I Abbreviations and definitions Symbols of water status are standardized as follows: '1' ... total water potential; '1'" osmotic or solute potential; 'I' P' turgor or pressure potential. Throughout this text the term adaptation refers to heritable modifications in structures or functions that increase the probability of an organism surviving and reproducing in a particular environment (121); adaptations can be constitutive (always expressed) or stress-induced. When discussing stress metabolism, we have followed Hochachka (92) in making use of the term "design principle" in an engineering sense. In this sense, the design principles for a system are set by what that system is supposed to do under various circumstances; the design principles then determine what components must be included in the system, and how they must interact. An illustrative example: in automobile engineering. distinct design principles are involved for the two safety objectives of collision avoidance and injury mitigation (125a). Collision-avoid­ ance design principles are set by the aim of preventing accidents, and lead to the inclusion of system components such as dual-braking systems. Injury-mitigation design principles follow from the goal of protecting occupants in case of an accident; their application results in introduction of components such as energy-absorbing steering columns.

Metabolic Responses of Mesophytes to Plant Water Deficits

The Genetics of Corn

Proline is a proteogenic amino acid and accumulates both under stress and non-stress conditions as a beneficial solute in plants. Recent discoveries point out that proline plays an important role in plant growth and differentiation across life cycle. It is a key determinant of many cell wall proteins that plays important roles in plant development. The role of extensins, arabinogalactan proteins and hydroxyproline- and proline-rich proteins as important components of cell wall proteins that play pivotal roles in cell wall signal transduction cascades, plant development and stress tolerance is discussed in this review. Molecular insights are also provided here into the plausible roles of proline transporters modulating key events in plant development. In addition, the roles of proline during seed developmental transitions including storage protein synthesis are discussed.

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Role of proline in cell wall synthesis and plant development and its implications in plant ontogeny.

Embryo-lethal mutants of Arabidopsis thaliana. A model system for genetic analysis of plant embryo development.

An embryo-lethal mutant of Arabidopsis thaliana is a biotin auxotroph

A conditional seedling lethal, monogenic recessive, endosperm mutant is described. Phenotypic can be accomplished when embryos are cultured in vitro on media supplemented with proline. The efficiency of the repair is proportional to the concentration of proline in the medium. Normal growth is resumed at a dose of 160 mg/l. All the data collected are most easily interpreted by assuming that the mutant, symbolized pro has a genetic block in the biosynthetic route leading to proline.This is probably the first case of a strict genetic requirement for an amino acid to be reported in Zea mays. The possible reasons for the difficulties encountered in isolating obligate auxotrophs in higher plants are briefly reviewed.

Milvia Nava-Racchi

Papers

A mutation causing proline requirement in Zea mays