Low phosphate signaling induces changes in cell cycle gene expression by increasing auxin sensitivity in the Arabidopsis root system

Lateral root development is an important morphogenetic process in plants, which allows the modulation root architecture and substantially determines the plant''s efficiency for water and nutrient uptake. Postembryonic root development is under the control of both endogenous developmental programs and environmental stimuli. Nutrient availability plays a major role among environmental signals that modulate root development. Phosphate (Pi) limitation is a constraint for plant growth in many natural and agricultural ecosystems. Plants posses Pi-sensing mechanisms that enable them to respond and adapt to conditions of limited Pi supply, including increased formation and growth of lateral roots. Root developmental modifications are mainly mediated by the plant hormone auxin. Recently we showed that the alteration of root system architecture under Pi-starvation may be mediated by modifications in auxin sensitivity in root cells via a mechanism involving the TIR1 auxin receptor. In this addendum, we provide additional novel evidence indicating that the low Pi pathway involves changes in cell cycle gene expression. It was found that Pi deprivation increases the expression of CDKA, E2Fa, Dp-E2F and CyCD3. In particular, E2Fa, Dp-E2F and CyCD3 genes were specifically upregulated by auxin in Pi-deprived Arabidopsis seedlings that were treated with the auxin transport inhibitor NPA, indicating that cell cycle modulation by low Pi signaling is independent of auxin transport and dependent on auxin sensitivity in the root.Key words: phosphate signaling, auxin transport, auxin sensitivity, roots     

Role of the Arabidopsis PIN6 Auxin Transporter in Auxin Homeostasis and Auxin-Mediated Development

Plant-specific PIN-formed (PIN) efflux transporters for the plant hormone auxin are required for tissue-specific directional auxin transport and cellular auxin homeostasis. The Arabidopsis PIN protein family has been shown to play important roles in developmental processes such as embryogenesis, organogenesis, vascular tissue differentiation, root meristem patterning and tropic growth. Here we analyzed roles of the less characterised Arabidopsis PIN6 auxin transporter. PIN6 is auxin-inducible and is expressed during multiple auxin–regulated developmental processes. Loss of pin6 function interfered with primary root growth and lateral root development. Misexpression of PIN6 affected auxin transport and interfered with auxin homeostasis in other growth processes such as shoot apical dominance, lateral root primordia development, adventitious root formation, root hair outgrowth and root waving. These changes in auxin-regulated growth correlated with a reduction in total auxin transport as well as with an altered activity of DR5-GUS auxin response reporter. Overall, the data indicate that PIN6 regulates auxin homeostasis during plant development.     

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.     

Ultrastructural Changes Caused by Fusarium oxysporum f. sp. lycopersici in Meloidogyne javanica Induced Giant Cells in Fusarium Resistant and Susceptible Tomato Cultivars

Tomato (Lycopersicon esculentum Mill.) seedlings, susceptible (cv. Pearson A-I Improved) and resistant (cv. Pearson Improved) to race 1 Fusarium oxysporum f. sp. lycopersici (Sacc.) Snyd &Hans., were inoculated with Meloidogyne javanica (Trueb) Chitwood second-stage juveniles and 3 weeks later with race 1 F. oxysporum f. sp. lycopersici spores. One week after fungal inoculation, no fungus was visible in root tissue of the tomato cultivars and the giant cells were normal. Two weeks after fungal inoculation, abundant hyphae were visible in xylem tissues of Fusarium-susceptible but not of Fusarium-resistant plants. In susceptible plants, giant cell degeneration occurred, characterized by membrane and organelle disruption. In addition, where hyphae were in direct contact with the giant cell, dissolution of the giant cell wall occurred. Three weeks after fungal inoculation, fungal hyphae and spores were visible inside xylem tissues and giant cells in Fusarium-susceptible plants and in xylem tissue of the resistant plants. In susceptible and resistant plants, giant cell degeneration was apparent. Giant cell walls were completely broken down in Fusarium-susceptible tomato plants. In both cultivars infected by Fusarium, giant cell nuclei became spherical and dark inclusions occurred within the chromatin material which condensed adjacent to the fragmented nuclear membrane. No such ultrastructural changes were seen in the giant cells of control plants inoculated with nematode alone. Giant cell deterioration in both cultivars is probably caused by toxic fungal metabolites.     

Characterization of MORE AXILLARY GROWTH Genes in Populus

Background

Strigolactones are a new class of plant hormones that play a key role in regulating shoot branching. Studies of branching mutants in Arabidopsis, pea, rice and petunia have identified several key genes involved in strigolactone biosynthesis or signaling pathway. In the model plant Arabidopsis, MORE AXILLARY GROWTH1 (MAX1), MAX2, MAX3 and MAX4 are four founding members of strigolactone pathway genes. However, little is known about the strigolactone pathway genes in the woody perennial plants.

