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Signal transduction systems
regulating fruit ripening
Lori Adams-Phillips
1
, Cornelius Barry
1
and Jim Giovannoni
1,2
1
Boyce Thompson Institute for Plant Research, Tower Road, Cornell Campus, Ithaca, NY 14853, USA
2
USDA-ARS Plant Soil and Nutrition Laboratory, Tower Road, Cornell Campus, Ithaca, NY 14853, USA
Fruit ripening is a unique aspect of plant development
with direct implications for a large component of the
food supply and related areas of human health and nutri-
tion. Recent advances in ripening research have given
insights into the molecular basis of conserved develop-
mental signals coordinating the ripening process and
suggest that sequences related to floral development
genes might be logical targets for additional discovery.
Recent characterization of hormonal and environmental
signal transduction components active in tomato fruit
ripening (particularly ethylene and light) show conserva-
tion of signaling components yet novel gene family size
and expression motifs that might facilitate complete and
timely manifestation of ripening phenotypes. Emerging
genomics tools and approaches are rapidly providing
new clues and candidate genes that are expanding the
known regulatory circuitry of ripening.
Fruit ripening is widely studied because of the specificity
of this developmental process to plant biology and the
practical importance of ripening to the human diet.
Ripening can be generally defined as the summation of
changes in tissue metabolism rendering the fruit organ
attractive for consumption by organisms that assist in seed
release and dispersal. Specific biochemical and physio-
logical attributes of ripening fruits vary among species
although generally include changes in color, texture,
flavor, aroma, nutritional content and susceptibility
to opportunistic pathogens (reviewed in Ref. [1]). Ripening
is influenced by internal and external cues, including
developmental gene regulation, hormones, light and tem-
perature, but until recently, significant molecular under-
standing was limited primarily to the role and regulation of
ethylene biosynthesis [2].
Ripening physiology has been classically defined as
either ‘climacteric’ or ‘non-climacteric’. Climacteric fruits
show a sudden increase in respiration at the onset of
ripening, usually in concert with increased production of
the gaseous hormone ethylene. Whereas ethylene is typi-
cally necessary for climacteric ripening, non-climacteric
fruits do not increase respiration at ripening and often
have no requirement for ethylene to complete maturation.
Earlier ripening research elucidated the role of ethylene
synthesis and regulation in climacteric ripening (reviewed
in Ref. [3]), which led to several new and important
questions that have begun to be addressed in recent years
and which are the subject of this review.
†Have modifications in the design or regulation of signal
transduction systems evolved that are important for
ripening compared with the model system (primarily
Arabidopsis thaliana) in which they were defined?
†What regulates ethylene production in climacteric fruit
and does this represent a conserved regulatory switch
among climacteric and non-climacteric fruit species?
†In the absence of increased ethylene synthesis, do
non-climacteric fruit still use the ethylene signaling
pathway (possibly via altered ethylene sensitivity or
cross-talk from other signal inputs) to regulate fruit
ripening?
These questions represent a logical progression toward
understanding early ripening regulatory events in addi-
tion to molecular details of those previously documented.
Fleshy and dry fruit
Fruit tissues are composed of enlarged floral components
including one or more carpels and, in some cases
(depending on species), include tissues derived from the
calyx, receptacle, bracts or floral tube (the basal region of
floral organ fusion). Mature fruits can be categorized
generally as either fleshy or dry; fleshy fruits typically
undergo ripening as defined above and dry fruits
(e.g. Arabidopsis, cereals and legumes) mature in a
process more akin to senescence and disperse their seeds
via abscission-like programs, including dehiscence or
shattering. Arabidopsis has proven an exceptional model
for gaining insight into the molecular regulation of early
steps in fruit formation and development [4,5] but does
not develop fleshy ripe fruits. Nevertheless, ethylene and
light signal transduction pathways defined primarily in
Arabidopsis [6,7] have proven extremely useful in advanc-
ing ripening research in fleshy fruit species such as
tomato. Tomato has emerged as the most tractable model
to date for the analysis of fleshy fruit development and
ripening, in part because of available mutants, excellent
genetics, routine transformation and numerous molecular
and genomics tools ([8,9],http://www.sgn.cornell.edu/).
For these reasons, tomato is the system that has been
used for many of the recent advances in ripening described
here.
