGP4 functions in the basipetal redirection of auxin from the root tip. Reporter gene studies showed that PGP4 was strongly expressed in root cap and epidermal cells. PGP4 exhibits apolar plasma membrane localization in the root cap and polar localization in tissues above. Root gravitropic bending and elongation as well as lateral root formation were reduced in pgp4 mutants compared with the wild type. pgp4 exhibited reduced basipetal auxin transport in roots and a small decrease in shoot-to-root transport consistent with a partial loss of the redirective auxin sink in the root. Seedlings overexpressing PGP4 exhibited increased shoot-to-root auxin transport. Heterologous expression of PGP4 in mammalian cells resulted in 1-N-naphthylthalamic acid-reversible net uptake of [ 3 H]indole-3-acetic acid. These results indicate that PGP4 functions primarily in the uptake of redirected or newly synthesized auxin in epidermal root cells. Roots thaliana Arabidopsis PGP4, an ATP Binding Cassette P-Glycoprotein, Catalyzes Auxin Transport in This information is current as of November 3, 2015 Supplemental Data http://www.plantcell.org/content/suppl/2005/10/05/tpc.105.035816.DC1.html References
Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll ... more Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll synthesis and induction of expression of the mRNAs encoding the major polypeptides of the light-harvesting complex of photosystem I1 (Lhcb genes). I n contrast, many conifers, including pines, firs, and spruces, can accumulate chlorophyll and the light-harvesting chlorophyll a/b-binding proteins of photosystem II in complete darkness. To understand the factors responsible for the regulation of expression of individual Lhcb mRNAs in the pine Pinus palustris, we have prepared sequence-specific cDNA probes for each of three family members, Lhcbf'Ppl, Lhcb2*Ppl, and Lhcb2*Pp2, and have studied the expression of two of these, Lhcbl'Ppl and Lhcb2*Pp2, in detail. The levels of expression of each sequence were disparate, and Lhcbl'Ppl-encoded transcripts were the most abundant i n the light. Both Lhcbl*Ppl and Lhcb2'PpZ mRNAs were expressed i n stems and cotyledons, but Lhcbl'Ppl mRNA was present at about 10-fold lower levels in stems than in cotyledons, i n contrast to Lhcb2*Pp2 mRNA, which was expressed at higher levels in stems than i n cotyledons. Both Lhcbl*Ppl and Lhcb2*Pp2 mRNAs were absent in embryos but were expressed during seedling development. The levels increased with age in both the light and the dark and in both cases were about 2-fold higher i n the light than in the dark. Despite the expression of Lhcbl'Ppl and Lhcb2*Pp2 mRNAs during development in darkness, the levels of both mRNAs increased in dark-grown seedlings given red light i n the low fluence range within 2 h of treatment.
Aminopeptidase M1 (APM1) is essential for embryonic, vegetative, and reproductive development in ... more Aminopeptidase M1 (APM1) is essential for embryonic, vegetative, and reproductive development in Arabidopsis (Arabidopsis thaliana). Here, we show that, like mammalian M1 proteases, APM1 appears to have distinct enzymatic and protein-protein interaction domains and functions as a homodimer. Arabidopsis seedlings treated with ezetimibe, an inhibitor of M1 proteinprotein interactions, mimicked a subset of apm1 phenotypes distinct from those resulting from treatment with PAQ-22, an inhibitor of M1 catalytic activity, suggesting that the APM1 catalytic and interaction domains can function independently. apm1-1 knockdown mutants transformed with catalytically inactive APM1 did not prevent seedling lethality. However, apm1-2 has a functional enzymatic domain but lacks the carboxyl (C) terminus, and transformation with catalytically inactive APM1 rescued the mutant. Overexpression of human insulin-responsive aminopeptidase/oxytocinase rescued all apm1 phenotypes, suggesting that the catalytic activity was sufficient to compensate for loss of APM1 function, while overexpression of catalytically inactive insulin-responsive aminopeptidase/oxytocinase only rescued apm1-2. Increased catalytic activity alone is not sufficient to compensate for loss of APM1 function, as overexpression of another Arabidopsis M1 family member lacking an extended C terminus did not rescue apm1-1. The protein interactions facilitating enzymatic activity appear to be dependent on the C terminus of APM1, as transformation with an open reading frame containing an internal deletion of a portion of the C terminus or a point mutation in a dileucine motif did not rescue the mutant. These results suggest that both the catalytic and interaction domains are necessary for APM1 function but that APM1 function and dimerization do not require these domains to be present in the same linear molecule.
