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Developmental Genetics in Lower Plants
John C. Wallace
University of New Hampshire, Durham, New Hampshire
803I. INTRODUCTION
A chapter on lower plant developmental genetics may seem out of place in a handbook largely devoted
to crop plants, but as the true complexity of physiological and developmental processes in higher plants
becomes ever more apparent, simpler model systems that can aid in the discovery of such processes are
worthy of inclusion. The relative simplicity of lower plants and ease of genetic analysis in the haploid
state can make them a model system to rival even Arabidopsis thalianafor some aspects of flowering
plant development. As genetic engineering of crop plants progresses beyond the addition of single, en-
tirely foreign genes to the manipulation and addition of entire pathways, knowledge of the truly funda-
mental aspects of plant development, which can best be discovered in lower plants, is more important than
ever.
This chapter focuses on the two lower plants (a volvox and a moss) for which there exist substantial
developmental genetic data; i.e., a variety of mutants whose normal ontogeny is altered have been iso-
lated and characterized. Recent advances in molecular approaches to the analysis of these organisms,
which offer even further advantages over higher plants, are already being exploited. A brief section on a
third plant (a fern) is also included.
Recent studies have shown that the simplest of the three, Volvox carteri, is almost as closely related
to animals as to plants [1], and one of the most fundamental questions that one hopes to answer by study-
ingVolvox, that of the origin of the germ-soma dichotomy, is not normally even relevant to plants. But it
remains classically defined as a plant, it is perhaps the simplest example in biology of how a single cell
differentiates into two cell types (and thus has fascinated biologists for a very long time, e.g., Ref. 2), and
work on it is beginning to answer some of these fundamental questions at the molecular level.
The moss Physcomitrella patensis much more obvious as a model for crop plants, and a healthy ar-
ray of developmental mutants from this species have been isolated [3,4]. The ease with which it can be
transformed [5,6] and the transformants grown to maturity are a great boon to its utility, and the discov-
ery that transgenes introduced into Physcomitrellahomologously recombine with their genomic counter-
parts at high frequency [7] has engendered some excitement in the plant research community [8,9].
The most complex of the three, the fern Ceratopteris richardii, has emerged as a model organism for
plant developmental genetics [10,11]. Its special appeal lies in the fact that both gametophyte and sporo-
phyte generations are free living, multicellular, and readily accessible for study, so the ease of genetic ma-