Corresponding author: Jim Giovannoni ( jjg33@cornell.edu).
Available online 15 June 2004
Review TRENDS in Plant Science Vol.9 No.7 July 2004
www.sciencedirect.com 1360-1385/$ - see front matter q2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tplants.2004.05.004
Ethylene signaling pathway defined in Arabidopsis
Much of what is known regarding the steps involved in
ethylene perception and signal transduction has been
realized through studies of the model plant species
Arabidopsis and it is therefore relevant to summarize
this work here (reviewed in Refs [6,10 –12]). In Arabi-
dopsis, ethylene is perceived by a family of five ethylene
receptors (ETR1,ETR2,ERS1,ERS2 and EIN4), similar
to bacterial two-component histidine kinase receptors
(reviewed in Refs [13,14]). Whereas dominant gain-of-
function mutations in single ethylene receptor genes
confer ethylene insensitivity, double, triple and quadruple
loss-of-function mutants result in constitutive ethylene
response phenotypes indicating their activities as redun-
dant and negative regulators of ethylene signaling
[15–17]. Acting downstream of the receptors is a putative
MAP-kinase kinase kinase (MAPKKK), termed CONSTI-
TUTIVE TRIPLE RESPONSE 1 (CTR1). CTR1 shares
homology to members of the Raf family of Ser/Thr kinases
and has been shown to possess intrinsic Ser/Thr protein
kinase activity [18]. Loss-of-function mutations in CTR1
result in constitutive activation of all the ethylene
responses examined, supporting the role of CTR1 as a
negative regulator of ethylene response [19]. In addition,
several lines of compelling evidence suggest CTR1 inter-
acts directly with receptor molecules to form a signaling
complex [20,21]. A MAP-kinase cascade has been impli-
cated in the mediation of the ethylene response down-
stream of CTR1, whereby a MAPKK [stress-induced
MAPKK (SIMKK)] activates an ethylene-inducible MAPK
protein (MPK6) [22]. However, to date, direct association of
CTR1 with SIMKK or any other MAPKK remains to be
demonstrated. Epistasis analysis places ETHYLENE
INSENSITIVE 2 (EIN2) downstream of CTR1 in the
ethylene signaling pathway [23]. EIN2 also appears to act
downstream or independently of MPK6 because ein2
mutants exhibit wild-type activation of MPK6 activity
upon treatment with and without ethylene [22]. Recent
experiments imply that the entire continuum of ethylene
phenotypes observed in receptor loss of function mutants
could be attributed to the unregulated activity of
EIN2 [16].
EIN2 encodes a protein with similarity to the Nramp
family of metal ion carriers [24] and based on indirect
evidence might represent a common convergence point
through which multiple hormone signal transduction
pathways, including abscisic acid [25,26], auxin [27],
cytokinin [28] and jasmonate [29] might act. However, the
mechanism by which EIN2 is activated remains unclear.
Considering the similarity of the EIN2 N-terminus to the
Nramp proteins, this domain might be important for
sensing or transporting a divalent cation, although no
metal-transporting capacity has been observed for EIN2
[24]. It is tempting to speculate that this cation might be
Ca
2?þ
given the role of this ion in ethylene-mediated
pathogenesis response [30]. Based on epistasis, EIN2
operates upstream of EIN3 and the EIL (EIN3-like) family
of nuclear localized trans-acting proteins [31,32]. EIN3
undergoes post-translational regulation by ethylene via
ubiquitin or proteasome-dependent proteolysis mediated
by two F-box proteins, EBF1 and EBF2 [33,34]. Homodimers
of EIN3, EIL1 and EIL2 bind to a defined target site in the
promoter region of the transcription factor, ETHYLENE
RESPONSE FACTOR 1 (ERF1)[32].ERF1 is part of a
large multi-gene family of transcription factors and is
important in the regulation of downstream ethylene
responsive genes via binding to the ‘GCC’ box promoter
element [35,36].
Tailoring ethylene signaling to the needs of a ripening
fruit
Several ethylene signal transduction components homolo-
gous to those identified in Arabidopsis have been isolated
from various plant species. Furthermore, sequence and
functional analysis is beginning to reveal that although
the basic machinery is apparently conserved, family
composition and regulation of ethylene signal transduc-
tion genes in fruit species such as tomato can vary
substantially (Figure 1).
