identified in mutant screens. Each of these genes encode proteins required for ovule
development. For example, theantmutant cannot make theintegumentsthat surround
the developing egg cell; thus the ANT gene product is required for development of the inte-
guments. Thebel1mutant is also defective in integuments, but does develop a collar of
tissue that surrounds the egg cell. Thusbel1mutants have an altered integument and the
function of the BEL1 protein is to specify integument identity within the developing
female gametophyte. Thesin1mutant also has altered integuments that are shorter. This
mutant is of special interest in that the SIN1 protein is a homolog of the DICER protein
that functions in generating small, interfering RNA molecules (siRNA) that suppress
gene expressionat the posttranscriptional level. The fact that a DICER-like enzyme is
Figure 4.3.Pollen development. [Reprinted from McCormick (2004), with permission from the
American Society of Plant Biologists.] See color insert.
4.2. EMBRYOGENESIS AND SEED GERMINATION 87