Variation in expression and protein localization of the PIN family of auxin efflux facilitator proteins in flavonoid mutants with altered auxin transport in Arabidopsis thaliana

The secondary metabolism of Arabidopsis thaliana: growing like a weed

Despite its small stature, short life-cycle and highly reduced genome, Arabidopsis thaliana has a complement of secondary metabolites that is every bit as numerous and diverse as those of other plant taxa. The list of secondary metabolites isolated from this model species has expanded more than five-fold in the past ten years, and many more substances are likely to be added in the near future. Among the classes of compounds recently discovered are coumarins, benzenoids and terpenoids. Many A. thaliana secondary metabolites appear to have internal roles within the plant instead of (or in addition to) mediating ecological interactions.

Plant development: auxin in loops

Concentration gradients of the hormone auxin are associated with various patterning events in plants. Recent work has refined our picture of the complex and dynamic system of auxin transport underlying the formation of these gradients.

Auxin: regulation, action, and interaction

BACKGROUND

The phytohormone auxin is critical for plant growth and orchestrates many developmental processes.

SCOPE

This review considers the complex array of mechanisms plants use to control auxin levels, the movement of auxin through the plant, the emerging view of auxin-signalling mechanisms, and several interactions between auxin and other phytohormones. Though many natural and synthetic compounds exhibit auxin-like activity in bioassays, indole-3-acetic acid (IAA) is recognized as the key auxin in most plants. IAA is synthesized both from tryptophan (Trp) using Trp-dependent pathways and from an indolic Trp precursor via Trp-independent pathways; none of these pathways is fully elucidated. Plants can also obtain IAA by beta-oxidation of indole-3-butyric acid (IBA), a second endogenous auxin, or by hydrolysing IAA conjugates, in which IAA is linked to amino acids, sugars or peptides. To permanently inactivate IAA, plants can employ conjugation and direct oxidation. Consistent with its definition as a hormone, IAA can be transported the length of the plant from the shoot to the root; this transport is necessary for normal development, and more localized transport is needed for tropic responses. Auxin signalling is mediated, at least in large part, by an SCFTIR1 E3 ubiquitin ligase complex that accelerates Aux/IAA repressor degradation in response to IAA, thereby altering gene expression. Two classes of auxin-induced genes encode negatively acting products (the Aux/IAA transcriptional repressors and GH3 family of IAA conjugating enzymes), suggesting that timely termination of the auxin signal is crucial. Auxin interaction with other hormone signals adds further challenges to understanding auxin response.

CONCLUSIONS

Nearly six decades after the structural elucidation of IAA, many aspects of auxin metabolism, transport and signalling are well established; however, more than a few fundamental questions and innumerable details remain unresolved.

Complex regulation of Arabidopsis AGR1/PIN2-mediated root gravitropic response and basipetal auxin transport by cantharidin-sensitive protein phosphatases

Polar auxin transport, mediated by two distinct plasma membrane-localized auxin influx and efflux carrier proteins/complexes, plays an important role in many plant growth and developmental processes including tropic responses to gravity and light, development of lateral roots and patterning in embryogenesis. We have previously shown that the Arabidopsis AGRAVITROPIC 1/PIN2 gene encodes an auxin efflux component regulating root gravitropism and basipetal auxin transport. However, the regulatory mechanism underlying the function of AGR1/PIN2 is largely unknown. Recently, protein phosphorylation and dephosphorylation mediated by protein kinases and phosphatases, respectively, have been implicated in regulating polar auxin transport and root gravitropism. Here, we examined the effects of chemical inhibitors of protein phosphatases on root gravitropism and basipetal auxin transport, as well as the expression pattern of AGR1/PIN2 gene and the localization of AGR1/PIN2 protein. We also examined the effects of inhibitors of vesicle trafficking and protein kinases. Our data suggest that protein phosphatases, sensitive to cantharidin and okadaic acid, are likely involved in regulating AGR1/PIN2-mediated root basipetal auxin transport and gravitropism, as well as auxin response in the root central elongation zone (CEZ). BFA-sensitive vesicle trafficking may be required for the cycling of AGR1/PIN2 between plasma membrane and the BFA compartment, but not for the AGR1/PIN2-mediated root basipetal auxin transport and auxin response in CEZ cells.

Sites and regulation of auxin biosynthesis in Arabidopsis roots

Auxin has been shown to be important for many aspects of root development, including initiation and emergence of lateral roots, patterning of the root apical meristem, gravitropism, and root elongation. Auxin biosynthesis occurs in both aerial portions of the plant and in roots; thus, the auxin required for root development could come from either source, or both. To monitor putative internal sites of auxin synthesis in the root, a method for measuring indole-3-acetic acid (IAA) biosynthesis with tissue resolution was developed. We monitored IAA synthesis in 0.5- to 2-mm sections of Arabidopsis thaliana roots and were able to identify an important auxin source in the meristematic region of the primary root tip as well as in the tips of emerged lateral roots. Lower but significant synthesis capacity was observed in tissues upward from the tip, showing that the root contains multiple auxin sources. Root-localized IAA synthesis was diminished in a cyp79B2 cyp79B3 double knockout, suggesting an important role for Trp-dependent IAA synthesis pathways in the root. We present a model for how the primary root is supplied with auxin during early seedling development.