Six ethylene receptors have been isolated in tomato, five
of which have been shown to bind ethylene (reviewed in
Refs [37,38]). Each tomato receptor gene has a distinct
pattern of expression including a subset (NEVER-RIPE or
NR and LeETR4) that is strongly induced during ripening
[39–41]. Interestingly, although a dominant mutation in
the ethylene binding site of NR confers ethylene insensi-
tivity and results in fruits that do not fully ripen [42,43],
analysis of transgenic loss-of-function mutations suggests
that NR is not necessary for ripening to proceed [44,45].
The molecular explanation for this result proved to be
compensatory up-regulation of another member of the
tomato receptor family, LeETR4, as a response to reduced
NR transcript. However, NR is not responsive to reduc-
tions in LeETR4 mRNA, indeed transgene-mediated
reduction in LeETR4 expression resulted in constitutive
ethylene responses including accelerated ripening [45].
Although reduced expression of the LeETR4 receptor
resulted in apparent increased ethylene sensitivity, over-
expression of the wild-type NR receptor in tomato resulted
in reduced sensitivity in seedlings and mature plants [46].
This is consistent with the model predicted in Arabidopsis
where ethylene receptors are thought to act as negative
regulators of ethylene signaling, thus reduced receptor
expression increases sensitivity to ethylene whereas
increased receptor expression decreases sensitivity [37,38].
However, neither the specificity of a single ethylene
receptor to a specific biological function, nor compensatory
regulation of receptor genes has been reported in
Arabidopsis and might represent the result of selective
pressures to insure maintenance of capacity to control
ethylene responses in a tissue whose normal development
is dependent on activity of this hormone.
Additional ethylene signaling components have been
defined in tomato, including a CTR1-like gene (LeCTR1)
that was shown through complementation of an Arabi-
dopsis ctr1 mutant to function in ethylene signaling [47].
Like NR and LeETR4,LeCTR1 mRNA is also up-regulated
during fruit ripening [47,48]. Only one CTR1 gene has
been identified in Arabidopsis. By contrast, additional
CTR genes (LeCTR2,LeCTR3 and LeCTR4) have been
identified in tomato (Figure 1)[48]. Further mining
of species-specific sequence databases indicates that a
Review TRENDS in Plant Science Vol.9 No.7 July 2004
332
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multi-gene family of likely CTR1 genes is not limited to
tomato [48].
Analysis of the Arabidopsis genome and extensive
screening for constitutive triple response mutants result-
ing in multiple allelic mutations in CTR1 suggests the
existence of a single CTR1 gene in Arabidopsis. Further-
more, Arabidopsis CTR1 has been assigned to a subclass
of MAPKKKs comprising six similar MAPKKK proteins
related to the Raf kinases [49]. Phylogenetic analysis
indicated that Arabidopsis CTR1 is more similar to
LeCTR1,LeCTR3 and LeCTR4 than to any of the other
five members of the Arabidopsis MAPKKK subfamily,
supporting the existence of a single CTR in Arabidopsis
and multiple CTRs in tomato [48]. Based on phylo-
genetic analysis, LeCTR2 (GenBank Accession number
AJ005077) shares more similarity with EDR1
Figure 1. Ethylene perception and signal transduction in tomato. Binding of ethylene to members of the receptor family (here represented by LeETR1, LeETR2, NR, LeETR4,
LeETR5, LeETR6) is mediated by a single copper ion (Cu), delivered by RAN1 (not shown). Ethylene negatively regulates the signal transduction pathway upon binding
to the receptor, possibly through direct interaction with the tomato CTR1 proteins (LeCTR1, LeCTR3 and LeCTR4). Upon inactivation of LeCTR protein(s), a putative MAPK
cascade (represented by LeSIMKK and LeMPK6 with candidate EST IDs shown in parentheses) is relieved from inhibition and activates ethylene signaling through a
cascade to downstream components including LeEIN2 (probably membrane localized but the specific sub-cellular membrane is currently unknown) and EIN3-like proteins,
LeEIL1– LeEIL4. LeEIL transcription factors probably initiate a transcription factor cascade through activation of secondary transcription (28txp) factors (represented as
LeERF1– LeERF4), which in turn activate ethylene-responsive target genes.
