The plant beneficial rhizobacterium Achromobacter sp. 5B1 influences root development through auxin signaling and redistribution

Roots provide physical and nutritional support to plant organs that are above ground and play critical roles for adaptation via intricate movements and growth patterns. Through screening the effects of bacterial isolates from roots of halophyte Mesquite (Prosopis sp.) on Arabidopsis thaliana, we identified Achromobacter sp. 5B1 as a probiotic bacterium that influences plant functional traits. Detailed genetic and architectural analyses in Arabidopsis grown in vitro and in soil, cell division measurements, auxin transport and response gene expression and brefeldin A treatments demonstrated that root colonization with Achromobacter sp. 5B1 changes the growth and branching patterns of roots, which were related to auxin perception and redistribution. Expression analysis of auxin transport and signaling revealed a redistribution of auxin within the primary root tip of wild‐type seedlings by Achromobacter sp. 5B1 that is disrupted by brefeldin A and correlates with repression of auxin transporters PIN1 and PIN7 in root provasculature, and PIN2 in the epidermis and cortex of the root tip, whereas expression of PIN3 was enhanced in the columella. In seedlings harboring AUX1, EIR1, AXR1, ARF7ARF19, TIR1AFB2AFB3 single, double or triple loss‐of‐function mutations, or in a dominant (gain‐of‐function) mutant of SLR1, the bacterium caused primary roots to form supercoils that are devoid of lateral roots. The changes in growth and root architecture elicited by the bacterium helped Arabidopsis seedlings to resist salt stress better. Thus, Achromobacter sp. 5B1 fine tunes both root movements and the auxin response, which may be important for plant growth and environmental adaptation.     

The axr4 auxin-resistant mutants of Arabidopsis thaliana define a gene important for root gravitropism and lateral root initiation

To understand the molecular mechanism of auxin action, mutants of Arabidopsis thaliana with altered responses to auxin have been identified and characterized. Here the isolation of two auxin-resistant mutants that define a new locus involved in auxin response, named AXR4, is reported. The axr4 mutations are recessive and map near the ch1 mutation on chromosome 1. Mutant plants are specifically resistant to auxin and defective in root gravitropism. Double mutants between axr4 and the recessive auxin-resistant mutants axr1-3 and aux1-7 were characterized to ascertain possible genetic interactions between the mutations. The roots of the axr4 axr1-3 double mutant plants are less sensitive to auxin, respond more slowly to gravity, and form fewer lateral roots than either parental single mutant. These results suggest that the two mutations have additive or even synergistic effects. The AXR1 and AXR4 gene products may therefore act in separate pathways of auxin response or perhaps perform partially redundant functions in a single pathway. The axr4 aux1-7 double mutant has the same sensitivity to auxin as the aux1-7 mutant but forms far fewer lateral roots than either parental single mutant. The aux1-7 mutation thus appears to be epistatic to axr4 with respect to auxin-resistant root elongation, whereas in lateral root formation, the effects of the two mutations are additive. The complexity of the genetic interactions indicated by these results may reflect differences in the mechanism of auxin action during root elongation and the formation of lateral roots. The AXR4 gene product, along with those of the AXR1 and AUX1 genes, is important for normal auxin sensitivity, gravitropic response in roots and lateral root formation.     

Role of AUX1 in the control of organ identity during in vitro organogenesis and in mediating tissue specific auxin and cytokinin interaction in <Emphasis Type="Italic">Arabidopsis</Emphasis>

Classic plant tissue culture experiments have shown that exposure of cell culture to a high auxin to cytokinin ratio promotes root formation and a low auxin to cytokinin ratio leads to shoot regeneration. It has been widely accepted that auxin and cytokinin play an antagonistic role in the control of organ identities during organogenesis in vitro. Since the auxin level is highly elevated in the shoot meristem tissues, it is unclear how a low auxin to cytokinin ratio promotes the regeneration of shoots. To identify genes mediating the cytokinin and auxin interaction during organogenesis in vitro, three allelic mutants that display root instead of shoot regeneration in response to a low auxin to cytokinin ratio are identified using a forward genetic approach in Arabidopsis. Molecular characterization shows that the mutations disrupt the AUX1 gene, which has been reported to regulate auxin influx in plants. Meanwhile, we find that cytokinin substantially stimulates auxin accumulation and redistribution in calli and some specific tissues of Arabidopsis seedlings. In the aux1 mutants, the cytokinin regulated auxin accumulation and redistribution is substantially reduced in both calli and specific tissues of young seedlings. Our results suggest that auxin elevation and other changes stimulated by cytokinin, instead of low auxin or exogenous auxin directly applied, is essential for shoot regeneration. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Participation of Auxin Transport in the Early Response of the <i>Arabidopsis</i> Root System to Inoculation with <i>Azospirillum brasilense</i>

