An expanded role for the TWN1 gene in embryogenesis: defects in cotyledon pattern and morphology in the twn1 mutant of Arabidopsis (Brassicaceae).

The suspensor is a specialized basal structure that differentiates early in plant embryogenesis to support development of the embryo proper. Suspensor differentiation in Arabidopsis is maintained in part by the TWIN1 (TWN1) gene, which suppresses embryogenic development in suspensor cells: twn1 mutants produce supernumerary embryos via suspensor transformation. To better understand mechanisms of suspensor development and further investigate the function of TWN1, we have characterized late-embryo and post-embryonic development in the twn1 mutant, using seedling culture, microscopy, and genetics. We report here that the twn1 mutation disrupts cotyledon number, arrangement, and morphology and occasionally causes partial conversion of cotyledons into leaves. These defects are not a consequence of suspensor transformation. Thus, in addition to its basal role in suspensor differentiation, TWN1 influences apical pattern and morphology in the embryo proper. To determine whether other genes can similarly affect both suspensor and cotyledon development, we looked for twinning in Arabidopsis mutants previously identified by their abnormal cotyledon phenotypes. One such mutant, amp1, produced a low frequency of twin embryos by suspensor transformation. Our results suggest that mechanisms that maintain suspensor identity also function later in development to influence organ formation at the embryonic shoot apex. We propose that TWN1 functions in cell communication pathways that convey local positional information in both the apical and basal regions of the Arabidopsis embryo.

A salinity-induced gene from the halophyte M. crystallinum encodes a glycolytic enzyme, cofactor-independent phosphoglyceromutase.

In the facultative halophyte Mesembryanthemum crystallinum (ice plant), salinity stress triggers significant changes in gene expression, including increased expression of mRNAs encoding enzymes involved with osmotic adaptation to water stress and the crassulacean acid metabolism (CAM) photosynthetic pathway. To investigate adaptive stress responses in the ice plant at the molecular level, we generated a subtracted cDNA library from stressed plants and identified mRNAs that increase in expression upon salt stress. One full-length cDNA clone was found to encode cofactor-independent phosphoglyceromutase (PGM), an enzyme involved in glycolysis and gluconeogenesis. Pgm1 expression increased in leaves of plants exposed to either saline or drought conditions, whereas levels of the mRNA remained unchanged in roots of hydroponically grown plants. Pgm1 mRNA was also induced in response to treatment with either abscisic acid or cytokinin. Transcription run-on experiments confirmed that Pgm1 mRNA accumulation in leaves was due primarily to increased transcription rates. Immunoblot analysis indicated that Pgm1 mRNA accumulation was accompanied by a modest but reproductible increase in the level of PGM protein. The isolation of a salinity-induced gene encoding a basic enzyme of glycolysis and gluconeogenesis indicates that adaptation to salt stress in the ice plant involves adjustments in fundamental pathways of carbon metabolism and that these adjustments are controlled at the level of gene expression. We propose that the leaf-specific expression of Pgm1 contributes to the maintenance of efficient carbon flux through glycolysis/gluconeogenesis in conjunction with the stress-induced shift to CAM photosynthesis.

Posttranscriptional and posttranslational control of enolase expression in the facultative Crassulacean acid metabolism plant Mesembryanthemum Crystallinum L.

During the induction of Crassulacean acid and metabolism by environmental stresses in the common ice plant (Mesembryanthemum crystallinum L.), enzyme activities involved in glycolysis and gluconeogenesis, including enolase (2-phospho-D-glycerate hydrolase), increase significantly. In this study, we describe two nearly identical cDNA clones (Pgh1a and Pgh1b) encoding enolase from the common ice plant. This cytoplasmically localized enzyme is encoded by a gene family of at least two members. The polypeptides encoded by these cDNAs share a high degree of amino acid sequence identity (86.7-88.3%) with other higher plant enolases. Enolase activity increased more than 4-fold in leaves during salt stress. This increase was accompanied by a dramatic increase in Pgh1 transcription rate and the accumulation of enolase transcripts in leaves. Pgh1 transcript levels also increased in leaves in response to low temperature, drought, and anaerobic stress conditions and upon treatment of unstressed plants with the plant growth regulators abscisic acid and 6-benzylaminopurine. In roots, enolase transcripts increased in abundance in response to salt, low and high temperature, and anaerobic stresses. Surprisingly, we observed no increase in enolase protein levels, despite the increased levels of mRNA and enzyme activity during salt stress. The stress-induced increase in enolase activity is therefore due to posttranslational regulation of steady-state enzyme pools. Our results demonstrate that the stress-induced shift to Crassulacean acid metabolism in the ice plant involves complex regulatory control mechanisms that operate at the transcriptional, posttranscriptional, and postranslational levels.