Annual Review of Plant Biology - Volume 46, 1995

Recent molecular and cellular data on the sexual cycle of Chlamydomonas reinhardtii, a unicellular green alga, is considered in the context of current theories on the origins and evolution of eukaryotic sex. The mating-type locus of C. reinhardtii controls gamete recognition and fusion, organelle inheritance, and sporulation, the three traits that characterize most sexual cycles in lower eukaryotes. The mating-type locus comprises approximately one megabase of DNA on linkage group VI, is highly rearranged in its central region, and contains identified genes that govern both recognition/fusion and chloroplast inheritance. Sporulation, a diploid program designed to negotiate changing environments, is analogous if not homologous to the somatic differentiation program of multicellular plants and animals, and its expression requires the fusion of compatible gametes.

The viviparous and germination mutants of maize and Arabidopsis thaliana illuminate the mechanism that integrates control of morphogenetic, maturation, dormancy, and germination pathways in seed development. Key elements of this mechanism include (a) developmental control of abscisic acid and gibberellin hormone synthesis and perception, (b) integration of maturation and anthocyanin pathways in the maize seed, (c) functions of the VP1 and ABI3 factors in abscisic acid-regulated gene expression, and (d) intrinsic developmental genes that couple seed maturation to the program of embryo morphogenesis. The scarcity of mutants that affect timing or tissue specificity of hormone synthesis in the seed is an important constraint to progress in understanding the role of hormone signals. The interactions among the abscisic acid-insensitive abil, abi2, abi3, abi4, and abi5 mutants of A. thaliana are consistent with multiple pathways of abscisic acid signal transduction in the seed. The maize Vp1 and A. thalianaAbi3 genes are functional homologs that mediate a seed-specific abscisic acid response necessary for maturation. The specific roles of these genes in controlling dormancy and anthocyanin synthesis in the seed have diverged since the evolutionary separation of maize and A. thaliana. The coupling of anthocyanin synthesis to maturation in maize may have resulted from changes in the downstream cl regulatory gene rather than a functional change in VPl. Functional analysis indicates that VP1 is a transcriptional activator of the Em and C1 genes in maize, although its specific role in abscisic acid signal transduction remains poorly understood. The lecl and fus3 mutants of A. thaliana and pleiotropic viviparous mutants of maize may identify intrinsic factors that couple the maturation pathway to embryo morphogenesis.

This review discusses how genetically manipulated plants are being used to study the regulation of metabolism in plants, using carbohydrate metabolism as an example. The molecular tools required are introduced, including the history of Agrobacterium tumefaciens-mediated gene transfer and other transformation techniques, the availability of promoters to achieve a specific or induced expression, strategies to target proteins to subcellular compartments of the cell, and the use of antisense or cosuppression to inhibit expression of endogenous genes. A discussion then follows of how such plants can be used in biochemical and physiological experiments to identify and quantify the importance of enzymes and processes that control metabolic fluxes, storage, and growth. These results are leading to a reassessment of ideas about metabolic regulation and have consequences for design of bioengineering strategies in plants. Emerging commercial applications are also surveyed.

A major advance in plant genome analysis has been the development of molecular marker maps. These, in combination with repeated sequence analysis, have given considerable insight into the organization and evolution of many plant genomes. Physical mapping and sequencing of the genomes of the model plant species Arabidopsis thaliana and rice are progressing rapidly. The physical maps facilitate the isolation of genes by map-based cloning and will enable the organization of whole chromosomes to be analyzed. Comparative mapping studies between related plant species, using common sets of molecular markers, have revealed extensive collinearity over short chromosomal segments or whole chromosomes. Thus, in the future, collinearity as well as homology may be used to clone genes from many plant species, maximally exploiting the A. thaliana or rice physical maps.

Analysis of gene function is of central importance for the understanding of physiological processes. Expression of genes in heterologous organisms has allowed the isolation of many important genes (e.g. for nutrient uptake and transport) and has contributed a lot to the functional analysis of the gene products. For animal research, expression in Xenopus oocytes and cell cultures are predominant techniques, whereas in the plant domain, yeast has become the prevalent expression system. This review provides a survey of this quickly developing field and intends to assist researchers in determining appropriate experimental approaches for specific biological questions. Because heterologous expression technology is of special value for the analysis of proteins that are difficult to handle biochemically, the examples given concentrate on membrane proteins, i.e. transporter proteins. Also included is a detailed discussion of the functional expression methodology and its use in identifying and characterizing genes and proteins.

This review focuses on light regulation of transcription, from light perception to changes in transcription, with an emphasis on regulatory elements in the promoters of light-regulated genes and proteins that bind to them. The abundance of over 100 mRNAs is regulated by light, and at least three photoreceptors influence transcription, but the importance of each photoreceptor may vary from gene to gene. Light-regulated promoters are composed of ubiquitous regulatory elements; the specific combination of elements appears to make a promoter light-regulated, and these combinations vary widely. Numerous proteins that bind to elements in light-regulated promoters have been identified and many have been cloned, but no cloned gene has been unequivocally assigned a function in light-regulated transcription. Substantial progress has been made in identifying steps in signal transduction from light perception to transcription, but these steps have yet to be assembled into complete pathways.

Research over the past three decades has greatly increased our understanding of starch biosynthesis in storage tissues, but our knowledge is still incomplete. Advances and areas of uncertainty are discussed with a focus on the maize mutants that have been useful in elucidating the process. This emphasis on mutant genes promises additional insights when we discover the role of several mutants known to affect starch synthesis and when we isolate mutations in genes encoding enzymes that have important roles in the process. The goal is a complete understanding of starch synthesis. This understanding will also facilitate the design of transgenic plants that might produce unique starches with promise as raw materials for industrial processes.