body building on land — morphological evolution of land plants

Available online at www.sciencedirect.com

Body building on land — morphological evolution of land plantsLiam Dolan

Land plants are derived from green algal ancestors and made

their first appearance on land 460 million years ago. The life

cycle of the land plant body comprises two multicellular

stages — one haploid (gametophyte) and the other diploid

(sporophyte). Recent discoveries suggest that the genes

controlling diploid development in ancestral green algal

zygotes diversified in the land plant lineage where they

control the development of the diploid body plan. There

are also numerous examples of the independent recruitment

of sets of genes to control the development of structures

that are morphologically and functionally similar. These

discoveries are giving insights into the mechanism by which

land plant morphologies changed over the past 460 million

years.

Address

Department of Cell and Developmental Biology, John Innes Centre,

Norwich NR4 7UH, United Kingdom

Corresponding author: Dolan, Liam ([email protected])

Current Opinion in Plant Biology 2009, 12:4–8

This review comes from a themed issue on

Growth and Development

Edited by Charles S. Gasser and Caroline Dean

Available online 26th December 2008

1369-5266/$ – see front matter

# 2008 Elsevier Ltd. All rights reserved.

DOI 10.1016/j.pbi.2008.12.001

Evolution of the land plant bodyThe first 50 million years of land plant life saw a gradual

increase in morphological diversity and was followed by

an explosion during which most of the morphological

variations that we see in land plants today evolved.

The earliest evidence of plant life on land is spores,

the haploid cells produced at meiosis surrounded by a

hard decay resistant wall in 460-year-old sediments [1].

The morphologies of these spores suggest that they were

produced by plants related to liverworts [2]. By 420

million years ago Cooksonia with aerial shoots with the

earliest water conducting cells bearing sporangia had

evolved [3]. The microphylls on Baragwanathia longifoliaindicate that leaves had evolved by 410 million years ago

[4]. Then during the Devonian period (approximately

420–350 million years ago) an explosion in the diversity of

plant morphologies occurred. This new diversity resulted

in plants spreading from the damp areas that had first

been colonised to upland drier habitats. Deep rooting


systems evolved and tall plants in the first forests covered

large areas [5].

The discovery of the roles of genes that control the de-

velopment of the seed plant body and the subsequent

demonstration of their roles in the development of earlier

diverged groups of plants provide insights into the mech-

anisms that arebehind this explosion in land plantdiversity.

Here I review recent developments in our understanding of

the genetic mechanisms that underpinned the evolution of

early land plant morphological diversity.

TALE proteins control the development of thediploid phase of the Chlamydomonasreinhardtii life cycleThere are haploid and diploid phases in the life cycle of

green plants. In land plants both of these phases are

multicellular. Gametes are produced by mitosis on multi-

cellular haploid gametophytes that develop from spores,

the haploid products of meiosis that occurs on a multi-

cellular diploid plant called the sporophyte. Whilst the

sporophytes of land plants are always multicellular, there

are a number of different life style arrangements to be

found among the green algae [6]. Charophyceae and land

plants are derived from a common ancestor, and the

charophyte gametophyte is multicellular whilst its spor-

ophyte is unicellular. Another group of green algae, the

Chlorophyceae, includes species in which both gameto-

phyte and sporophytes are multicellular and others in

which both phases are unicellular. The latter includes the

alga, Chlamydomonas reinhardtii, in which haploid cells

fuse to form a diploid cell which then undergoes meiosis.

The genetic switch that controls the developmental

changes that occur when C. reinhardtii becomes diploid

has recently been discovered [7,8]. There are two mating

types of C. reinhardtii, plus and minus. Gsp1 and Gsm1 are

transcription factors that accumulate during the devel-

opment of plus and minus haploid strains, respectively.

Upon cell fusion the two proteins are present in the same

diploid cell and form dimers. This Gsm1–Gsp1 complex

moves into the nucleus where it modulates the transcrip-

tion of genes that promote meiosis and other aspects of

the diploid developmental program. Ectopic expression

of Gsp1 in the minus background (containing Gsm1) and

ectopic expression of Gsm1 in a plus background (contain-

ing Gsp1) induce the diploid developmental program and

meiosis. This suggests that coexpression of Gsp1 and

Gsm1 is sufficient for diploid development.

