Download My favourite flowering image: a cob of pod corn

Journal of Experimental Botany, Vol. 65, No. 22, pp. 6751–6754, 2014
doi:10.1093/jxb/ert461 Flowering Newsletter Review
My favourite flowering image: a cob of pod corn
Günter Theißen*
Department of Genetics, Friedrich Schiller University Jena, Philosophenweg 12, D-07743 Jena, Germany
* To whom correspondence should be addressed. E-mail: [email protected]
Abstract
For good reasons scientists usually do not report the personal circumstances of their work when publishing their
results. This means, however, that the scientific facts being reported may not accurately reflect the personal importance of the respective work for the individual scientists. Pictures of pod corn (or Tunicate maize) have been on my
mind for much of my life, through good and through bad times. This is why…
Key words: Domestication, inflated calyx syndrome, MADS-box gene, maize, STMADS11-like gene, SVP, Tunicate mutant,
Zea mays.
I would probably never have been obsessed by pod corn without
my father. When he became chronically ill in the early 1970s
he started reading frantically a wide variety of books. One of
these books (Grebenscikov, 1959) was about the domestication
of maize (Zea mays ssp. mays). In contrast to the strange novels
my father was reading, it immediately caught my interest. I was
impressed by the enormous agronomic and religious importance of maize for many Native American Indian tribes. I was
even more provoked by the fact that maize does not exist in the
wild. [We now know that maize is morphologically so different from its wild ancestor, teosinte (Zea mays ssp. parviglumis)
that ancestry is not easily recognizable.] I was most fascinated,
however, by a picture of a strange mutant of maize, pod corn
(Tunicate maize) that was, at this time, hypothesized to represent an ancestral form of maize (Fig. 22 in Grebenscikov, 1959).
Due to its amazing phenotype, pod corn has been of religious
importance for certain Native American Indian tribes since
pre-Columbian times, who believed in its curative and magical
powers (Cutler, 1944; Mangelsdorf, 1948, 1958). It seemed that
I was not the only one who was fascinated by pod corn.
The most prominent phenotypic feature of pod corn is a
foliaceous elongation of the glumes, such that they cover the
kernels in the ears (Fig. 1; cob on the right). This is different
from ordinary maize varieties in which glumes are not present
or are so short that they are invisible in the mature ear (Fig. 1;
cob on the left).
As a postdoctoral student in the early 1990s, I seriously
considered continuing my work as a molecular biologist
to try to work for the benefit of mankind by helping to
cure AIDS or other devastating diseases (‘It’s the economy,
stupid!’). However, when Heinz Saedler at the Max Planck
Institute for Breeding Research in Cologne offered me the
chance to work on the domestication of maize, I remembered that strange book of my father’s and couldn’t
resist the opportunity. Among a collection of maize cobs
in Heinz’s office I saw the ‘real thing’ for the first time.
I was impressed again. It didn’t look like one of those
usual mutants where one immediately gets the feeling that
there is something completely wrong. On the contrary, the
mutant phenotype was of bizarre beauty and appeared
quite functional. There is nothing wrong with wrapping
fruits in long glumes, I thought, almost all grasses do this.
Suddenly wild-type maize with its naked kernels appeared
stranger to me than the Tunicate mutant. (One should
say that only the cobs of heterozygous plants are appealing; mutants homozygous for the Tunicate allele are less
impressive.)
After some initial detours, I started a project in Heinz’s
department studying the contribution of MADS-box genes to
the domestication of maize and their potential use for rational
crop plant design. cDNAs and genomic fragments of these
genes were cloned and their chromosomal map positions were
determined in order to see whether they mapped to any of the
chromosomal regions that John Doebley and his coworkers
had identified as the regions relevant for the origin of maize
(Doebley et al., 1990). This naïve candidate gene approach was
© The Author 2014. Published by Oxford University Press on behalf of the Society for Experimental Biology. All rights reserved.
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6752 | Theißen
Fig. 1. An ear from ‘wild type’ maize (left) and pod corn (right).
The pod corn ear is from a heterozygous Tu/+ plant; homozygous
plants have an even stronger mutant phenotype, but have, in the
author’s view, less beauty. (Picture taken from Wingen et al., 2012.
The molecular basis of pod corn (Tunicate maize). Proceedings of
the National Academy of Sciences, USA 109, 7115–7120.)
initially lots of fun but, in the long run, exhausting, not least
because we had initially underestimated the total number of
MADS-box genes in maize by about an order of magnitude.
One day in the late 1990s we got a new set of mapping data.
Thomas Münster, a postdoctoral student in the laboratory,
told me that, yet again, none of our genes mapped to one of
the ‘Doebley regions’. He also told me that one of the genes
mapped closely to a mutant of maize termed ‘Tunicate’ and
he asked me whether this made any sense to me. It did! I knew
that Tunicate is based on a dominant, gain-of-function mutation and that many of these mutations bring about their
effects because of ectopic expression of the gene, so I predicted that the Tunicate gene is expressed in the glumes of
maize in the Tunicate mutant, but not in the wild type.