TRENDS in Plant Science
LeEIN2? (AF243474)
PERE
LeEIL1
LeEIL3 LeEIL2
LeERF4
GCC
LeETR1
NR
NR
LeCTR1
LeCTR3
LeCTR4
LeEIL4
LeERF2
LeERF3
LeERF1
Nucleus
ER
Membrane?
Cu Cu Cu Cu Cu Cu
LeETR1
LeETR2
LeETR2
LeETR4
LeETR4
LeETR5
LeETR5
LeETR6
LeETR6
LeSIMKK? (TC118386)
LeMPK6? (TC125770, TC125769)
Ethylene response genes
e.g. PG, E8, E4, NR
Phytoene synthase
ACO, ACS
2o txp factors (LeERF1–LeERF4)?
Review TRENDS in Plant Science Vol.9 No.7 July 2004 333
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(ENHANCED DISEASE RESISTANCE 1) than with
CTR1 [48,50].InArabidopsis,EDR1 is involved in
pathogen response but not in ethylene signaling [50].
Transient silencing of the LeCTR1 gene resulted in
plants with constitutive ethylene phenotypes, confirming
the physiological role of LeCTR1 in negatively regulating
ethylene responses in tomato [51]. It is worth noting that
the LeCTR1 sequence used in these experiments has
sufficient homology to, and thus could have silenced, the
subsequently discovered LeCTR3 and LeCTR4 genes. In
addition, although LeCTR1 is induced during ripening, all
three genes are known to be expressed at this stage of fruit
development [48]. Individual silencing of each LeCTR gene
will be necessary to assess individual gene function and to
address the question of redundancy in tomato.
The presence of multiple CTRs in plants raises many
questions about how signal outputs from individual
receptors are transduced. Whether or not specific tomato
CTRs interact with specific tomato receptors remains to be
demonstrated. Assuming tomato ethylene receptors and
CTRs interact, as in Arabidopsis, the interaction kinetics
between the various CTRs and the receptors, in conjunc-
tion with the varying ratio of receptors and CTRs encoded
by different family members (and for different tissues and
responses), might represent a mechanism for optimizing
fidelity of ethylene responses in tomato and other species
with multiple CTR genes.
Lastly, homologs of Arabidopsis EIN3,EIL and ERF
genes have also been identified and characterized in
tomato. Three tomato EIL genes were isolated and shown
to be functionally redundant, regulating multiple ethylene
responses throughout plant development [52]. A fourth
tomato EIL gene (LeEIL4) exhibiting ripening-induced
expression has been recently cloned, although functional
characterization has still to be completed [53]. Four
members of the ERF family (LeERF1–LeERF4) have
also been isolated in tomato and their levels of expression
have been characterized in response to wound and
ethylene treatments and in an assortment of develop-
mental stages including ripening [54].LeERF2 exhibited
ripening-associated expression and did not accumulate in
several ripening mutants, suggesting a specific role in
ripening. Proteins derived from all four LeERFs were
capable of binding to a GCC-box containing cis-elements.
Although the GCC-box has been shown to function in
mediating ethylene-inducible expression in several sys-
tems, evidence of the involvement of this element in
regulating ripening-related gene expression is lacking.
Although functional characterization of the ERF gene
family in tomato remains to be completed, ethylene-
inducible ripening expression of genes encoding multiple
steps in the tomato ethylene signal pathway strongly
suggest a selective advantage for amplifying ethylene
signaling machinery during climacteric fruit ripening.
The triple-response screen in combination with the
powerful genetic tools derived from sequencing the
Arabidopsis genome has enabled researchers to begin to
unravel the intricacies of ethylene signaling in plants.
Many of the loci characterized to date have encoded global
regulators of ethylene responses in plants. Some tissue-
specific ethylene response mutants have been identified
also, for example, ethylene insensitive root (eir1), hookless1
(hls1), and weak ethylene-insensitive (wei2,wei3)[23,55,56].
In tomato, the epinastic (epi) mutant displays a ctr-like
seedling phenotype but none of the LeCTR loci identified
to date map to the same chromosomal location as epi
[48,57,58]. Further characterization of the epi mutant was
carried out through double mutant analysis with the
dominant ethylene insensitive receptor mutant, Nr [58].