Gsp1 and Gsm1 are members of the TALE (three

amino acid loop extension) superclass of homeodomain

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Morphological evolution of land plants Dolan 5

containing transcription factors, which includes the class 1

KNOX, class 2 KNOX, BELLRINGER class and BELL-

RINGER-related class of proteins [7,9,10]. Class 1

KNOX and BELLRINGER proteins play important roles

in the control of the development of the shoot meristem

in Arabidopsis and BELLRINGER-related proteins have

only been found in the green algae and are absent from

land plants. Given the role of the members of this group

of transcription factors in diploid development in C.reinhardtii it might be expected that TALE family mem-

bers could control diploid-specific aspects of develop-

ment in land plants.

TALE proteins control the development of thediploid phase of the land plant life cycleTALE proteins have long been known to control the

development of shoot meristems in the sporophytes of

Arabidopsis and maize [9]. To determine the role of

TALE proteins in the control of the development in

sporophytes of non-seed land plants, two different

approaches were taken. In the first one the function of

TALE proteins was determined in moss by mutation and

in the second one the expression patterns of TALE gene

expression were determined in the ferns.

Mosses have two types of shoot-like axes. The sporo-

phyte develops from the zygote and consists of an

elongated axis, the seta, which supports a capsule where

meiosis and spore formation take place. The aerial axis of

the haploid gametophyte, the gametophore, develops

from cells in the filamentous colony that forms upon

germination of the haploid spore. It has an apical cell

that divides to produce cells that form the body of the

aerial axis including the single cell layered leaves. Phys-comitrella patens plants harbouring loss of function

mutations in class 1 KNOX genes, MOSS KNOX2(MKN2) and MKN4, have defective sporophyte develop-

ment [11]. Consistent with this role in sporophyte de-

velopment is the observation that MKN2 and MKN4 are

expressed in the sporophyte, but not in the gametophyte.

Together this indicates that these class 1 KNOX genes are

required for the development of the sporophyte and not

the gametophyte. Furthermore, loss of function

mutations in the class 2 KNOX gene, MKN1-3, also have

defective sporophyte development showing for the first

time that class 2 KNOX genes play a role in development

[11]. MKN1-3 is expressed throughout the diploid plant,

but not in the sporophyte. This is the first demonstration

of a role for a class 2 KNOX gene in plant development and

it also suggests that class 2 KNOX genes control the

development of the diploid phase of the life cycle and

not the haploid phase.

Since shoot axes with leaves develop in the gametophyte

stage of the moss life cycle and since KNOX genes are

neither expressed nor functional in the haploid organism,

it suggests that the molecular mechanism controlling the


development of shoot axes is different in moss gameto-

phyte and sporophyte. Identification of genes controlling

the development of the haploid shoot system will be

instructive in understanding genetic mechanisms under-

pinning convergent evolution of morphologies.

Further confirmation that the role of TALE proteins is

restricted to the control of sporophyte development

comes from the characterisation of gene expression pat-

terns during the life cycle of the fern Ceratopteris richardii[12]. Three class 1 KNOX genes are expressed in the

meristem, leaf primordia, vascular bundles and leaf mar-

gins of the sporophyte and no expression was detected in

the gametophyte. A single C. richardii class 2 KNOX genes

is expressed throughout the body of the diploid plant but

again no expression was detected in the gametophyte.

This suggests that the function of KNOX gene is restricted

to the development of structures in the diploid phase of

the life cycle.

Polar auxin transport occurs in the axis of the moss

sporophyte, but not in the gametophyte

In seed plants, there is a greater flux of auxin from the top

to the bottom (basipetal movement) of shoots than in the

opposite direction (acropetal movement) [13]. This polar

transport of auxin is involved in the development of a

variety of aspects of the shoot and the root systems.

Despite the superficial morphological similarities be-

tween the diploid shoots of vascular plants and the leafy

haploid moss gametophore, no polar auxin transport sys-

tem exists in the latter. In contrast, polar auxin transport

was observed in the axis of the moss sporophyte, where an

elongated leafless stalk carries a sporangium at its tip.

Therefore it is unlikely that auxin is involved in the

development of the gametophyte shoots of moss. At least

it is not likely to have the same role in haploid gameto-

phore and the diploid shoot. This suggests that auxin-

regulated axis development is specific to the diploid

phase and in the last common ancestor of the mosses

and the seed plants.