Methodology/Principal Finding

Here we report the identification of MAX homologues in the woody model plant Populus trichocarpa. We identified the sequence homologues for each MAX protein in P. trichocarpa. Gene expression analysis revealed that Populus MAX paralogous genes are differentially expressed across various tissues and organs. Furthermore, we showed that Populus MAX genes could complement or partially complement the shoot branching phenotypes of the corresponding Arabidopsis max mutants.

Conclusion/Significance

This study provides genetic evidence that strigolactone pathway genes are likely conserved in the woody perennial plants and lays a foundation for further characterization of strigolactone pathway and its functions in the woody perennial plants.     

Auxin controls the division of root endodermal cells

The root endodermis forms a selective barrier that prevents the free diffusion of solutes into the vasculature; to make this barrier, endodermal cells deposit hydrophobic compounds in their cell walls, forming the Casparian strip. Here, we showed that, in contrast to vascular and epidermal root cells, endodermal root cells do not divide alongside the root apical meristem in Arabidopsis thaliana. Auxin treatment induced division of endodermal cells in wild-type plants, but not in the auxin signaling mutant auxin resistant3-1. Endodermis-specific activation of auxin responses by expression of truncated AUXIN-RESPONSIVE FACTOR5 (ΔARF5) in root endodermal cells under the control of the ENDODERMIS7 promoter (EN7::ΔARF5) also induced endodermal cell division. We used an auxin transport inhibitor to cause accumulation of auxin in endodermal cells, which induced endodermal cell division. In addition, knockout of P-GLYCOPROTEIN1 (PGP1) and PGP19, which mediate centripetal auxin flow, promoted the division of endodermal cells. Together, these findings reveal a tight link between the endodermal auxin response and endodermal cell division, suggesting that auxin is a key regulator controlling the division of root endodermal cells, and that PGP1 and PGP19 are involved in regulating endodermal cell division.

The endodermal auxin response, which is regulated by centripetal auxin flow, determines division of the endodermal cells.     

Lateral root stimulation in the early interaction between Arabidopsis thaliana and the ectomycorrhizal fungus Laccaria bicolor: Is fungal auxin the trigger?

Lateral root (LR) stimulation during early signal exchange between plant roots and ectomycorrhizal (ECM) fungi has recently been shown to be achieved by modulation of auxin gradients. We suggested that this modulation could occur through altered polar auxin transport (PAT) and through activation of auxin signalling pathways in the root. However, it remains unclear, which fungal molecules alter auxin pathways inside the plant partner. It has been suggested in previous studies that auxin released by the fungus could trigger observed plant responses during early signal exchange and later on during root colonization. Here we focus on the early interaction and we provide evidence for an alternative mechanism. Indeed, LR stimulation by the fungus in Arabidopsis thaliana followed a totally different timing than with exogenously applied auxin. Furthermore, experimental conditions that excluded the exchange of soluble molecules while allowing exchange of volatile(s) between the plant and the fungus were sufficient for LR induction, therefore questioning the role of secreted fungal auxin. These data suggest that volatiles released by the fungus and sensed by the plant may act upstream of altered auxin signaling in the plant.Key words: mycorrhiza, ectomycorrhiza, lateral root, auxin, volatiles, ethylene, jasmonic acidInteractions of plant roots with symbiotic, ectomycorrhizal soil fungi lead to lateral root (LR) stimulation during the very early interaction phase.¹ This LR stimulation has recently been shown to be independent of root colonization and to occur as well in non-mycorrhizal plants, such as Arabidopsis suggesting that fungal signals have a broad perception spectrum.^1,2 However, little is known about the type of signals exchanged between fungi and their plant partners during this early interaction phase. Several studies have proposed a role for the phytohormone auxin produced and secreted by ECM fungi as the signalling molecule during ECM fungus/plant signaling.^2–7 Recently we studied changes in auxin response and auxin transport in poplar and Arabidopsis thaliana roots during contact with the ECM fungus Laccaria bicolor.¹ We demonstrated that the presence of the fungus enhances the auxin response and distribution at the root apex and that this, as well as LR stimulation, is reliant on polar auxin transport through AtPIN2 and probably through PtPIN9 in poplar. Here, using Arabidopsis thaliana, whose LR stimulation by Laccaria bicolor has been demonstrated, we propose that not yet identified fungal volatiles may regulate auxin homeostasis in the plant, questioning the contribution of the auxin released by the fungus on the induction of LR.     