Interestingly, in the epi/epi Nr/Nr double mutant,
vegetative growth resembles that of epi, whereas petal
senescence, pedicel abscission and fruit ripening are
similar to Nr inhibition of these processes. This result
suggests a role for epi in the regulation of a specific subset
of ethylene responses controlling vegetative growth and
development, or in an independent pathway that cross-
talks with the ethylene signaling network [58]. A fruit-
specific ethylene response mutant remains to be confirmed
and reported in the literature.
ESTs provide candidates for filling in the gaps in fruit
ethylene signal transduction
The generation of vast amounts of sequence information
through EST and whole genome sequencing efforts is
providing plant biologists with new tools to dissect
development and response processes. Of particular inter-
est for ripening are the fruit EST collections derived
from tomato and grape (http://www.sgn.cornell.edu/,
http://www.tigr.org). These collections provide resources
for comparative studies of gene expression between non-
climacteric (grape) and climacteric fruits (http://ted.bti.
cornell.edu/).
In a recent report, Ashraf El-Kereamy and co-authors
described enhanced anthocyanin accumulation in grape
berries following ethylene treatment, suggesting ethylene-
responsive ripening characteristics in non-climacteric
fruit [59]. The responsiveness of at least some non-
climacteric fruits to ethylene, particularly in the area of
color development, is well documented and has commer-
cial application (e.g. in promotion of color development in
citrus peel). Examination of the grape EST collection
indicates that genes homologous to Arabidopsis ETR1,
EIN2,SIMKK and MAPK6 are expressed in grape berries.
This observation, combined with the results of El-Kereamy
et al., support the intriguing hypothesis that although
ethylene synthesis does not increase during the ripening of
non-climacteric fruits, alterations in ethylene responsive-
ness might be able to mediate physiological changes
associated with ripening.
ESTs representing candidate EIN2,SIMKK and
MAPK6 genes are also present in the tomato fruit EST
collections (Figure 1). Furthermore, mining of microarray
and EST prevalence data from the Tomato Expression
Database (http://ted.bti.cornell.edu/) suggests that a
MAPK6 homolog (TC125769) is up-regulated during
ripening, consistent with other genes encoding ethylene
signaling components in maturing tomato fruit. Expan-
sion of EST resources from these and other fruit crops
should reveal candidates for ethylene signal transduction
from additional species but can also facilitate compara-
tive analysis of family size and expression levels (this is
true only when ESTs are derived from non-normalized,
Review TRENDS in Plant Science Vol.9 No.7 July 2004
334
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non-subtracted cDNAs, as is the case for existing tomato
and grape ESTs). In short, accumulating EST and
associated expression data should enable more-accurate
prediction of orthologs to known genes, in addition to
facilitating identification of candidates for aspects of
ethylene signaling that remain poorly defined (e.g. MAP-
kinase cascade components downstream of CTR1 and
ultimate activators of ethylene-regulated gene expression
during fruit ripening).
Light signal transduction impacts ripe fruit pigmentation
and is a target for nutritional enhancement
In contrast to ethylene, which is required for completion of
most, if not all, ripening processes in climacteric fruit, the
impact of light during fruit ripening appears to be specific
to regulation of pigment accumulation [60]. Tomato high-
pigment mutations (hp1,hp2) result in elevated carotenoid
and flavonoid accumulation because of increased sensi-
tivity to light but have little impact on other ripening
characteristics [61,62]. The genes responsible for both
mutations have been cloned and represent tomato homo-
logs of light signal transduction genes previously
described in Arabidopsis. Specifically, hp1 results from a
lesion in a gene homologous to UV-DAMAGED DNA
BINDING PROTEIN 1 (DDB1) and hp2 is mutated in the
tomato DE-ETIOLATED1 (DET1) ortholog [63,64]. The
corresponding Arabidopsis proteins are capable of inter-
action [65] and analysis of single and hp1 hp2 double
mutants suggests that the same is likely to be true in
tomato [63].
