Gad Galili

Seed development and germination

Part 1 Seed morphology and development: the seed - structure and function morphogenetic processes in embryo development of maize. Part 2 Storage compounds - synthesis and accumulation: transport and accumulation of reserve materials in developing seeds - the seed as a sink seed storage proteins in cereals seed storage proteins in legumes regulation of seed storage protein gene expression synthesis of embryo specific protein during seed development carbohydrate biosynthesis in seeds lipid biosynthesis in developing seeds synthesis and transport of oil body proteins (oleosins) in seeds mechanisms and regulation of mineral storage during seed development. Part 3 From seed development to germination: mechanisms of dessication tolerance in seeds regulatory mechanisms in the transition from seed development to germination genetic control of hormone role in seed development and germination water relations in germination control of seed dormancy and germination in the gaseous environment (O2, CO2, C2H4) recent advances in the control of germination by light and temperature synthesis and secretion of hydrolytic enzymes during germination carbohydrate degradation during germination control of mobilization of storage compounds following germination of dicotyledonous seeds. Part 4 Ecology of germination: pollinators and seed production maternal and environmental effects on seed quality and germination seed bank ecology inhibition and promotion of seed germination in soil - dormancy cycles modelling seed germination of weeds seed germination of root parasitic plants seed germination of halophytes allelopathy and germination seed germination of desert plants. Part 5 Seed quality improvement - technological advances: loss of seed viability in storage - cytological, biochemical, and genetic aspects the implications of seed-associated mycoflora during storage biochemical processes during osmotic priming of seeds improvement of protein quality in seeds overproduction of essential amino acids in seeds artificial seeds.

Overexpression of a Plasma Membrane Aquaporin in Transgenic Tobacco Improves Plant Vigor under Favorable Growth Conditions but Not under Drought or Salt Stress

Most of the symplastic water transport in plants occurs via aquaporins, but the extent to which aquaporins contribute to plant water status under favorable growth conditions and abiotic stress is not clear. To address this issue, we constitutively overexpressed the Arabidopsis plasma membrane aquaporin, PIP1b, in transgenic tobacco plants. Under favorable growth conditions, PIP1b overexpression significantly increased plant growth rate, transpiration rate, stomatal density, and photosynthetic efficiency. By contrast, PIP1b overexpression had no beneficial effect under salt stress, whereas during drought stress it had a negative effect, causing faster wilting. Our results suggest that symplastic water transport via plasma membrane aquaporins represents a limiting factor for plant growth and vigor under favorable conditions and that even fully irrigated plants face limited water transportation. By contrast, enhanced symplastic water transport via plasma membrane aquaporins may not have any beneficial effect under salt stress, and it has a deleterious effect during drought stress.

Arabidopsis seed development and germination is associated with temporally distinct metabolic switches

While the metabolic networks in developing seeds during the period of reserve accumulation have been extensively characterized, much less is known about those present during seed desiccation and subsequent germination. Here we utilized metabolite profiling, in conjunction with selective mRNA and physiological profiling to characterize Arabidopsis (Arabidopsis thaliana) seeds throughout development and germination. Seed maturation was associated with a significant reduction of most sugars, organic acids, and amino acids, suggesting their efficient incorporation into storage reserves. The transition from reserve accumulation to seed desiccation was associated with a major metabolic switch, resulting in the accumulation of distinct sugars, organic acids, nitrogen-rich amino acids, and shikimate-derived metabolites. In contrast, seed vernalization was associated with a decrease in the content of several of the metabolic intermediates accumulated during seed desiccation, implying that these intermediates might support the metabolic reorganization needed for seed germination. Concomitantly, the levels of other metabolites significantly increased during vernalization and were boosted further during germination sensu stricto, implying their importance for germination and seedling establishment. The metabolic switches during seed maturation and germination were also associated with distinct patterns of expression of genes encoding metabolism-associated gene products, as determined by semiquantitative reverse transcription-polymerase chain reaction and analysis of publicly available microarray data. When taken together our results provide a comprehensive picture of the coordinated changes in primary metabolism that underlie seed development and germination in Arabidopsis. They furthermore imply that the metabolic preparation for germination and efficient seedling establishment initiates already during seed desiccation and continues by additional distinct metabolic switches during vernalization and early germination.

New Insights into the Shikimate and Aromatic Amino Acids Biosynthesis Pathways in Plants

The aromatic amino acids phenylalanine, tyrosine, and tryptophan in plants are not only essential components of protein synthesis, but also serve as precursors for a wide range of secondary metabolites that are important for plant growth as well as for human nutrition and health. The aromatic amino acids are synthesized via the shikimate pathway followed by the branched aromatic amino acids biosynthesis pathway, with chorismate serving as a major intermediate branch point metabolite. Yet, the regulation and coordination of synthesis of these amino acids are still far from being understood. Recent studies on these pathways identified a number of alternative cross-regulated biosynthesis routes with unique evolutionary origins. Although the major route of Phe and Tyr biosynthesis in plants occurs via the intermediate metabolite arogenate, recent studies suggest that plants can also synthesize phenylalanine via the intermediate metabolite phenylpyruvate (PPY), similarly to many microorganisms. Recent studies also identified a number of transcription factors regulating the expression of genes encoding enzymes of the shikimate and aromatic amino acids pathways as well as of multiple secondary metabolites derived from them in Arabidopsis and in other plant species.

