Articles
https://doi.org/10.1038/s41477-017-0082-9
© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
1
Division of Evolutionary Biology, National Institute for Basic Biology, Okazaki 444-8585, Japan.
2
Department of Basic Biology, School of Life Science,
SOKENDAI (The Graduate University for Advanced Studies), Okazaki 444-8585, Japan.
3
Graduate School of Natural Science and Technology, Kanazawa
University, Kanazawa 920-1192, Japan.
4
School of Life Science and Technology, Tokyo Institute of Technology, Yokohama 226-8501, Japan.
5
JST CREST,
Yokohama 226-8501, Japan.
6
Department of Cell Biology and Anatomy, Graduate School of Medicine, The University of Tokyo, Bunkyo 113-0033, Japan.
7
The Earth-Life Science Institute, Tokyo Institute of Technology, Meguro 152-8550, Japan.
8
Functional Genomics Facility, National Institute for Basic
Biology, Okazaki 444-8585, Japan.
9
School of Food Industrial Sciences, Miyagi University,Sendai 982-0215, Japan. *e-mail: mhasebe@nibb.ac.jp
M
ADS-box genes are conserved in green plants, metazoans
and fungi
1,2
, and encode proteins containing the MADS
domain, which comprises approximately 60 amino acid
residues with DNA binding activity for transcriptional regula
-
tion
3,4
. In addition to the MADS-domain, MIKC
C
-type MADS-box
genes
5
encode I, K and C domains for protein interactions. MIKC
C
-
type MADS-domain proteins interact with distinct sets of other
MADS-domain proteins, other transcription factors and chroma
-
tin remodelling factors
6,7
. These interactions enable MIKC
C
-type
MADS-domain proteins to specifically regulate different sets of
genes, including those encoding various transcription factors and
proteins involved in hormone biosynthesis, signalling and growth
regulation
8
. As a result, MIKC
C
-type MADS-box genes function in a
range of developmental processes in gametophytes, seeds, embryos,
roots, leaves, inflorescences and fruits
9
.
Although approximately 40 MIKC
C
-type MADS-box genes are
present in angiosperm species, single MIKC
C
-type MADS-box genes
have been found in species of the Zygnematophyceae, Charales and
Coleochaetales algae
10
, of which Zygnematophyceae is inferred to
be the most closely related lineage to extant land plants
11
. The moss
Physcomitrella patens is a member of a basal lineage of land plants,
and possesses six MIKC
C
-type MADS-box genes
12
, which diverged
in the moss lineage
13
. These results suggest that the common ances-
tor of land plants had a single MIKC
C
-type MADS-box gene, and
that the increase in the number of genes through gene and genome
duplications and subsequent neo- and sub-functionalization has
corresponded to the evolution of body organization in the land
plants
14,15
.
Because MIKC
C
-type MADS-box genes have diverged remark-
ably in land plants and are involved in various developmental
processes in angiosperms, their functions have been characterized
in the green algae and bryophytes to infer their original role in the
common land plant ancestor, as well as their subsequent functional
evolution. Based on expression analyses, the MIKC
C
-type MADS-
box genes are speculated to regulate gamete and gametangium
formation in the green algae and bryophytes
10, 16–18
; however, loss-
and gain-of-function experiments have not been reported in the
green algae, and three mutant lines lacking single Physcomitrella
genes are indistinguishable from wild type, which is probably
because of genetic redundancy
17
. In addition, downregulation of the
Physcomitrella MADS-box gene PPM1 using antisense RNA causes
variable defects in the gametangia and sporophytes, although the
ppm1 deletion mutant resembles the wild type
17
. Therefore, the
functions of MIKC
C
-type MADS-box genes in green algae and
Physcomitrella remain unclear.
In this study, to elucidate the precise functions of the MIKC
C
-
type MADS-box genes in Physcomitrella and infer their evolution
in the land plants, we analysed the spatiotemporal protein localiza
-
tion patterns of all six Physcomitrella MIKC
C
-type MADS-box genes
and characterized the morphology and development of deletion
and overexpression mutant lines. We found that the MIKC
C
-type
MADS-box genes are necessary for proper fertilization in two ways:
the regulation of the length of the gametophore internodes affects
external water uptake, which enables the sperm to swim to the egg,
and the regulation of motile flagellum formation in sperm. The for
-
mer function appears to have been maintained in the angiosperms,
since internode length is determined by the regulation of cell divi
-
sion and growth and certain angiosperm MIKC
C
-type MADS-box
genes regulate cell division and growth
19–21
. The latter function was
lost, consistent with the loss of sperm in the angiosperms.
Physcomitrella MADS-box genes regulate water
supply and sperm movement for fertilization
Shizuka Koshimizu
1,2
, Rumiko Kofuji
1,3
, Yuko Sasaki-Sekimoto
4,5
, Masahide Kikkawa
6
, Mie Shimojima
4
,
Hiroyuki Ohta
4,5,7
, Shuji Shigenobu
2,8
, Yukiko Kabeya
1
, Yuji Hiwatashi
1,2,9
, Yosuke Tamada
1,2
,
Takashi Murata
1,2
and Mitsuyasu Hasebe
1,2
*
MIKC classic (MIKC
C
)-type MADS-box genes encode transcription factors that function in various developmental processes,
including angiosperm floral organ identity. Phylogenetic analyses of the MIKC
C
-type MADS-box family, including genes from
non-flowering plants, suggest that the increased numbers of these genes in flowering plants is related to their functional diver-
gence; however, their precise functions in non-flowering plants and their evolution throughout land plant diversification are
unknown. Here, we show that MIKC
C
-type MADS-box genes in the moss Physcomitrella patens function in two ways to enable
fertilization. Analyses of protein localization, deletion mutants and overexpression lines of all six genes indicate that three
MIKC
C
-type MADS-box genes redundantly regulate cell division and growth in the stems for appropriate external water conduc-
tion, as well as the formation of sperm with motile flagella. The former function appears to be maintained in the flowering plant
lineage, while the latter was lost in accordance with the loss of sperm.
© 2017 Macmillan Publishers Limited, part of Springer Nature. All rights reserved.
NATURE PLANTS | VOL 4 | JANUARY 2018 | 36–45 | www.nature.com/natureplants
36
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