Differential combinatorial interactions of cis-acting elements recognized by R2R3-MYB, BZIP, and BHLH factors control light-responsive and tissue-specific activation of phenylpropanoid biosynthesis genes

Chalcone synthase (CHS), chalcone flavanone isomerase (CFI), flavanone 3-hydroxylase (F3H) and flavonol synthase (FLS) catalyze successive steps in the biosynthetic pathway leading to the production of flavonols. We show that in Arabidopsis thaliana all four corresponding genes are coordinately expressed in response to light, and are spatially coexpressed in siliques, flowers and leaves. Light regulatory units (LRUs) sufficient for light responsiveness were identified in all four promoters. Each unit consists of two necessary elements, namely a MYB-recognition element (MRE) and an ACGT-containing element (ACE). C1 and Sn, a R2R3-MYB and a BHLH factor, respectively, known to control tissue specific anthocyanin biosynthesis in Z. mays, were together able to activate the AtCHS promoter. This activation of the CHS promoter required an intact MRE and a newly identified sequence designated R response element (RREAtCHS) containing the BHLH factor consensus binding site CANNTG. The RRE was dispensable for light responsiveness, and the ACE was not necessary for activation by C1/Sn. These data suggest that a BHLH and a R2R3-MYB factor cooperate in directing tissue-specific production of flavonoids, while an ACE-binding factor, potentially a BZIP, and a R2R3-MYB factor work together in conferring light responsiveness.

Endocytotic cycling of PM proteins

Plasma membrane protein internalization and recycling mechanisms in plants share many features with other eukaryotic organisms. However, functional and structural differences at the cellular and organismal level mandate specialized mechanisms for uptake, sorting, trafficking, and recycling in plants. Recent evidence of plasma membrane cycling of members of the PIN auxin efflux facilitator family and the KAT1 inwardly rectifying potassium channel demonstrates that endocytotic cycling of some form occurs in plants. However, the mechanisms underlying protein internalization and the signals that stimulate endocytosis of proteins from the cell-environment interface are poorly understood. Here we summarize what is known of endocytotic cycling in animals and compare those mechanisms with what is known in plants. We discuss plant orthologs of mammalian-trafficking proteins involved in endocytotic cycling. The use of the styryl dye FM4-64 to define the course of endocytotic uptake and the fungal toxin brefeldin A to dissect the internalization pathways are particularly emphasized. Additionally, we discuss progress in identifying distinct endosomal populations marked by the small GTPases Ara6 and Ara7 as well as recently described examples of apparent cycling of plasma membrane proteins.

AtMYB32 is required for normal pollen development in Arabidopsis thaliana

AtMYB32 gene is a member of the R2R3 MYB gene family coding for transcription factors in Arabidopsis thaliana. Its expression pattern was analysed using Northern blotting, in situ hybridization and promoter-GUS fusions. AtMYB32 is expressed in many tissues, but most strongly in the anther tapetum, stigma papillae and lateral root primordia. AtMYB32-GUS was induced in leaves and stems following wounding, and in root primordia by auxin. T-DNA insertion populations were screened and two insertion mutants were identified, both of which were partially male sterile, more than 50% of the pollen grains being distorted in shape and lacking cytoplasm. AtMYB4 is closely related to AtMYB32 and represses the CINNAMATE 4-HYDROXYLASE gene. Distorted pollen grains were produced in both AtMYB4 insertion mutant and overexpression lines. In an AtMYB32 insertion mutant, the transcript levels of the DIHYDROFLAVONOL 4-REDUCTASE and ANTHOCYANIDIN SYNTHASE genes decreased while the level of the CAFFEIC ACID 0-METHYLTRANSFERASE transcript increased. Change in the levels of AtMYB32 and AtMYB4 expression may influence pollen development by changing the flux along the phenylpropanoid pathways, affecting the composition of the pollen wall.

The PIN and LAX families of auxin transport genes in Medicago truncatula

Auxin transport proteins may be involved in nodule development. As a prelude to investigating the roles of these proteins in nodule development, we took advantage of the genetic and molecular resources available in the legume Medicago truncatula to characterize the gene families encoding auxin efflux and influx carriers. We identified ten auxin efflux carrier sequences (MtPINs) and five auxin influx/permease sequences (MtLAXs). The genomic sequence of each of these fifteen genes was determined, the genes were mapped on the publicly available map of M. truncatula, and their expression was examined in shoot and root tissue of nodulating plants. With one exception, transcripts of all MtPIN genes were detected. The expression of MtPIN2 was limited to nodulating roots, while transcripts of all other expressed genes were detected in both shoots and roots. Both the PIN and LAX gene families contain more members in M. truncatula than in Arabidopsis, but the gene families are not significantly expanded. Sequence comparison of the M. truncatula PIN and LAX genes with PIN and LAX genes from other dicots and monocots indicates that both gene families share a common overall structure, with areas of high homology both within M. truncatula and across species boundaries. Molecular phylogenies of both the PIN and LAX gene families were constructed. Combined with intron position and expression data, the phylogenies were used to assign relationships between MtPIN and MtLAX genes and the orthologous Arabidopsis PIN and LAX genes. MtPIN2 and MtPIN7 appear to be the result of a recent gene duplication with subsequent divergence of expression patterns. These results set the stage for the use of these genes in research on the role of auxin in nodulation.