It took me a while to convince someone in the laboratory
to work on Tunicate, since such a project did not follow our
major goals. Luzie Wingen, a PhD student in my group at
that time, eventually agreed to embark on this endeavor. She
just needed a few weeks to grow the plants and to do some
initial Northern hybridization experiments. The results were
clear-cut: in all of the wild-type plants she tested, the gene was
expressed only in vegetative tissues, especially in leaves, while
in the Tunicate mutant there was strong ectopic expression in
male and female inflorescences. Luzie confirmed her findings
by detailed in situ hybridization experiments. Since ectopic
expression was not observed for other, closely related genes,
we were convinced that we had cloned the Tunicate gene.
The mapping result was also interesting for another reason:
it associated a mutant phenotype and hence a function with
a new subfamily of MADS-box genes that was unknown at
that time. Wolfram Faigl, a technician in our laboratory, and
Thomas had cloned several subfamily members from maize
and termed them ‘Faimadse’. When the first subfamily member was published from potato as STMADS11 (Carmona
et al., 1998), we applied the priority rule for naming clades of
MADS-box genes (Theißen et al., 1996; Becker and Theißen,
2003) and termed the whole clade ‘STMADS11-like genes’
(Becker et al., 2000; Becker and Theißen, 2003). The gene
SHORT VEGETATIVE PHASE (SVP) from Arabidopsis
thaliana, cloned by Peter Huijser and co-workers (Hartmann
et al., 2000), is probably the best-known STMADS11-like
gene. It turned out that STMADS11-like genes have diverse
and interesting functions in phase transitions and other developmental processes and hence they have become the subject
of intense research to the present day (Khan and Ali, 2013).
Following a tradition in our laboratory (Theißen et al.,
1995), the Tunicate candidate gene was termed ZMM19.
Further work corroborated our hypothesis that ZMM19
represents the Tunicate locus. We found, for example, that
ZMM19 is a duplicate locus in Tunicate plants but not in
wild-type plants and has rearrangements in the promoter
regions of the mutant. These findings fitted nicely to some
classical articles on maize genetics maintaining that the
mutant Tu locus is complex and compound (Mangelsdorf
and Galinat, 1964).
In March 2000, Luzie presented our results at the 42nd
Annual Maize Genetics Conference in Cœur d’Alene, Idaho,
USA (Wingen et al., 2000). In a detailed report about this
conference, Running et al. (2000) devoted a whole section to our findings, starting with the statement that ‘Luzie
Wingen…described the cloning of the Tunicate1 (tu1) gene’.
I was convinced that we would publish our findings within a
few months but I was wrong.
We decided to do some additional experiments to corroborate our findings. For example, we cloned and characterized
some ‘weak’ Tunicate alleles with the help of Hans Sommer.
Our additional experiments all finally worked out nicely, but
slowly. The fact that some of us, including Luzie and me, left
Cologne and moved on to new challenging and time-consuming jobs did not speed up the project either.
Only a few brief messages were sent to the world about
our findings. In a paper in Maydica about MADS-box genes
in maize, we reported that ZMM19 mapped closely to the
Tunicate locus and that several other independent lines of evidence strongly supported the view that ZMM19 indeed represented the Tunicate gene (Münster et al., 2002). A bit later,
Chaoying He, Thomas Münster, and Heinz summarized our
findings in an article about ‘evolutionary novelties’ (He et al.,
My favourite flowering image | 6753
2004). The piece was termed a ‘minireview’ but, in the case of
the Tunicate story, it reported results almost a decade before
they were actually published so that ‘preview’ would have
been a more appropriate term. In the same year we submitted
the sequence of ZMM19 from Tunicate to GenBank. This
obviously helped others to uncover the molecular basis of the
Tunicate gene (see Acknowledgements in Han et al., 2012).
Even though most of our data on Tunicate were not yet
published, they nevertheless inspired others to look for similar phenomena elsewhere. Heinz wondered whether orthologues of a gene that makes glumes elongate upon ectopic
expression may have a similar effect on sepals. Along these
lines of thought he started to investigate the ‘inflated calyx’
trait of the Solanaceae. Heinz’s intuition was right and, since
he had teamed up with co-workers that were on a faster publication track, the initial work on the inflated calyx phenomenon was published many years before our work on Tunicate
(He and Saedler, 2005).
While the inflated calyx story generated a tsunami of
publications (He and Saedler, 2007; He et al., 2007; Hu and
Saedler, 2007; Khan et al., 2009, 2012; Zhang et al., 2012), the
original source of inspiration made little waves. That we may
have missed a chance to get a good publication was therefore
hard to deny. Nevertheless, it took us a while to draft the first
manuscript as most of us were busy with other duties that
appeared more important at that time. Since I still thought
that we had cloned one of the coolest genes on this planet
we decided, in 2005, to send the manuscript to one of these
prestigious journals you all know (journal title available upon
request). However, reviewers told us that our manuscript
was ‘just a cloning paper’ and thus only of limited interest.
