Plants have evolved complex molecular, cellular and physiological mechanisms to respond to environmental stressors. Because of the inherent complexity of this response, genetic manipulation to substantially improve water deficit tolerance, particularly in agricultural crops, has been largely unsuccessful, as the improvements are frequently accompanied by slower growth and delayed reproduction. Here, we ectopically express two abiotic stress-responsive bZIP AREB/ABF transcription factor orthologs, Arabidopsis ABF3 and Gossypium hirsutum ABF2D, in G. hirsutum, to compare the effects of exogenous and endogenous AREB/ABF transgene overexpression on dehydration resilience. Our results show that ectopic expression of each of these orthologs increases dehydration resilience, although these increases are accompanied by slower growth. These phenotypic effects are proportional to the ectopic expression level in the GhABF2D transgenic plants, while the phenotypes of all of the AtABF3 transgenic plants are similar, largely independent of ectopic expression level, possibly indicating differential post-transcriptional regulation of these transgenes. Our results indicate that overexpression of exogenous and endogenous ABF homologs in G. hirsutum substantially increases drought resilience, primarily through stomatal regulation, negatively impacting transpiration and photosynthetic productivity.
Gibberellic acid (GA) is both necessary and sufficient to promote fiber elongation in cultured fertilized ovules of the upland cotton variety Coker 312. This is likely due to the temporal and spatial regulation of GA biosynthesis, perception, and subsequent signal transduction that leads to alterations in gene expression and morphology. Our results indicate that the initiation of fiber elongation by the application of GA to cultured ovules corresponds with increased expression of genes that encode xyloglucan endotransglycosylase/hydrolase (XTH) and expansin (EXP) that are involved in promoting cell elongation. To gain a better understanding of the GA signaling components in cotton, that lead to such changes in gene expression, two GA receptor genes (GhGID1a and GhGID1b) and two DELLA protein genes (GhSLR1a and GhSLR1b) that are orthologous to the rice GA receptor (GID1) and the rice DELLA gene (SLR1), respectively, were characterized. Similar to the GA biosynthetic genes, expression of GhGID1a and GhGID1b is under the negative regulation by GA while GA positively regulates GhSLR1a. Recombinant GST-GhGID1s showed GA-binding activity in vitro that was augmented in the presence of GhSLR1a, GhSLR1b, or rice SLR1, indicating complex formation between the receptors and repressor proteins. This was further supported by the GA-dependent interaction of these proteins in yeast cells. Ectopic expression of the GhGID1a in the rice gid1-3 mutant plants rescued the GA-insensitive dwarf phenotype, which demonstrates that it is a functional GA receptor. Furthermore, ectopic expression of GhSLR1b in wild type Arabidopsis led to reduced growth and upregulated expression of DELLA-responsive genes.
Sucrose phosphate synthase (SPS) catalyzes the first step in the synthesis of sucrose in photosynthetic tissues. We characterized the expression of three different isoforms of SPS belonging to two different SPS gene families in alfalfa (Medicago sativa L.), a previously identified SPS (MsSPSA) and two novel isoforms belonging to class B (MsSPSB and MsSPSB3). While MsSPSA showed nodule-enhanced expression, both MsSPSB genes exhibited leaf-enhanced expression. Alfalfa leaf and nodule SPS enzymes showed differences in chromatographic and electrophoretic migration and differences in Vmax and allosteric regulation. The root nodules in legume plants are a strong sink for photosynthates with its need for ATP, reducing power and carbon skeletons for dinitrogen fixation and ammonia assimilation. The expression of genes encoding SPS and other key enzymes in sucrose metabolism, sucrose phosphate phosphatase and sucrose synthase, was analyzed in the leaves and nodules of plants inoculated with Sinorhizobium meliloti. Based on the expression pattern of these genes, the properties of the SPS isoforms and the concentration of starch and soluble sugars in nodules induced by a wild type and a nitrogen fixation deficient strain, we propose that SPS has an important role in the control of carbon flux into different metabolic pathways in the symbiotic nodules.
This chapter discusses the cotton fibre development and cellulose synthesis (fibre initiation, elongation and primary cell wall development, fibre secondary cell wall development and fibre maturation), fibre properties (physiochemical composition and structural properties of cellulose and physical properties), regulation of fibre development (metabolic control and environmental effects on fibre development), hormonal regulation of fibre development and the Bt ovule-culture system, molecular genetics of fibre development and advances in cotton genomics and functional genomics. The different genetic engineering efforts to alter fibre characteristics, such as manipulation of metabolic pathways, modification of hormone levels/signalling and their pathways, targeting of cell wall genes, introduction of novel traits to the cotton fibre and the use of model plant systems in cotton fibre research, are also highlighted.
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