Leaves evolved independently in thelycophytes and euphyllophytesShoots are indeterminate structures that produce new

cells at their tip in meristems where a stem cell population

is located. Lateral appendages such as leaves develop

from small subpopulations of cells on the flanks of mer-

istems. The growth of these lateral structures is determi-

nate. That is, they grow to a certain size and then stop.

Class 1 KNOX genes and BELLRINGER are expressed in

and around the meristem and a number of these genes

have been shown to be required for the growth and

development of the meristem [9]. Class 1 KNOX gene

expression disappears in groups of founder cells just

before lateral organs emerge on the sides of the meristem

through the cell proliferation. Class 1 KNOX expression

remains repressed during the subsequent growth of these


6 Growth and Development

lateral organ primordia. Experimentally induced ectopic

expression of class 1 KNOX genes in primordia results in

defects in leaf development, indicating that repression is

required for leaf development [9]. This repression of the

class 1 KNOX genes and BELLRINGER is mediated by

the activity of a Myb transcription factor [14,15]. In

Arabidopsis this Myb protein is encoded by ASY-METRIC1, in maize by rough sheath 2 and in Antirrhinum

by Phantastica and collectively these proteins are known

as ARPs [16–18]. Class 1 KNOX genes are ectopically

expressed in mutants that lack ARP function resulting in

the development of defective leaves similar to those that

form when class 1 KNOX genes are experimentally mis-

expressed in leaf primordia.

The fossil record indicates that both lycophytes (in-

cluding extant Lycopodium, Selaginella and Isoetes) and

euphyllophytes (including extant ferns, horsetails and

seed plants) are derived from ancestors in which the

shoots had meristems but no leaves [4]. Two different

types of leaves evolved in the two lineages. Microphylls

are defined as leaves with a single vascular trace evolved

in the lycophyte lineage. Megaphylls have a complex

venation pattern in which there is more than one vascular

trace and evolved at least twice in the euphyllophyte

clade. Given that lycophytes and euphyllophytes and

their common ancestor had meristems at the tips of their

shoots, it is no surprise that the pattern of class 1 KNOXgene expression is similar in lycophytes and euphyllo-

phytes [19]; different members of the class 1 KNOX genes

are expressed in and around the meristem in both groups

of plants [9]. Nevertheless, given that microphylls and

megaphylls evolved independently it might have been

expected that different mechanisms of KNOX-repression

in lateral organs would have evolved during the indepen-

dent evolution of megaphylls and microphylls. Contrary

to that expectation, ARP genes are expressed in the

lateral primordia of both lycophytes and eutracheophytes

[19]. This suggests that ARP genes repress class 1 KNOXgene expression in lateral primordia in both groups of

plants. This suggests that a similar mechanism evolved

independently to repress class 1 KNOX expression during

microphyll and megaphyll development in lycophytes

and eutracheophytes, respectively.

Whilst the patterns of gene expression in developing

lateral primordia indicate that ARP genes were recruited

independently to control the development of leaves there

is also evidence for regulatory differences in the devel-

opment of microphylls and megaphylls. Arabidopsis classIII HD-ZIP genes control the initiation of lateral appen-

dages, the differentiation of the top from the bottom of

the leaf (dorsiventrality) and the development of vascu-

lature [20]. Whilst class III HD-ZIP genes were recruited

to control the development of lateral organs in both

microphyllous and megaphyllous species, their patterns

of gene expression in the developing leaf and vasculature


are dramatically different in the lycophytes and eutra-

cheophytes [21,22]. This suggests that these genes may

have different roles during the development of mega-

phylls and microphylls.

There are two take-home messages from the comparative

genetics of shoot and leaf development. The first one is

that despite the independent evolution of microphylls,

both organ types utilise at least some of the same genes to

control development. That is, there has been indepen-

dent co-option of ARP and class III HD-ZIP genes during

microphyll and megaphyll evolution. The second one is

that whilst similar genes were recruited, their precise

developmental roles may differ in during microphyll

and megaphyll development. Therefore whilst the

expression pattern of the ARP genes suggests that they

may repress KNOX gene expression in primordia in both

microphylls and megaphylls, and the role of the class IIIHD-ZIP genes in the development of the two leaf types is

different.

