Growth and development of Mesembryanthemum crystallinum (Aizoaceae)

This review describes the life cycle of Mesembryantheum crystallinum L. (the common ice plant, Aizoaceae, Caryophyllales), a halophyte with a developmentally programmed switch from C₃ photosynthesis to Crassulacean acid metabolism (CAM) which is accelerated by salinity and drought. Since there has been controversy regarding the interplay between genes and environmental stimuli during the development of M. crystallinum, it is timely to summarize the life cycle for a defined set of conditions. We seek to establish the framework whereby five stages of development can be described in terms of morphology, physiology, and molecular biology. Stages 1 and 2, representing germination and growth of a juvenile form, show a determinate pattern of growth. Although specific genes for salt tolerance can be induced at these stages, stress early in development prevents progression to the mature form (stages 3-5) in which the plants advance to mature growth, flowering, and seed development. Growth in stage 3 is indeterminate in the absence of stress, but development and flowering are accelerated by environmental stresses, and CAM is constitutively expressed. Depending on the severity of the stress, plants start to flower (stage 4) and then die from the roots, ultimately with only seed capsules remaining viable, with salt sequestered into large epidermal bladder cells (stage 5). We highlight responses to salinity leading to compartmentation of ions and compatible solutes, turgor maintenance, and CAM. Finally, the molecular genetics of the ice plant are characterized, emphasizing selected genes and their products. We conclude with an analysis of the multiple stages of growth as an ecological adaptation to progressive stress. The initial determinate and inflexible juvenile phase provides a critical mass of plant material which supports the indeterminate, mature phase. Depending on the degree of stress, the mature form is then propelled towards flowering and seedset. CONTENTS Summary 171 I. Introduction 172 II. Standardizing methodology 173 III. Growth and development 175 IV. Effects of salt stress on developmental physiology 176 V. Ionic composition of cells during development 179 VI. Water transport within the plant 179 VII. The switch from C₃ to Crassulacean Acid Metabolism 181 VIII. Stress and plant-growth regulators 182 IX. Molecular biology 183 X. Genetics, ploidy and mutants 184 XI. Conclusions and future directions 185 Acknowledgements 187 References 187.