Polar transport of the phytohormone auxin and the establishment of localized auxin maxima regulat... more Polar transport of the phytohormone auxin and the establishment of localized auxin maxima regulate embryonic development, stem cell maintenance, root and shoot architecture, and tropic growth responses. The past decade has been marked by dramatic progress in efforts to elucidate the complex mechanisms by which auxin transport regulates plant growth. As the understanding of auxin transport regulation has been increasingly elaborated, it has become clear that this process is involved in almost all plant growth and environmental responses in some way. However, we still lack information about some basic aspects of this fundamental regulatory mechanism. In this review, we present what we know (or what we think we know) and what we do not know about seven auxin-regulated processes. We discuss the role of auxin transport in gravitropism in primary and lateral roots, phototropism, shoot branching, leaf expansion, and venation. We also discuss the auxin reflux/fountain model at the root tip, flavonoid modulation of auxin transport processes, and outstanding aspects of post-translational regulation of auxin transporters. This discussion is not meant to be exhaustive, but highlights areas in which generally held assumptions require more substantive validation.
Dwarfism traits in Zea mays are regulated by multiple factors including the hormone auxin. Dwarf ... more Dwarfism traits in Zea mays are regulated by multiple factors including the hormone auxin. Dwarf brachytic2 (br2) mutants harbour lesions in the gene encoding an orthologue of Arabidopsis thaliana ABCB1 which functions in auxin efflux out of meristematic regions in the shoot and root. br2 mesocotyls and coleoptiles exhibit reduced auxin transport. However, the dwarf stature of br2 derives from shortened lower internodes whilst the upper portion of the plant is completely normal. As such, it is counter-intuitive to attribute br2 dwarfism exclusively to reduced auxin export out of the shoot apex. Arabidopsis abcb1 mutants exhibit only minor reductions in auxin transport and plant height unless combined with mutations in the ABCB19 auxin transporter. Phylogenetic modelling analysis excludes the possibility that BR2 is more closely related to ABCB19 which has three more closely related orthologues in maize. BR2 is expressed in nodal meristems, and analyses of auxin transport and content indicate that BR2 function in these grass-specific tissues is analogous to ABCB1 function in the shoot and root apex of Arabidopsis. These results indicate that ABCB1/BR2 function is conserved between dicots and monocots, but also suggests that this function must be understood in the context of the segmental organization of grass plants.
Indole-3-acetic acid (IAA) is a primary phytohormone that regulates multiple aspects of plant dev... more Indole-3-acetic acid (IAA) is a primary phytohormone that regulates multiple aspects of plant development. Because polar transport of IAA is an essential determinant of organogenesis and dynamic tropic growth, methods to monitor IAA movement in vivo are in demand. A self-referencing electrochemical microsensor was optimized to non-invasively measure endogenous IAA flux near the surface of Zea mays roots without the addition of exogenous IAA. Enhanced sensor surface modification, decoupling of acquired signals, and integrated flux analyses were combined to provide direct, real time quantification of endogenous IAA movement in B73 maize inbred and brachytic2 (br2) auxin transport mutant roots. BR2 is localized in epidermal and hypodermal tissues at the root apex. br2 roots exhibit reduced shootward IAA transport at the root apex in radiotracer experiments and reduced gravitropic growth. IAA flux data indicates that maximal transport occurs in the distal elongation zone of maize roots, and net transport in/out of br2 roots was decreased compared to B73. Integration of short term real time flux data in this zone revealed oscillatory patterns, with B73 exhibiting shorter oscillatory periods and greater amplitude than br2. IAA efflux and influx were inhibited using 1-N-naphthylphthalamic acid (NPA), and 2-naphthoxyacetic acid (NOA), respectively. A simple harmonic oscillation model of these data produced a correlation between modeled and measured values of 0.70 for B73 and 0.69 for br2. These results indicate that this technique is useful for real-time IAA transport monitoring in surface tissues and that this approach can be performed simultaneously with current live imaging techniques.
Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll ... more Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll synthesis and induction of expression of the mRNAs encoding the major polypeptides of the light-harvesting complex of photosystem I1 (Lhcb genes). I n contrast, many conifers, including pines, firs, and spruces, can accumulate chlorophyll and the light-harvesting chlorophyll a/b-binding proteins of photosystem II in complete darkness. To understand the factors responsible for the regulation of expression of individual Lhcb mRNAs in the pine Pinus palustris, we have prepared sequence-specific cDNA probes for each of three family members, Lhcbf'Ppl, Lhcb2*Ppl, and Lhcb2*Pp2, and have studied the expression of two of these, Lhcbl'Ppl and Lhcb2*Pp2, in detail. The levels of expression of each sequence were disparate, and Lhcbl'Ppl-encoded transcripts were the most abundant i n the light. Both Lhcbl*Ppl and Lhcb2'PpZ mRNAs were expressed i n stems and cotyledons, but Lhcbl'Ppl mRNA was present at about 10-fold lower levels in stems than in cotyledons, i n contrast to Lhcb2*Pp2 mRNA, which was expressed at higher levels in stems than i n cotyledons. Both Lhcbl*Ppl and Lhcb2*Pp2 mRNAs were absent in embryos but were expressed during seedling development. The levels increased with age in both the light and the dark and in both cases were about 2-fold higher i n the light than in the dark. Despite the expression of Lhcbl'Ppl and Lhcb2*Pp2 mRNAs during development in darkness, the levels of both mRNAs increased in dark-grown seedlings given red light i n the low fluence range within 2 h of treatment.
Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll ... more Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll synthesis and induction of expression of the mRNAs encoding the major polypeptides of the light-harvesting complex of photosystem I1 (Lhcb genes). I n contrast, many conifers, including pines, firs, and spruces, can accumulate chlorophyll and the light-harvesting chlorophyll a/b-binding proteins of photosystem II in complete darkness. To understand the factors responsible for the regulation of expression of individual Lhcb mRNAs in the pine Pinus palustris, we have prepared sequence-specific cDNA probes for each of three family members, Lhcbf'Ppl, Lhcb2*Ppl, and Lhcb2*Pp2, and have studied the expression of two of these, Lhcbl'Ppl and Lhcb2*Pp2, in detail. The levels of expression of each sequence were disparate, and Lhcbl'Ppl-encoded transcripts were the most abundant i n the light. Both Lhcbl*Ppl and Lhcb2'PpZ mRNAs were expressed i n stems and cotyledons, but Lhcbl'Ppl mRNA was present at about 10-fold lower levels in stems than in cotyledons, i n contrast to Lhcb2*Pp2 mRNA, which was expressed at higher levels in stems than i n cotyledons. Both Lhcbl*Ppl and Lhcb2*Pp2 mRNAs were absent in embryos but were expressed during seedling development. The levels increased with age in both the light and the dark and in both cases were about 2-fold higher i n the light than in the dark. Despite the expression of Lhcbl'Ppl and Lhcb2*Pp2 mRNAs during development in darkness, the levels of both mRNAs increased in dark-grown seedlings given red light i n the low fluence range within 2 h of treatment.
Metallopeptidases of the M1 family are found in all phyla (except viruses) and are important in t... more Metallopeptidases of the M1 family are found in all phyla (except viruses) and are important in the cell cycle and normal growth and development. M1s often have spatiotemporal expression patterns which allow for strict regulation of activity. Mutations in the genes encoding M1s result in disease and are often lethal. This family of zinc metallopeptidases all share the catalytic region containing a signature amino acid exopeptidase (GXMXN) and a zinc binding (HEXXH[18X]E) motif. In addition, M1 aminopeptidases often also contain additional membrane association and/or protein interaction motifs. These protein interaction domains may function independently of M1 enzymatic activity and can contribute to multifunctionality of the proteins. A brief review of M1 metalloproteases in plants and animals and their roles in the cell cycle is presented. In animals, human puromycin-sensitive aminopeptidase (PSA) acts during mitosis and perhaps meiosis, while the insect homologue puromycin-sensiti...