Roots: evidence for the recruitment ofgametophyte genes into the sporophyteLand plants are held in place by a variety of rooting

structures. In the bryophyte gametophytes, rhizoids

anchor the plant to the growth substrate. These rhizoids

are multicellular filaments in which the distal most-cell

expands by tip growth and then divides in the mosses and

hornworts. Liverworts on the other hand develop uni-

cellular rhizoids. Bryophyte sporophytes do not develop a

rooting structure because they are embedded in gameto-

phyte tissue. Rhizoids also form not only on vascular plant

gametophytes but also on rhizomes and roots (as root

hairs) in the sporophyte.

Roots evolved independently in the lycophyte and in the

euphyllophyte clades, but it is still unclear how many

times roots evolved in each of these clades [4]. Roots

therefore evolved at least twice but may have evolved

many more times. The developmental genetics of the

shoot and root meristems suggest that it is likely that roots

evolved from ancestral shoots through modification. The

fact that shoots and horizontally growing leafless rhizomes

are present in the fossil record before roots is consistent

with such a model. The oldest aerial shoot in the fossil

record is the Cooksonia sporophyte found in 420 million

years old sediments whilst the first root does not occur in

the fossil record until much later, around 390 million years

ago [3]. Furthermore, the ontogeny of lycophyte ‘roots’

(rhizomorphs) such as those found in extant Isoetes species

and extinct arborescent lycophytes such as Paurodendronspecies suggests that these rooting structures have

evolved from shoots [23,24].

A pair of basic helix–loop–helix transcription factors,

ROOT HAIR DEFECTIVE6 and ROOT HAIR

DEFECTIVE SIX LIKE1 (RSL1) positively regulate


Morphological evolution of land plants Dolan 7

the differentiation of root hair cells in Arabidopsis [25].

Mutant plants that lack RHD6 and RSL1 function do not

develop root hairs. RHD6 and RSL1 proteins accumulate

in the developing hair cell before the emergence of the tip

growing hair where it is postulated that they promote the

transcription of genes required for hair growth. Similar

genes were found in the moss Physcomitrella patens and

designated PpRDL1 and PpRSL2. Plant lacking both

PpRSL1 and PpRSL2 function developed very few short

rhizoids indicating that these genes controlled the de-

velopment of rhizoids [25]. Furthermore, the Pprsl1Pprsl2double mutant also did not develop caulonema, and tip

growing cells that form during the filamentous phase of

the moss life cycle that are responsible for colonising the

environment in which the spore germinates. Therefore

these basic helix–loop–helix transcription factors control

the development of cells with rooting functions in mosses

as well as flowering plants. It remains to be seen if these

genes control the development of root hair and rhizoid

cells in lycophyte ferns and horsetails.

Given that Physcomitrella and Arabidopsis last shared a

common ancestor 420 million years ago, the demon-

stration that similar genes control the development of

cells with a rooting function suggests that the mechanism

is ancient [3,25]. If we assume that the sporophytes of

ancestral extinct bryophytes were rhizoidless like extant

species, it suggests that the genes controlling the for-

mation of these rooting cells co-opted from the gameto-

phyte of ancestral plants into the sporophyte as the

sporophyte rose to dominance during the Devonian

explosion. An alternative scenario is that ancestral bryo-

phytes developed rhizoids in the gametophyte as well as

in the sporophyte. This could be the case if the sporo-

phyte of the earliest bryophytes was free living and not

reduced as it is in extant species. If this were the case then

it would suggest that the RHD6 mechanism was ancient

and inherited by all land plant sporophytes and gameto-

phytes. Nevertheless no fossil bryophytes with sporo-

phyte rhizoids have been described in the fossil record.

In the absence of such evidence the simplest explanation

is that the early bryophytes resembled extant species in

which the sporophyte grows on the semi-parasitically

gametophyte.

Model: mechanisms of morphologicalmodification of the land plant body during theDevonianIt is likely that land plants are derived from an algal

ancestor in which the diploid phase of the life cycle was

a single cell (zygote) that underwent meiosis [26]. Then

prior to or soon after the evolution of the land plants the

diploid phase of the life cycle became multicellular and

the zygote underwent numerous rounds of mitosis

before meiosis occurred. During the subsequent course

of land plant evolution the sporophyte became larger

and more complex than the gametophyte above. Here


four processes that provide insight into the genetic

changes that underpin the increase in morphological

diversity that occurred during the evolution of land

plant are outlined.