LjABCB1, an ATP-binding cassette protein specifically induced in uninfected cells of Lotus japonicus nodules

Legume plants develop root nodules through symbiosis with rhizobia, and fix atmospheric nitrogen in this symbiotic organ. Development of root nodules is regulated by many metabolites including phytohormones. Previously, we reported that auxin is strongly involved in the development of the nodule vascular bundle and lenticel formation on the nodules of Lotus japonicus. Here we show that an ATP-binding cassette (ABC) protein, LjABCB1, which is a homologue of Arabidopsis auxin transporter AtABCB4, is specifically expressed during nodulation of L. japonicus. A reporter gene analysis indicated that the expression of LjABCB1 was restricted to uninfected cells adjacent to infected cells in the nodule, while no expression was observed in shoot apical meristems or root tips, in which most auxin transporter genes are expressed. The auxin transport activity of LjABCB1 was confirmed using a heterologous expression system.     

Endophytic fungus Falciphora oryzae promotes lateral root growth by producing indole derivatives after sensing plant signals

The endophytic fungus Falciphora oryzae was initially isolated from wild rice (Oryza granulata) and colonizes many crop species and promotes plant growth. However, the molecular mechanisms underlying F. oryzae-mediated growth promotion are still unknown. We found that F. oryzae was able to colonize Arabidopsis thaliana. The most dramatic change after F. oryzae inoculation was observed in the root architecture, as evidenced by increased lateral root growth but reduced primary root length, similar to the effect of auxin, a significant plant growth hormone. The expression of genes responsible for auxin biosynthesis, transport, and signalling was regulated in Arabidopsis roots after F. oryzae cocultivation. Indole derivatives were detected at significantly higher levels in liquid media after cocultivation compared with separate cultivation of Arabidopsis and F. oryzae. Consistently, the expression of indole biosynthetic genes was highly upregulated in F. oryzae upon treatment with Arabidopsis exudates. Global analysis of Arabidopsis gene expression at the early stage after F. oryzae cocultivation suggested that signals were exchanged to initiate Arabidopsis–F. oryzae interactions. All these results suggest that signalling molecules from Arabidopsis roots are perceived by F. oryzae and induce the biosynthesis of indole derivatives in F. oryzae, consequently stimulating Arabidopsis lateral root growth.     

Expression of EuNOD-ARP1 encoding auxin-repressed protein homolog is upregulated by auxin and localized to the fixation zone in root nodules of Elaeagnus umbellata

Root nodule formation is controlled by plant hormones such as auxin. Auxin-repressed protein (ARP) genes have been identified in various plant species but their functions are not clear. We have isolated a full-length cDNA clone (EuNOD-ARP1) showing high sequence homology to previously identified ARP genes from root nodules of Elaeagnus umbellata. Genomic Southern hybridization showed that there are at least four ARP-related genes in the genome of E. umbellata. The cDNA clone encodes a polypeptide of 120 amino acid residues with no signal peptide or organelle-targeting signals, indicating that it is a cytosolic protein. Its cytosolic location was confirmed using Arabidopsis protoplasts expressing a EuNOD-ARP1:smGFP fusion protein. Northern hybridization showed that EuNOD-ARP1 expression was higher in root nodules than in leaves or uninoculated roots. Unlike the ARP genes of strawberry and black locust, which are negatively regulated by exogenous auxin, EuNOD-ARP1 expression is induced by auxin in leaf tissue of E. umbellata. In situ hybridization revealed that EuNOD-ARP1 is mainly expressed in the fixation zone of root nodules.     

Aeromonas punctata PNS-1: a promising candidate to change the root morphogenesis of Arabidopsis thaliana in MS and sand system

A model system of sand, comprising Arabidopsis plants inoculated with Aeromonas punctata PNS-1 strain, was used to evaluate the bacterial effect in modulation of plant root structure at second-order lateral root level. In MS media, the root morphogenesis was changed only at first-order lateral root level when inoculated with PNS-1 strain. Inoculation with PNS-1 strain was subjected to significant (P < 0.01) increase in primary root length and lateral root density in both MS and sand system. However, this strain modulated the root structure in the sand environment in a complex manner that may be helpful for incitation of the plant–microbe interaction close to natural environment. In order to determine whether this change in root morphology was due to bacterial auxin, Arabidopsis transgenic line (DR5:GUS) was used to reveal the change in homeostasis of endogenous auxin. In PNS-1 inoculated transgenic seedlings of Arabidopsis plant (DR5:GUS), endogenous auxin in primary root apices and lateral roots was enhanced. For confirmation, PNS-1 was evaluated for auxin production in vitro, showed an increase in auxin production after supplementation of l-tryptophan. The presence of ACC deaminase activity in PNS-1 showed its possible involvement in primary root elongation. In the present study Aeromonas punctata PNS-1 is the potential candidate for triggering the change in root morphogenesis of Arabidopsis thaliana with the involvement of auxin and ACC deaminase production.