The plasma membrane is the interface between the cytosol and the external environment. The protei... more The plasma membrane is the interface between the cytosol and the external environment. The proteins that reside and function on the plasma membrane regulate the cellular entrance and exit of bioactive molecules, actuate signaling cascades in response to external stimuli, and potentiate interactions between cells. The presence and abundance of proteins on the plasma membrane is regulated by anterograde and retrograde intracellular vesicular trafficking, exocytosis, and endocytosis. The cytoskeleton is an integral component of cellular trafficking mechanisms, as the vesicles and endosomes move on actin filaments or microtubules. Selection and movement of the protein cargo to be trafficked to and from the plasma membrane depends to a great extent on signature organellar targeting motifs within the proteins themselves as well as interactions with various adaptor proteins. Endocytosis is essential not only to the recycling/turnover of plasma membrane proteins, but it also functions in dynamic processes that recycle proteins back to the plasma membrane. Some evidence suggests that transcytotic trafficking mechanisms function in plants, although these are distinct from basolateral -apical redirection mechanisms characterized in animal cells.
Members of the ABC (for ATP binding cassette) superfamily of integral membrane transporters funct... more Members of the ABC (for ATP binding cassette) superfamily of integral membrane transporters function in cellular detoxification, cell-to-cell signaling, and channel regulation. More recently, members of the multidrug resistance P-glycoprotein (MDR/PGP) subfamily of ABC transporters have been shown to function in the transport of the phytohormone auxin in both monocots and dicots. Here, we report that the Arabidopsis thaliana MDR/PGP PGP4 functions in the basipetal redirection of auxin from the root tip. Reporter gene studies showed that PGP4 was strongly expressed in root cap and epidermal cells. PGP4 exhibits apolar plasma membrane localization in the root cap and polar localization in tissues above. Root gravitropic bending and elongation as well as lateral root formation were reduced in pgp4 mutants compared with the wild type. pgp4 exhibited reduced basipetal auxin transport in roots and a small decrease in shoot-to-root transport consistent with a partial loss of the redirective auxin sink in the root. Seedlings overexpressing PGP4 exhibited increased shoot-to-root auxin transport. Heterologous expression of PGP4 in mammalian cells resulted in 1-N-naphthylthalamic acid-reversible net uptake of [ 3 H]indole-3-acetic acid. These results indicate that PGP4 functions primarily in the uptake of redirected or newly synthesized auxin in epidermal root cells. Roots thaliana Arabidopsis PGP4, an ATP Binding Cassette P-Glycoprotein, Catalyzes Auxin Transport in This information is current as of November 3, 2015 Supplemental Data http://www.plantcell.org/content/suppl/2005/10/05/tpc.105.035816.DC1.html References
Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll ... more Angiosperms require light for multiple aspects of chloroplast development, including chlorophyll synthesis and induction of expression of the mRNAs encoding the major polypeptides of the light-harvesting complex of photosystem I1 (Lhcb genes). I n contrast, many conifers, including pines, firs, and spruces, can accumulate chlorophyll and the light-harvesting chlorophyll a/b-binding proteins of photosystem II in complete darkness. To understand the factors responsible for the regulation of expression of individual Lhcb mRNAs in the pine Pinus palustris, we have prepared sequence-specific cDNA probes for each of three family members, Lhcbf'Ppl, Lhcb2*Ppl, and Lhcb2*Pp2, and have studied the expression of two of these, Lhcbl'Ppl and Lhcb2*Pp2, in detail. The levels of expression of each sequence were disparate, and Lhcbl'Ppl-encoded transcripts were the most abundant i n the light. Both Lhcbl*Ppl and Lhcb2'PpZ mRNAs were expressed i n stems and cotyledons, but Lhcbl'Ppl mRNA was present at about 10-fold lower levels in stems than in cotyledons, i n contrast to Lhcb2*Pp2 mRNA, which was expressed at higher levels in stems than i n cotyledons. Both Lhcbl*Ppl and Lhcb2*Pp2 mRNAs were absent in embryos but were expressed during seedling development. The levels increased with age in both the light and the dark and in both cases were about 2-fold higher i n the light than in the dark. Despite the expression of Lhcbl'Ppl and Lhcb2*Pp2 mRNAs during development in darkness, the levels of both mRNAs increased in dark-grown seedlings given red light i n the low fluence range within 2 h of treatment.