The potential of Plant Growth Promoting Rhizobacteria (PGPR) has been demonstrated in the case of plant inoculation with bacteria of the genus Azospirillum which improves yield. A. brasilense produces a wide variety of molecules, including the natural auxin indole-3-acetic acid (IAA), as well as other phytoregulators. However, several studies have suggested that auxin induces changes in plant development during their interaction with the bacteria. The effects of A. brasilense Sp245 on the development of Arabidopsis thaliana root were investigated to help explain the molecular basis of the interaction. The results obtained showed a decrease in primary root length from the first day and remained so throughout the exposure, accompanied by a stimulation of initiation and maturation of lateral root primordia and an increase of lateral roots. An enhanced auxin response was evident in the vascular tissue and lateral root meristems of inoculated plants. However, after five days of bacterization, the response disappeared in the primary root meristems. The role of polar auxin transport (PAT) in auxins relocation involved the PGP1, AXR4-1, and BEN2 proteins, which apparently mediated A. brasilense-induced root branching of Arabidopsis seedlings.     

Arabidopsis AXR6 encodes CUL1 implicating SCF E3 ligases in auxin regulation of embryogenesis

The AXR6 gene is required for auxin signaling in the Arabidopsis embryo and during postembryonic development. One of the effects of auxin is to stimulate degradation of the Aux/IAA auxin response proteins through the action of the ubiquitin protein ligase SCF(TIR1). Here we show that AXR6 encodes the SCF subunit CUL1. The axr6 mutations affect the ability of mutant CUL1 to assemble into stable SCF complexes resulting in reduced degradation of the SCF(TIR1) substrate AXR2/IAA7. In addition, we show that CUL1 is required for lateral organ initiation in the shoot apical meristem and the inflorescence meristem. These results indicate that the embryonic axr6 phenotype is related to a defect in SCF function and accumulation of Aux/IAA proteins such as BDL/IAA12. In addition, we show that CUL1 has a role in auxin response throughout the life cycle of the plant.     

Synergistic action of auxin and cytokinin mediates aluminum‐induced root growth inhibition in Arabidopsis

Auxin acts synergistically with cytokinin to control the shoot stem‐cell niche, while both hormones act antagonistically to maintain the root meristem. In aluminum (Al) stress‐induced root growth inhibition, auxin plays an important role. However, the role of cytokinin in this process is not well understood. In this study, we show that cytokinin enhances root growth inhibition under stress by mediating Al‐induced auxin signaling. Al stress triggers a local cytokinin response in the root‐apex transition zone (TZ) that depends on IPTs, which encode adenosine phosphate isopentenyltransferases and regulate cytokinin biosynthesis. IPTs are up‐regulated specifically in the root‐apex TZ in response to Al stress and promote local cytokinin biosynthesis and inhibition of root growth. The process of root growth inhibition is also controlled by ethylene signaling which acts upstream of auxin. In summary, different from the situation in the root meristem, auxin acts with cytokinin in a synergistic way to mediate aluminum‐induced root growth inhibition in Arabidopsis.     

SHY2 as a node in the regulation of root meristem development by auxin,brassinosteroids, and cytokinin

In multicellular organisms, the balance between cell division and differentiation determines organ size, and represents a central unknown in developmental biology. In Arabidopsis roots, this balance is mediated between cytokinin and auxin through a regulatory circuit converging on the IAA3/SHORT HYPOCOTYL 2 (SHY2) gene. Here, we show that crosstalk between brassinosteroids (BRs) and auxin occurs in the vascular transition zone to promote root meristem development. We found that BR increases root meristem size by up‐regulating expression of the PINFORMED 7 (PIN7) gene and down‐regulating expression of the SHY2 gene. In addition, BES1 could directly bind to the promoter regions of both PIN7 and SHY2, indicating that PIN7 and SHY2 mediate the BR‐induced growth of the root meristem by serving as direct targets of BES1. Moreover, the PIN7 overexpression and loss‐of‐function SHY2 mutant were sensitive to the effects of BR and could partially suppress the short‐root phenotypes associated with deficient BR signaling. Interestingly, BRs could inhibit the accumulation of SHY2 protein in response to cytokinin. Taken together, these findings suggest that a complex equilibrium model exists in which regulatory interactions among BRs, auxin, and cytokinin regulate optimal root growth.     

Improving of oilseed rape lateral root formation by physiological analogues of auxin

The aim of the present work was to study the effect of auxin physiological analogue TA-12 [1-(2-chloroethoksicarbonylmethyl)-4-naphthalenesulfonic acid calcium salt] on the formation of oilseed rape lateral root and on the mitotic activity of apical meristem cells. Spring oilseed rape (Brassica napus L. ssp. oleifera annua Metzg.) cultivar ‘Mascot’ was chosen as a test object. Anatomical, cytological and histological studies on root development suggest that compound TA-12 induces the activity of parent root pericycle cells, stimulates the formation of lateral roots and enhances the division of apical meristem cells. The auxin transport inhibitor 2,3,5-triiodobenzoic acid suppresses the division of apical meristem cells, while this process is restored by the auxin physiological analogue TA-12 and naphthaleneacetic acid. The compound TA-12, by stimulating primary root growth and lateral root induction, optimised the formation of the oilseed rape root system.     

