required for normal ovule development strongly suggests that posttranscriptional regulation
of ovule identity genes is important for maternal development.
The phenotypes of these mutants help build a model of the ovule developmental
pathway. They suggest that during the process of flowering the ovule primordia initiate
and then gain ovule identity. For example, primordia initiation must include ANT function,
which is then followed by the action of genes that specify the integuments like BEL1. In
this model, SIN1 function would follow, giving rise to the normal shape and size of the
integuments. Thus, by using a combination of genetic and molecular approaches, develop-
mental biologists can order gene function in the development of specific tissues.
4.2.2 Fertilization
The beginning of a plant’s life starts with fertilization of the haploid (1N) egg cell within
the ovule by one of the two haploid sperm nuclei carried by the pollen tube of the pollen
grain (see Chapter 2 and Fig. 4.2). Development will produce a 2Nplant embryo sur-
rounded by maternal tissues within the carpels. Plants actually undergo a separate fertiliza-
tion event that creates the 3N endosperm. The endosperm results from fusion of the other
1 N spermnuclei with the twopolar nuclei(2N) within the central cell of the ovule. The
resulting endosperm tissue can transfer nutrients into the developing embryo. Thus plants,
like animals, have a food supply handy for the developing embryo. The triploid nature of
the endosperm has been speculated to be a mechanism for controlling gene dosage or a way
for maternal control of embryo development (Berger et al. 2006). An interesting phenom-
ena calledendoreplication,orendoduplication, occurs at an increased rate within the endo-
sperm. This process involves DNA replication in the absence of cell division, resulting in a
highNnumber within certain cells of the endosperm.
Studies onEphedra trifurca, a nonflowering seed plant that is a close relative of the
angiosperms, have revealed key differences in fertilization. This plant, from which
Mormon tea is made, has a second fertilization event that leads to formation of a second
embryo instead of endosperm development. This difference has prompted speculation
that the modern endosperm of today’s plants may have evolved from a second embryo
like that found inEphedra. We know that fertilization and development of the embryo
and endosperm in angiosperms are dependent on each other; that is to say that normally
the endospermmustdevelop in order for the embryo to develop. However, there is a
mutant that has been identified where fertilization of the endosperm occurs in the
absence of embryo fertilization and development. This mutant, calledfie(fertilization-
independent endosperm), suggests a connection between endosperm development and
chromatin as the FIE gene product is a type ofpolycombprotein. Polycomb proteins
were first discovered inDrosophila melanogasterand act by “locking” chromatin into
accessible or nonaccessible forms that dramatically alter gene expression in the next gener-
ation. Thus, the FIE polycomb gene product may be necessary to “lock in” the appropriate
chromatin pattern for the communication between the embryo and the endosperm develop-
mental processes (Twell 2006).
4.2.3 Fruit Development
Fertilization is also important to consider in plant biotechnology as it directly impacts the
process of fruit development. Fertilization is the trigger for growth of the ovary that then can
develop into a fruit. The termfruitcan be used to describe any ovary that initiates a growth
88 PLANT DEVELOPMENT AND PHYSIOLOGY