TALE evolution is responsible for the increase in

sporophyte diversity

TALE transcription factors both control the develop-

ment of the zygote in the green alga C. reinhardtii and

regulate the development of the body plan of the land

plant sporophyte. Since there are more TALE genes in

the genomes of land plants than in C. reinhardtii because

of the diversification of the class 1 KNOX and BELLRIN-GER genes, it has been suggested the diversification of

this gene family can account for morphological diversity

in the sporophytes of land plants. The increased number

of TALE proteins in land plants compared to algae would

allow for new interactions and combinations of regulatory

proteins that could promote the development of ‘new’

body parts. This increase in the number of some groups of

TALE proteins is coupled with the disappearance of the

BELLRINGER-related genes in the land plants. Given

that BELLRINGER-related genes are found in the algae,

their absence from land plants has been interpreted as

allowing the latter to escape the ‘rut’ of the algal life

cycle.

Neither TALE proteins nor polar auxin transport is

required for axis formation in the moss gametophyte or

what controls axis formation in the moss gametophyte?

TALE proteins control the development of the shoot axis

of the land plant sporophyte. Even though leafy axes

evolved in the gametophytes of bryophytes the TALEgenes are not involved in the development of these axes.

This suggests that the TALE proteins have a diploid-

specific function which may relate to their history in the

control of diploid development in algal ancestors. Sim-

ilarly sporophyte axes developed the ability to undergo

polar auxin transport in the sporophyte of bryophytes and

euphyllophytes, but gametophyte axes of bryophytes

show no evidence of polarised movement of auxin. Since

neither the TALE proteins nor polar auxin transport

controls the development of axes in the gametophytes

of bryophyte, a novel genetic mechanism controlling axis

formation must operate in these organs.

Leaves evolved independently

Megaphylls and microphylls evolved independently and

in each case similar genes were recruited to control the

development of these leafy organs. Some genes, such as

the ARPs, may carry out similar functions in both leaf

types, where it is proposed that they repress KNOX gene

expression. Nevertheless at least some of these genes,

namely the HD-ZIP class III genes carry out very different

roles in the two leaf types. So whilst similar genes were

recruited during the independent evolution their func-

tions may be very different in the two leaf types.


8 Growth and Development

Roots are shoots with hairs

It is likely that early in land plant evolution that genes

that had promoted the development of cells with rooting

functions in the gametophytes of bryophytes such as

rhizoids and caulonema became expressed in the spor-

ophyte of vascular plants or their ancestors. Once

expressed in the sporophyte they promoted the devel-

opment of rhizoids on rhizomes (shoots) and root hairs on

roots. Given that both the fossil record and comparative

genetics suggest that roots are derived shoots, it seems

that the root is a mosaic; the genetic network controlling

root axis development is derived from the shoot and that

controlling the development of filamentous epidermal

cells (rhizoids and root hairs) is derived from the haploid

phase of the life cycle in ancestors.

Conflict of interestThere is no conflict of interest relating to this article.

AcknowledgementsResearch in my laboratory is funded by the Biotechnology and BiologicalResearch Council, Natural Environment Research Council and HumanFrontiers in Science Program.

References1. Wellman CH, Gray J: The microfossil record of early land plants.

Philos Trans R Soc Lond B Biol Sci 2000, 355:717-731 discussion731–712.

2. Wellman CH, Osterloff PL, Mohiuddin U: Fragments of theearliest land plants. Nature 2003, 425:282-285.

3. Edwards D, Feehan J: Records of Cooksonia-type sporangiafrom late Wenlock strata in Ireland. Nature 1980, 287:41-42.

4. Kenrick P, Crane PR: The Origin and Early Diversification of LandPlants: A Cladistic Study Washington and London: SmithsonianInstitution Press; 1997.

5. Bateman RM, Crane PR, DiMichele WA, Kenrick PR, Rowe NP,Speck T, Stein WE: Early evolution of land plants: phylogeny,physiology, and ecology of the primary terrestrial radiation.Annu Rev Ecol Syst 1998, 29:263-292.