Temporal root responses in <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> L. to chromate reveal structural and regulatory mechanisms involving the SOLITARY ROOT/IAA14 repressor for maintenance of identity meristem genes

The Arabidopsis root system is modified in response to stress generated by high concentrations of nonessential ions such as chromate [Cr(VI)]. In this work, the distribution of auxin and its transporters PIN1 and PIN7, as well as the expression of genes that maintain the identity of the root meristem, were analyzed in Arabidopsis thaliana wild-type (WT) seedlings and in a mutant affected in the SOLITARY ROOT (SLR1/IAA14) locus, which is required for root response to Cr(VI). We show that primary root inhibition, auxin transporter levels, and expression of meristem identity genes were maintained in the slr-1 mutants but not in WT plants in response to Cr(VI) in a time- and concentration-dependent manner. Notably, the outermost single cell layer of the lateral root cap, which normally dies and tends to peel off, remains viable and increases in size following exposure of WT plants, but not slr-1 mutants, to Cr(VI). Our results suggest that (1) the primary root tip senses Cr(VI), (2) the external lateral root cap may play a protective role during Cr(VI) exposure, and (3) Cr(VI) impacts cell division in root meristems via auxin redistribution and SLR1/IAA14 function, influencing the expression of root meristem genes.     

Auxin Flow in Anther Filaments is Critical for Pollen Grain Development through Regulating Pollen Mitosis

It was well known that auxin is critical for anther/pollen grain development, however, the clear distribution and detailed effects of auxin during floral development are still unclear. We have shown here that, through analyzing GUS activities of Arabidopsis lines harboring auxin response elements DR5-GUS, auxin was mainly accumulated in the anther during flower stages 10–12. Further studies employing the indoleacetic acid–lysine synthetase (iaaL) coding gene from Pseudomonas syringae subsp. savastanoi under control of the promoter region of Arabidopsis phosphatidylinositol monophosphate 5-kinase 1 gene, which conducts the anther filament-specific expression, showed that block of auxin flow of filaments resulted in shortened filaments and significantly defective pollen grains. Similar phenotype was observed in tobacco plants transformed with the same construct, confirming the effects of auxin flow in filaments on anther development. Detailed studies further revealed that the meiosis process of pollen grain was normal while the mitosis at later stage was significantly defected, indicating the effects of auxin flow in filaments on pollen grain mitosis process. Analysis employing [¹⁴C]IAA, as well as the observation on the expression of AtPIN1, coding for auxin efflux carrier, demonstrated the presence of polar auxin transport in anther filaments and pollen grains.     

Expression and regulation of theRSI-1 gene during lateral root initiation

The tomato geneRSI-1 was previously identified as a molecular marker for auxin-induced lateral root initiation. We have further characterized the expression mode of theRSI-1 gene in tomato andArabidopsis thaliana. Northern blot analyses revealed that the gene was induced specifically by auxin in tomato roots and hypocotyls. For experiments with transgenic plants, the 5′ flanking region of theRSI-1 gene was linked to a GUS reporter gene, then transformed into tomato andArabidopsis. In these transgenic tomato plants, GUS activity was detected at the sites of initiation for lateral and adventitious roots. Expression of the fusion gene was auxin-dependent and tissue-specific. This was consistent with results from the northern blot analyses. In transgenicArabidopsis, the overall expression pattern of theRSI-GUS gene, including tissue specificity and auxin inducibility, was comparable to that in transgenic tomato seedlings. These results indicate that an identical regulatory mechanism for lateral root initiation might be conserved in both plants. Thus, the expression mode of theRSI-CUS gene inArabidopsis mutants defective in lateral root development should be investigated to provide details of this process.     

MM31/EIR1 promotes lateral root formation in Arabidopsis

Lateral root formation in Arabidopsis provides a model for the study of auxin function. Tryptophan (Trp) is a precursor of the auxin indoleacetic acid (IAA). To study the physiological function of Trp in auxin-related phenotypes, we examined the effect of Trp on lateral root formation. We found that Trp treatment enhanced lateral root formation and, by screening for mutants in which the effect of Trp on lateral root formation was enhanced, we isolated the mm31 mutant. Based on genetic and physiological analyses, we propose that MM31/EIR1 modulates lateral root formation by regulating the IAA polar transport system, and that auxin transport from the shoot to the root regulates lateral root formation.Key words: lateral root formation, Arabidopsis, EIR1, IAA, auxin