I felt I was still playing Classic Rock (gene cloning) at a time
when everyone was already listening to Hip Hop and House
(genomics). I actually wanted proudly to present the paper to
my father, but time was running out: After decades of illness
he died at the end of 2005 before our work was published.
For the reasons mentioned before it took us quite some
time to rewrite the manuscript and to send it to the next prestigious journal. This time—in 2008—the Editor did not even
send it out for review (Classic Rock was only a faint memory
by then).
But I couldn’t get these pictures of pod corn out of my
mind. Luzie also started to push me. When I learned that the
52nd Annual Maize Genetics Conference was to be held in
Europe for the first time and at such a nice location as Riva at
the northern end of Lake Garda in Italy, I decided to advertise our work on Tunicate once again. This time, 10 years after
Luzie’s presentation in Idaho, a poster presentation was all
I could get, but I was more than pleased. In Riva, I met JongJin Han from Rob Martienssen’s group at the Cold Spring
Harbor Laboratory in New York. I learned that he was also
working on Tunicate and that he had for quite a while hypothesized that Tunicate encodes a microRNA but he was now
also convinced that the MADS-box gene ZMM19 represents
the Tunicate locus.
Being confirmed in this way is somehow a mixed blessing.
We gave the prestigious journal approach a final try and sent
the manuscript to PNAS in 2011. This time we were lucky
in that John Doebley got to handle it—if there is anyone on
this planet that has sympathy for work on maize, it is probably him, and, amazingly, one of our reviewers remembered
Luzie’s talk 12 years ago and strongly argued that publication
of our data was overdue. That was hard to deny. All in all,
the reviewer’s requests were tough, but fair, and thus revision
of the manuscript was hard and time-consuming, but doable. The letter explaining our changes to the manuscript that
Luzie and I produced was finally almost as long as the whole
manuscript.
During that time my mother suddenly became very ill.
Thus I had to do much of the work required for revision and
the correction of the galley proofs during long train rides. At
that time I often felt that working on bizarre plant mutants
doesn’t make any sense at all, because it doesn’t help to
reduce misery and pain. One day I was sitting at my mother’s
bed that she could not leave anymore with a version of the
paper published online ahead of print. I wanted to show it
to her but she was too weak to recognize what it was about.
She probably would not have been interested in it anymore
anyway.
About 15 years had passed since we had cloned the Tunicate
gene, and I should have been happy that we finally managed
to get it published in a decent journal. Mission accomplished!
But I felt nothing but emptiness. Our paper was finally published in print on the first of May 2012 (Wingen et al., 2012).
All’s well that ends well! Two days later my mother died.
PS: Jong-Jin’s very detailed work was published two
months later in The Plant Cell (Han et al., 2012)—he even
got the cover! The paper not only confirmed all of our key
findings but also had a number of interesting new results to
report. For example, the authors demonstrated that the rearrangement in the promoter region of ZMM19 in Tunicate is
based on a 1.8 Mb chromosomal inversion that fused a gene
expressed in inflorescences to the 5’ regulatory region of
ZMM19; that the two ZMM19 copies in Tunicate are about
30 kb apart (we had wondered about this for a long time); and
that transgenic lines in which ZMM19 expression is driven by
the rearranged promoter region phenocopy the Tunicate trait.
Addendum: I am fully aware that the picture I have chosen
does not show a flower, but two infructescences. I apologize
for any confusion that this may cause. My only excuse: There
is just no flower picture that means so much to me.
Acknowledgements
I thank Wim Deleu and Hans Sommer for their contributions to the Tunicate project. Special thanks go to Wolfram
Faigl and Thomas Münster for playing Klotto and picking
Faimadse clone wf4123, that was renamed into SUPER
TECHNICAL ASSISTANT GENE1 (STAG1), that was
renamed into TM078, that was renamed into ZMM19,
and eventually turned out to be the TUNICATE gene. I am
especially grateful to Heinz Saedler without whose continuous support the project would not even have been initiated
and to Luzie Wingen, for without her final enthusiasm for
the Tunicate work our data may never have been published.
6754 | Theißen
I thank Jong-Jin Han for a long talk in Riva. I am indebted
to John Doebley for giving us a chance. Many thanks also
to Wolfram, Thomas, and Luzie for helpful comments on
a previous version of the manuscript. Special thanks go
to Lydia Gramzow for improving my English and to Lars
Hennig for inviting me to write about my favourite flower
image and for his patience. This paper is dedicated to my
parents, Heiner Theißen (1940–2005) and Marlies Theißen
(1939–2012).
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