8/10/2019 Cervantes 2001 Trends in Plant Science 2001

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TRENDS in Plant Science Vol.6 No.4 April 2001

http://plants.trends.com 1360-1385/01/$ see front matter 2001 Elsevier Science Ltd. All rights reserved.

141News&Comment

About 70% of the nitrogen available to plants

originates through nitrogen fixation by

symbiotic bacteria. A group of Gram-negativebacteria, the rhizobia, are housed within the

root nodules of legumes. Nodulation is

initiated by the presence of appropriate

rhizobial endosymbionts in the rhizosphere.

To effect this, plant roots continuously release

elicitors of bacterialNodgene expression,

and respond proactively to the presence of

bacterial Nod factors by initiating signalling

and developmental pathways that lead to

infection and nodule morphogenesis. Early

cellular responses to Nod factors include

membrane depolarization, Ca2+ influx and the

initiation of multiple, transient increases incytoplasmic Ca2+concentration (termed Ca2+

waves or Ca2+spikes). The phenotypic

characterization of non-nodulating mutants

ofMedicago truncatulaand pea by Rebecca

Wais and colleagues1and Simon Walker and

colleagues2has shows that Ca2+ spiking is an

early intracellular signal that leads to nodule

development,

The presence of Nod factors elicits Ca2+

spiking in root hairs of legumes after

~10min. During each spike, the cytoplasmic

Ca2+ concentration ([Ca2+]cyt

) increases

transiently by 200500nM. The oscillationsin [Ca2+]

cythave a periodicity of 12min and

persist for several hours. After ~6 hours,

root hair deformation occurs. This is

followed by the induction of early nodulin

(ENOD) genes. No mutant defective in theearliest stages of nodulation was found to

exhibit Ca2+ spiking. Thus, the products of

theM.truncatulaDMI1and DMI2genes,

and the pea SYM1,SYM8and SYM10genes,

are apparently involved in establishing Ca2+

spiking. Mutants in these genes do not

exhibit root deformation or the induction of

ENODgenes. By contrast, Ca2+ spiking was

observed in the mutants dmi3,sym9and

sym30.These mutants do not exhibit root

deformation or the induction of ENOD

genes either, but the products of the DMI3,

SYM9and SYM30genes must actsubsequently to Ca2+ spiking on the

nodulation pathway. Ca2+spiking was also

exhibited by the M. truncatulamutants nsp

and hcland the pea mutants sym2A and

sym7. These mutants are compromised in

events subsequent to root hair deformation,

and their phenotype supports the

hypothesis that Ca2+spiking is an early

event in the signalling pathway leading to

nodulation. Interestingly, theM. truncatula

mutants dim1,dim2anddim3and the pea

mutants sym8, sym9,sym19and sym30are

also unable to form associations withmycorrhizal fungi. This suggests that the

development of mycorrhizal associations

shares common signalling elements with

nodulation, including Ca2+ spiking. Thus,

Ca2+

spiking could be an integral componentof several signalling pathways.

The observation that several genes are

required for Ca2+ spiking and nodulation

establishes a strong correlation between

these two phenomena. Nevertheless, the

authors are careful to caveat that Ca2+ spiking

might be a correlative event activated by

steps in common with nodulation. A

comprehensive genetic study would enable

a causal dissection of the processes initiating

root nodulation, as well as providing an

insight into the roles for Ca2+ spiking in other

cellular processes. The characterization offurther mutants with contrasting Ca2+ spiking

phenotypes is eagerly anticipated, as is the

cloning of theDMIandSYMgenes.

1 Wais, R.J . et al. (2000) Genetic analysis of

calcium spiking responses in nodulation

mutants ofMedicago truncatula.Proc. Natl.

Acad. Sci. U.S.A. 97, 1340713412

2 Walker, S.A.et al. (2000) Dissection of

nodulation signaling using pea mutants

defective for calcium spiking induced by Nod

factors and chitin oligomers. Proc. Natl. Acad.

Sci. U.S. A.97, 1341313418

Philip J .White

philip-j.white@hri.ac.uk

A Nod and a wave: calciumsignals during nodulation

Haemoglobin is one of the best known and

more completely characterized proteins in

biochemistry. In mammals, its function is

the bi-directional transport of oxygen from

lungs to tissues and of carbon dioxide from

tissues to lungs. Thus, haemoglobin travelsin the circulatory system through veins and

capillaries inside the erythrocytes in the

blood stream. Haemoglobin was described

in plants a long time ago in the legume root

nodule, and its function was suggested to

be associated with the oxygen supply to the

symbiotic bacteroid. However, the question

remains as to how plant haemoglobin can

exert this function, being enclosed in the

static plant cell system.

In a recent article, Xxxxxx Kawashima and

colleagues1describe the existence of

multiple symbiotic haemoglobins

(leghemoglobins) in pea. These are grouped

into two types, PsLbA and PsLbB, which

differ in oxygen affinity as well as in cellular

localization. PsLb5-10, the only protein

representative of the PsLbA type, has a

higher oxygen-binding capacity and the

corresponding transcripts are detected

throughout the central tissue of effectivenodules. The transcripts for PsLb120-1,

which is representative of the PsLbB type, are

localized in a region from infection zoneII to

the distal part of nitrogen fixation zone.

The presence of two types of

leghemoglobins of different oxygen affinity

could serve to create and maintain an

oxygen gradient across the nodule tissue.

This primary gradient might be enriched by

the existence of multiple leghemoglobins of

each type, but also the oxygen affinities of

the leghemoglobins might be modulated in

response to the metabolic reactions that

occur inside the cells. For example, recently

two carbonic anhidrases were described of

different cellular expression across the

nodule section in alfalfa (Medicago sativa).

Carbonic anhidrases might modify the pH

status of the cells, affecting in turn the

oxygen-binding capacities of haemoglobins

(the Bohr effect).Thus, the possibility exists that a

coordinate action between multiple

symbiotic haemoglobins and carbonic

anhidrases could be important in the

creation and maintenance of oxygen and

carbon dioxide gradients required for

nodule metabolism.

1 Kawashima K. et al. (2001) Two types of pea

leghemoglobin genes showing different O2-

binding affinities and distinct patterns of spatial

expression in nodules. Plant Physiol.

125, 641651

Emilio Cervantes

ecervant@gugu.usal.es

Oxygen transport in the static plant cell system