Ripe fruit pigments including carotenoids and flavon-
oids have antioxidant properties that assist in neutralizing
the effects of photo-oxidation while also having nutritional
significance to humans [66–68]. Because mutations in the
light signaling pathway positively influence pigmentation
of ripe fruit, targeting the light signaling pathway might
be an effective means of engineering fruit nutritional
quality. Although carotenoid accumulation in edible plant
tissues has been manipulated by altering corresponding
biosynthetic enzymes (e.g. Golden Rice, [68]), the outcome
of such approaches has typically fallen short of expec-
tations. This is probably because of a lack of understanding
regarding endogenous mechanisms of regulation and accu-
mulation of carotenoids and/or undesirable side effects on
non-target metabolites derived from the altered pathway
[68,69]. Engineering of an existing signal transduction
network already capable of regulating flux through the
carotenoid synthesis pathway in a biologically viable
manner might represent a simplified alternative to opti-
mizing carotenoid-associated nutritional benefit in plant
tissues such as fruit. Indeed, recently it has been shown
that manipulating tomato light signal transduction genes
homologous to HY5 and COP1 from Arabidopsis can result
in modified fruit carotenoid accumulation [63].
Developmental regulation of ripening: moving up the
regulatory cascade
Insights into the molecular basis of ethylene synthesis and
perception in climacteric fruit logically lead to questions
concerning ripening regulation upstream of ethylene
synthesis and response. As stated at the onset: what
regulates ethylene during climacteric fruit ripening?
Answers to this question could also conceivably lead to
the discovery of conserved regulatory mechanisms shared
by climacteric and non-climacteric species.
Three spontaneous tomato ripening mutations, ripening-
inhibitor (rin), non-ripening (nor) and Colorless non-
ripening (Cnr) are particularly interesting in this regard
because their physiology is suggestive of roles in ripening
regulation before ethylene synthesis. Fruit homozygous
for either rin or nor, or carrying a dominant Cnr allele,
undergo complete fruit expansion and yield mature seed,
yet fail to proceed in any significant way to ripening.
Mature and unripe rin,nor or Cnr fruit do not demonstrate
climacteric respiration nor elevated ethylene synthesis
[70,71]. However, both rin and nor are capable of ethylene
synthesis in response to wounding [72], suggesting that
the lack of ripening ethylene in these two mutants is
because of a deficiency in appropriate developmental
signals, as opposed to genetic lesions in ethylene biosyn-
thetic genes. Data on Cnr wound ethylene has not been
reported. Application of endogenous ethylene does not
restore ripening to rin,nor or Cnr fruit, but does result in
induction of ethylene-regulated genes [71,73]. This last
observation is particularly intriguing because it suggests
that rin,nor and Cnr have a broader influence on aspects
of climacteric ripening than those aspects controlled solely
by ethylene (Figure 2) and such mechanisms might be
expected to be conserved between both climacteric and
non-climacteric species [9].
Figure 2. Model for regulation of climacteric ripening via coordinated signaling
pathways. Transcription factors including LeMADS-RIN, LeNOR, likely additional
MADS-box proteins, CNR and factors remaining to be discovered (?) represent
the developmental signaling system that initiates ripening in climacteric fruit.
Some components, such as those homologous to LeMADS-RIN can be used in
non-climacteric species as well. The developmental signaling system regulates
ethylene synthesis that is itself autocatalytic, in addition to non-ethylene-mediated
ripening responses (represented by the red broken arrow). Light influences ripen-
ing, at least in tomato, only in relation to carotenoid accumulation and through
activity of the DET1 (hp2) and DDB1 (hp1) gene products.
TRENDS in Plant Science
LeMADS-RIN
LeNOR
LeMADS-?
H
H
H
H
Receptors
Light
?
Developmental
transcription factors
CNR
?
Ethylene synthesis
MAP kinase
cascade and
Nramp
Fruit ripening genes
Ethylene response
transcription factors
C=C
Review TRENDS in Plant Science Vol.9 No.7 July 2004 335
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Positional cloning efforts have resulted in the isolation
of both the rin and nor loci and molecular characterization
of both mutations. rin results from deletion of the last exon
of a tomato MADS-box transcription factor gene desig-
nated LeMADS-RIN [9,74]. The nor locus harbors a gene
with structural features suggestive of a transcription
factor, although not a member of the MADS-box family
(J. Vrebalov and J. Giovannoni, unpublished).
