A Cytokinin Perception Mutant Colonized by Rhizobium in the Absence of Nodule Organogenesis From: 2007 Jan SCIENCE Vol315 5.

A Cytokinin Perception Mutant Colonized by Rhizobium in the Absence of Nodule Organogenesis

From: 2007 Jan SCIENCE Vol315 5

Now We Known

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Sample Screen Lotus japonicus har1-1 hypernodulation.

Hyperinfected 1(HIT1) low nodulation, excessive formation of IT.

(hit1-1 har1-1), (hit1-1) Mesorhizobium loti carry reporter gene.

hit1-1 har1-1 10dai

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hit1-1 har1-1 14dai

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hit Lots IT, but less or no nodule organogenesis. ª›®œ•Œ QuickTimeý ©M

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Cortex Epidermis

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Role of HIT1 in NP organogenesis ENOD40(cortical cell divison, mRNA) Nin(IT formation, initiation of primordia)

Role of HIT1 in NP organogenesis

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inoculate

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Role of HIT1 in NP organogenesis

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Role of HIT1 in NP organogenesis L. japonicus root hairless( Ljrhl1-1 )

- lots uncolonized NPs. - no root hairs and ITs. ( Ljrhl1-1 ) and (hit1-1) double mutant.

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Role of HIT1 in NP organogenesis

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Jens Stougaard’s group

- L. japonicus Spontaneous nodule formation (Lotus hitidine kinase - Lhk)

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har + AR hit har + AR hit har + Lhk1

Expression of Lhk1 in various L. japonicus tissues as assayed by RT-

PCR.

Phylogenetic analysis of LHK1 and LHK1-like proteins.

Analysis of the full-length Lhk1 cDNA revealeda 2979–base pair open reading frameencoding a predicted protein of 993 amino acids

The LHK1 protein had 64% identitywith the Arabidopsis cytokinin histidine kinasereceptor AHK4, and 49 and 45% identity withAHK2 and AHK3, respectively. Similar toLHK1, LHK2 was more closely related toAHK4 than to other Arabidopsis cytokininreceptors

Lhk1 cDNA confers cytokinin responsiveness to yeast cells.

cells were transformed with either p415CYC carrying the Lhk1 cDNA, the hit1-2 cDNA or the vector alone

hit1-1 roots are insensitive to exogenously applied cytokinin.

We further investigated a role for the HIT1

locus in NP organogenesis by studying Early

Nodulin 40 (ENOD40) and Nin expression,

two markers for NP initiation

50 nM benzyl adenine (BA) increased the steady-state level of Nin transcript

qPCR showing significant attenuation of Nin mRNA in hit1-1 versus wild-type roots upon exogenous application of 50nM BA. BA-stimulated accumulation of Nin transcripts in hit1 roots was significantly higher in comparison to water treatments

Nodule organogenesis

The LHK1 function as cytokinin receptors

hit1-1 roots are insensitive to exogenously applied cytokinin

50 nM benzyl adenine (BA) increased the steady-state level of Nin transcript

Nin mRNA in hit1-1 less then in wild-type

The LHK1 function as cytokinin receptors, which may explain a leaky (formation of some nodules) symbiotic phenotype and lack of more general developmental abnormalities in mutants carrying hit1 alleles.

cytokinin biosynthesis and/or action are crucial downstream targets of NF perception recruitment of a cytokinin receptor could have been an essential event during the evolution of nitrogen-fixing nodule symbiosis

Nin expression, supporting IT formation, may be regulated by an Lhk1-independent mechanism in the root epidermis, possibly involving another cytokinin receptor.

祥霖

Arabidopsis ahk2 ahk3 ahk4 triple mutants display a reduced number of root vascular cell files→ impaired periclinal procambial cell divisions

snf2 mutant?

Phenotypic characterization of the snf2 mutant

wt

snf2

5 week time 0 6 day 3 week

On hormone free medium

Phenotypic characterization of the snf2 mutant

cytokinin

auxi

n

ARR(type-A response regulator) gene was Cytokinin-induced cellular processes in plants

ARR5 promoter-GUS was expressed during nodulation

L266F receptor protein→ cytokinin- independent activity in E. coli

Lotus Lrr5?

Expression of Lrr5, Lhk1, and Nin in intact wild-type and snf2 roots

Expression of Lrr5 in wild-type and snf2 root explants

Medium with or without 0.5 mg/ml of BAP(cytokinin) for 10 days

Expression of Nin in wild-type and snf2 rootexplants

Medium with or without 0.5 mg/ml of BAP(cytokinin) for 10 days

Root length of wt and snf2 plants in the presence of increasing concentration of BAPBAP: 0 3 6 10 ng/ml

BAP: 30 50 100 500 ng/ml

Wt/snf2

• snf2 mutants growth were strongly affected by external cytokinin

The phenotype of snf2 mutants suggests that cytokinin signaling acts downstream of Nod-factor signal transduction

signal Nod-factor snf2 nod formation

Signal transduction： symRK mutants lacking Ca2+ spiking

ccamk (Ca2+- and calmodulin-dependent protein kinase)

develop spontaneous nodules in snf2 transgenic roots

symRKsnf2 nod formation

ccamk

nfr1, nfr5, symrk, ccamk mutants→ unable to form nodules in response to rhizobia inoculation

nin, nsp2 mutants→ arrested before initiation of cell division induced by Nod-factor signaling, no spontaneous nodules were observed in snf2 transgenic rootsCytokinin signal perception acts upstream

of cell division initiation

symRKnod formation

ccamk

nin

nsp2snf2

snf1-1 mutants(CCaMK protein impaired in autophosphorylation)→ 7±0.9

snf2 mutants→ 3±0.5 snf1-1+snf2→ 17±0.9

• Parallel signaling

• Deregulated signaling in snf1 results in a local increase in cytokinin levels transcriptionally up-regulating snf2

har1-1(hypernodulating)→ no nodule snf2 mutant→ 3±0.5 snf2+har1-1 double mutants→ 14±1.4

•Only a few cells dedifferentiate •Only a few dedifferentiated cells sustain cell divisions during the snf2 nodule-initiation process

Model for functional role of Lhk1 in nodulation

Discuss Applications of auxin transport inhibitors resulted in empty nodule-like structures, which suggested that local inhibition of auxin transport sensitizes cells for division

Externally supplied cytokinin induces cortical cell division and activation genes, and expression of cytokinin biosynthesis gene in a nodulation-deficient Sinorhizobium meliloti resulted in nodule-like structures

Conclude cytokinin signaling plays an important role in plant meristem formation and is directly involved in initiating root nodule organogenesis