Aminopeptidase M1 (APM1) is essential for embryonic, vegetative, and reproductive development in ... more Aminopeptidase M1 (APM1) is essential for embryonic, vegetative, and reproductive development in Arabidopsis (Arabidopsis thaliana). Here, we show that, like mammalian M1 proteases, APM1 appears to have distinct enzymatic and protein-protein interaction domains and functions as a homodimer. Arabidopsis seedlings treated with ezetimibe, an inhibitor of M1 proteinprotein interactions, mimicked a subset of apm1 phenotypes distinct from those resulting from treatment with PAQ-22, an inhibitor of M1 catalytic activity, suggesting that the APM1 catalytic and interaction domains can function independently. apm1-1 knockdown mutants transformed with catalytically inactive APM1 did not prevent seedling lethality. However, apm1-2 has a functional enzymatic domain but lacks the carboxyl (C) terminus, and transformation with catalytically inactive APM1 rescued the mutant. Overexpression of human insulin-responsive aminopeptidase/oxytocinase rescued all apm1 phenotypes, suggesting that the catalytic activity was sufficient to compensate for loss of APM1 function, while overexpression of catalytically inactive insulin-responsive aminopeptidase/oxytocinase only rescued apm1-2. Increased catalytic activity alone is not sufficient to compensate for loss of APM1 function, as overexpression of another Arabidopsis M1 family member lacking an extended C terminus did not rescue apm1-1. The protein interactions facilitating enzymatic activity appear to be dependent on the C terminus of APM1, as transformation with an open reading frame containing an internal deletion of a portion of the C terminus or a point mutation in a dileucine motif did not rescue the mutant. These results suggest that both the catalytic and interaction domains are necessary for APM1 function but that APM1 function and dimerization do not require these domains to be present in the same linear molecule.
Polar transport of the phytohormone auxin and the establishment of localized auxin maxima regulat... more Polar transport of the phytohormone auxin and the establishment of localized auxin maxima regulate embryonic development, stem cell maintenance, root and shoot architecture, and tropic growth responses. The past decade has been marked by dramatic progress in efforts to elucidate the complex mechanisms by which auxin transport regulates plant growth. As the understanding of auxin transport regulation has been increasingly elaborated, it has become clear that this process is involved in almost all plant growth and environmental responses in some way. However, we still lack information about some basic aspects of this fundamental regulatory mechanism. In this review, we present what we know (or what we think we know) and what we do not know about seven auxin-regulated processes. We discuss the role of auxin transport in gravitropism in primary and lateral roots, phototropism, shoot branching, leaf expansion, and venation. We also discuss the auxin reflux/fountain model at the root tip, flavonoid modulation of auxin transport processes, and outstanding aspects of post-translational regulation of auxin transporters. This discussion is not meant to be exhaustive, but highlights areas in which generally held assumptions require more substantive validation.
Dwarfism traits in Zea mays are regulated by multiple factors including the hormone auxin. Dwarf ... more Dwarfism traits in Zea mays are regulated by multiple factors including the hormone auxin. Dwarf brachytic2 (br2) mutants harbour lesions in the gene encoding an orthologue of Arabidopsis thaliana ABCB1 which functions in auxin efflux out of meristematic regions in the shoot and root. br2 mesocotyls and coleoptiles exhibit reduced auxin transport. However, the dwarf stature of br2 derives from shortened lower internodes whilst the upper portion of the plant is completely normal. As such, it is counter-intuitive to attribute br2 dwarfism exclusively to reduced auxin export out of the shoot apex. Arabidopsis abcb1 mutants exhibit only minor reductions in auxin transport and plant height unless combined with mutations in the ABCB19 auxin transporter. Phylogenetic modelling analysis excludes the possibility that BR2 is more closely related to ABCB19 which has three more closely related orthologues in maize. BR2 is expressed in nodal meristems, and analyses of auxin transport and content indicate that BR2 function in these grass-specific tissues is analogous to ABCB1 function in the shoot and root apex of Arabidopsis. These results indicate that ABCB1/BR2 function is conserved between dicots and monocots, but also suggests that this function must be understood in the context of the segmental organization of grass plants.
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