MADS-box genes are ubiquitous among eukaryotes and
are predominantly associated with floral determination
and development in plants [75]. MADS-box proteins are
capable of forming heterodimers and higher-order multi-
mers, suggesting additional MADS-box genes might
participate in ripening (Figure 2;[76]). Indeed, several
MADS-box genes expressed in ripening tomato fruit
have been identified and are logical candidates for
functional analysis related to fruit ripening ([9], see also
http://ted.bti.cornell.edu/ for expression data on tomato
MADS-box genes). Even more intriguing is the use of the
LeMADS-RIN cDNA to recover a similar sequence from
strawberry, a non-climacteric fruit, suggesting a conserved
link between climacteric and non-climacteric ripening
control [74]. Orthologous genes from agriculturally
important fruit species are now likely to be targeted as
tools for engineering fruit quality and shelf-life.
Identification of two putative transcription factors
regulating ripening in tomato through induction of
climacteric ethylene biosynthesis and additional non-
ethylene-regulated processes, represents a higher rung
in the ladder of fruit ripening control as well as candidates
for conserved molecular mechanisms governing climac-
teric and non-climacteric ripening. Isolation of the Cnr
locus should contribute to a greater appreciation of the
developmental component of ripening regulation. Under-
standing the relationships among the Cnr,rin and nor
gene products represents a logical next target for under-
standing the molecular basis of ripening control.
Emerging genomics tools including ESTs and expres-
sion arrays are also likely to accelerate the discovery of
homologous genes from additional species and the identi-
fication of additional novel ripening regulators, particu-
larly when their evolutionary conservation is established
via comparative genomics approaches. For example, a
recent comparison of ripening-related gene expression in a
non-climacteric fruit species (grape) versus a climacteric
species (tomato) resulted in identification of ripening-
related transcription factor sequences from families that
previously had not been associated with ripening [77].
Specifically, EST abundance was used as a measure of gene
expression in ripening tomatoes and grapes. Subsets of
ripening-related genes from both species were compared
at the level of predicted peptide homology to identify
homologous genes with parallel expression patterns in
maturing fruit tissues from both tomato and grape.
Although ,20 ripening-related putative transcription
factor sequences were identified in each species, three
were highly homologous and thus represent candidates for
conserved regulation of ripening in climacteric and non-
climacteric species. The three common transcription factor
sequences included members of the MADS-box, B-zip, and
zinc-finger families; B-zip and zinc-finger have not been
previously associated with ripening. Functional charac-
terization of these genes, and additional regulatory
candidates likely to result from continued genomics-
based experiments should enable researchers studying
ripening to identify broadly conserved and more-specific
genetic regulators of ripening in the near future.
Acknowledgements
Research in the Giovannoni laboratory is supported by grants from the
National Science Foundation, BARD, IFAFS and the US Department of
Agriculture NRICGP.
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Endeavour
the quarterly magazine for the history
and philosophy of science
You can access Endeavour online via
ScienceDirect, where you’ll find a
collection of beautifully illustrated
articles on the history of science, book
reviews and editorial comment.
Featuring
Sverre Petterssen and the Contentious (and Momentous) Weather Forecasts for D-Day, 6 June 1944 by J.R. Fleming
Food of Paradise: Tahitian breadfruit and the Autocritique of European Consumption by P. White and E.C. Spary
Two Approaches to Etiology: The Debate Over Smoking and Lung Cancer in the 1950s by M. Parascandola
Sicily, or sea of tranquility? Mapping and naming the moon by J. Vertesi
The Prehistory of the Periodic Table by D. Rouvray
Two portraits of Edmond Halley by P. Fara
and coming soon
Fighting the ‘microbe of sporting mania’: Australian science and Antarctic exploration in the early twentieth century
by P. Roberts
Learning from Education to Communicate Science as a Good Story by A. Negrete and C. Lartigue
The Traffic and Display of Body Parts in the Early-19th Century by S. Alberti and S. Chaplin
The Rise, Fall and Resurrection of Group Selection by M. Borrello
Pomet’s great ‘‘Compleat History of Drugs’’ by S. Sherman
Sherlock Holmes: scientific detective by L. Snyder
The Future of Electricity in 1892 by G.J.N. Gooday
The First Personal Computer by J. November
Baloonmania: news in the air by M.G. Kim
and much, much more . . .
Locate Endeavour on ScienceDirect (http://www.sciencedirect.com)
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