Journalartikel

Jahr der Veröffentlichung: 2004

Seiten: 129-148

Zeitschrift: Critical Reviews in Plant Sciences

Bandnummer: 23

Heftnummer: 2

ISSN: 0735-2689

eISSN: 1549-7836

DOI Link: https://doi.org/10.1080/07352680490433240

Verlag: Taylor and Francis Group

Abstract: 
Class B floral homeotic genes play a key role in specifying the identity
of male reproductive organs (stamens) and petals during the development
of flowers. Recently, close relatives (orthologues) of these genes have
been found in diverse gymnosperms, the sister group of the flowering
plants (angiosperms). The fact that such genes have not been found so
far, despite considerable efforts, in mosses, ferns or algae, has been
taken as evidence to suggest that B genes originated 300–400 million
years ago in a lineage that led to extant seed plants. Gymnosperms do
not develop petals, and their male reproductive organs deviate
considerably from angiosperm stamens. So what is the function of
gymnosperm B genes? Recent experiments revealed that B genes from
diverse extant gymnosperms are exclusively expressed in male
reproductive organs (microsporophylls). At least for some of these genes
it has been shown that they can partially substitute for the Arabidopsis B genes AP3 and PI
in ectopic expression experiments, or even partially substitute these
genes in different class B floral organ identity gene mutants. This
functional complementation, however, is restricted to male organ
development. These findings strongly suggest that gymnosperm and
angiosperm B genes have highly related interaction partners and
equivalent functions in the male organs of their different host species.
It seems likely that in extant gymnosperms B genes have a function in
specifying male reproductive organs. This function was probably
established already in the most recent common ancestor of extant
gymnosperms and angiosperms (seed plants) 300 million years ago and thus
represents the ancestral function of seed plant B genes, from which
other functions (e.g., in specifying petal identity) might have been
derived. This suggests that the B gene function is part of an ancestral
sex determination system in which B gene expression specifies male
reproductive organ development, while the absence of B gene expression
leads to the formation of female reproductive organs. Such a simple
switch mechanism suggests that B genes might have played a central role
during the origin of flowers. In the out-of-male and out-of-female
hypotheses changes in B gene expression led to the origin of
hermaphroditic flower precursors out of male or female gymnosperm
reproductive cones, respectively. We compare these hypotheses with other
recent molecular hypotheses on the origin of flowers, in which C/D and FLORICAULA/LEAFY-like genes is given a more prominent role, and we suggest how these hypotheses might be tested in the future.

Harvard-Zitierstil: Becker, A. and Theißen, G. (2004) Gymnosperm Orthologues of Class B Floral Homeotic Genes and Their Impact on Understanding Flower Origin, Critical Reviews in Plant Sciences, 23(2), pp. 129-148. https://doi.org/10.1080/07352680490433240

APA-Zitierstil: Becker, A., & Theißen, G. (2004). Gymnosperm Orthologues of Class B Floral Homeotic Genes and Their Impact on Understanding Flower Origin. Critical Reviews in Plant Sciences. 23(2), 129-148. https://doi.org/10.1080/07352680490433240