6. Graham LE, Wilcox LW: Algae. Saddle River, NJ: Prentice-Hall,Inc; 2000.

7. Lee JH, Lin H, Joo S, Goodenough U: Early sexual origins ofhomeoprotein heterodimerization and evolution of the plantKNOX/BELL family. Cell 2008, 133:829-840.

8. Zhao H, Lu M, Singh R, Snell WJ: Ectopic expression of aChlamydomonas mt+-specific homeodomain protein in mtSgametes initiates zygote development without gamete fusion.Genes Dev 2001, 15:2767-2777.

9. Hake S, Smith HM, Holtan H, Magnani E, Mele G, Ramirez J: Therole of knox genes in plant development. Annu Rev Cell Dev Biol2004, 20:125-151.


10. Bertolino E, Reimund B, Wildt-Perinic D, Clerc RG: A novelhomeobox protein which recognizes a TGT core andfunctionally interferes with a retinoid-responsive motif. J BiolChem 1995, 270:31178-31188.

11. Singer SD, Ashton NW: Revelation of ancestral roles of KNOXgenes by a functional analysis of Physcomitrella homologues.Plant Cell Rep 2007, 26:2039-2054.

12. Sano R, Juarez CM, Hass B, Sakakibara K, Ito M, Banks JA,Hasebe M: KNOX homeobox genes potentially have similarfunction in both diploid unicellular and multicellular meristems,but not in haploid meristems. Evol Dev 2005, 7:69-78.

13. Fujita T, Sakaguchi H, Hiwatashi Y, Wagstaff SJ, Ito M, Deguchi H,Sato T, Hasebe M: Convergent evolution of shoots in landplants: lack of auxin polar transport in moss shoots. Evol Dev2008, 10:176-186.

14. Schneeberger R, Tsiantis M, Freeling M, Langdale JA: The roughsheath2 gene negatively regulates homeobox geneexpression during maize leaf development. Development 1998,125:2857-2865.

15. Byrne ME, Simorowski J, Martienssen RA: ASYMMETRICLEAVES1 reveals knox gene redundancy in Arabidopsis.Development 2002, 129:1957-1965.

16. Byrne ME, Barley R, Curtis M, Arroyo JM, Dunham M, Hudson A,Martienssen RA: Asymmetric leaves1 mediates leaf patterningand stem cell function in Arabidopsis. Nature 2000, 408:967-971.

17. Tsiantis M, Schneeberger R, Golz JF, Freeling M, Langdale JA: Themaize rough sheath2 gene and leaf development programs inmonocot and dicot plants. Science 1999, 284:154-156.

18. Waites R, Selvadurai HR, Oliver IR, Hudson A: ThePHANTASTICA gene encodes a MYB transcription factorinvolved in growth and dorsoventrality of lateral organs inAntirrhinum. Cell 1998, 93:779-789.

19. Harrison CJ, Corley SB, Moylan EC, Alexander DL, Scotland RW,Langdale JA: Independent recruitment of a conserveddevelopmental mechanism during leaf evolution. Nature 2005,434:509-514.

20. Bowman JL, Floyd SK: Patterning and polarity in seed plantshoots. Annu Rev Plant Biol 2008, 59:67-88.

21. Floyd SK, Zalewski CS, Bowman JL: Evolution of class IIIhomeodomain-leucine zipper genes in streptophytes.Genetics 2006, 173:373-388.

22. Floyd SK, Bowman JL: Distinct developmental mechanismsreflect the independent origins of leaves in vascular plants.Curr Biol 2006, 16:1911-1917.

23. Stewart WN, Rothwell GW: Paleobotany and The Evolution ofPlants. edn 2. Cambridge: Cambridge University Press; 1993.

24. Rothwell GW, Erwin DM: The rhizomorph of Paurodendron,implications for homologies among the rooting organs of theLycopsida. Am J Bot 1985, 72:86-98.

25. Menand B, Yi K, Jouannic S, Hoffmann L, Ryan E, Linstead P,Schaefer DG, Dolan L: An ancient mechanism controls thedevelopment of cells with a rooting function in land plants.Science 2007, 316:1477-1480.

26. Bower FO: The Origin of A Land Flora: A Theory Based Upon theFacts of Alternation. edn 1. London: Macmillan; 1908.


body building on land — morphological evolution of land plants

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