Cloning and characterization of a novel Mg2+/H+ exchanger

Cellular functions require adequate homeostasis of several divalent metal cations, including Mg2+ and Zn2+. Mg2+, the most abundant free divalent cytoplasmic cation, is essential for many enzymatic reactions, while Zn2+ is a structural constituent of various enzymes. Multicellular organisms have to balance not only the intake of Mg2+ and Zn2+, but also the distribution of these ions to various organs. To date, genes encoding Mg2+ transport proteins have not been cloned from any multicellular organism. We report here the cloning and characterization of an Arabidopsis thaliana transporter, designated AtMHX, which is localized in the vacuolar membrane and functions as an electrogenic exchanger of protons with Mg2+ and Zn2+ ions. Functional homologs of AtMHX have not been cloned from any organism. Ectopic overexpression of AtMHX in transgenic tobacco plants render them sensitive to growth on media containing elevated levels of Mg2+ or Zn2+, but does not affect the total amounts of these minerals in shoots of the transgenic plants. AtMHX mRNA is mainly found at the vascular cylinder, and a large proportion of the mRNA is localized in close association with the xylem tracheary elements. This localization suggests that AtMHX may control the partitioning of Mg2+ and Zn2+ between the various plant organs.

Regulation of Lysine and Threonine Synthesis.

Human and monogastric animals cannot synthesize 10 out of the 20 amino acids and therefore need to obtain these from their diet. Among the essential amino acids, lysine and threonine are considered to be exceedingly important in that they are the most limiting essential amino acids in cereal grains, which represent the largest source of food worldwide (Bright and Shewry, 1983). Because of the nutritional importance of lysine and threonine, the regulation of their metabolism has been studied extensively at the biochemical, genetic, and, more recently, molecular levels. Like many bacterial species, higher plants synthesize lysine and threonine from aspartate using two different branches of the aspartate family pathway, as shown schematically in Figure 1. The enzymes involved in lysine and threonine synthesis have been reviewed in detail (Bryan, 1980). In this review, I discuss the complex biochemical, cellular, developmental, physiological, and environmental controls of the synthesis of lysine and threonine. I also focus on specific enzymes that play major regulatory roles in the synthesis of these amino acids.

Improving the Content of Essential Amino Acids in Crop Plants: Goals and Opportunities

The inability of humans and many farm animals to synthesize certain amino acids has long triggered tremendous interest in increasing the levels of these so-called essential amino acids in crop plants. Knowledge obtained from basic genetic and genetic engineering research has also been successfully

Seed desiccation: a bridge between maturation and germination

The development of orthodox seeds concludes by a desiccation phase. The dry seeds then enter a phase of dormancy, also called the after-ripening phase, and become competent for germination. We discuss physiological processes as well as gene expression and metabolic programs occurring during the desiccation phase in respect to their contribution to the desiccation tolerance, dormancy competence and successful germination of the dry seeds. The transition of developing seeds from the phase of reserve accumulation to desiccation is associated with distinct gene expression and metabolic switches. Interestingly, a significant proportion of the gene expression and metabolic signatures of seed desiccation resemble those characterizing seed germination, implying that the preparation of the seeds for germination begins already during seed desiccation.

Increased Lysine Synthesis Coupled with a Knockout of Its Catabolism Synergistically Boosts Lysine Content and Also Transregulates the Metabolism of Other Amino Acids in Arabidopsis Seeds

To elucidate the relative significance of Lys synthesis and catabolism in determining Lys level in plant seeds, we expressed a bacterial feedback-insensitive dihydrodipicolinate synthase (DHPS) in a seed-specific manner in wild-type Arabidopsis as well as in an Arabidopsis knockout mutant in the Lys catabolism pathway. Transgenic plants expressing the bacterial DHPS, or the knockout mutant, contained ∼12-fold or ∼5-fold higher levels, respectively, of seed free Lys than wild-type plants. However, the combination of these two traits caused a synergistic ∼80-fold increase in seed free Lys level. The dramatic increase in free Lys in the knockout mutant expressing the bacterial DHPS was associated with a significant reduction in the levels of Glu and Asp but also with an unexpected increase in the levels of Gln and Asn. This finding suggested a special regulatory interaction between Lys metabolism and amide amino acid metabolism in seeds. Notably, the level of free Met, which competes with Lys for Asp and Glu as precursors, was increased unexpectedly by up to ∼38-fold in the various transgenic and knockout plants. Together, our results show that Lys catabolism plays a major regulatory role in Lys accumulation in Arabidopsis seeds and reveal novel regulatory networks of seed amino acid metabolism.

Principal Transcriptional Programs Regulating Plant Amino Acid Metabolism in Response to Abiotic Stresses

Using a bioinformatics analysis of public Arabidopsis (Arabidopsis thaliana) microarray data, we propose here a novel regulatory program, combining transcriptional and posttranslational controls, which participate in modulating fluxes of amino acid metabolism in response to abiotic stresses. The program includes the following two components: (1) the terminal enzyme of the module, responsible for the first catabolic step of the amino acid, whose level is stimulated or repressed in response to stress cues, just-in-time when the cues arrive, principally via transcriptional regulation of its gene; and (2) the initiator enzyme of the module, whose activity is principally modulated via posttranslational allosteric feedback inhibition in response to changes in the level of the amino acid, just-in-case when it occurs in response to alteration in its catabolism or sequestration into different intracellular compartments. Our proposed regulatory program is based on bioinformatics dissection of the response of all biosynthetic and catabolic genes of seven different pathways, involved in the metabolism of 11 amino acids, to eight different abiotic stresses, as judged from modulations of their mRNA levels. Our results imply that the transcription of the catabolic genes is principally more sensitive than that of the biosynthetic genes to fluctuations in stress-associated signals. Notably, the only exception to this program is the metabolic pathway of Pro, an amino acid that distinctively accumulates to significantly high levels under abiotic stresses. Examples of the biological significance of our proposed regulatory program are discussed.