SUMMARYVery long chain lipids are important components of the plant cuticle that establishes the boundary surface of aerial organs. In addition, these lipids were detected in the extracellular pollen coat (tryphine), where they play a crucial role in appropriate pollen-stigma communication. As such they are involved in the early interaction of pollen with the stigma. A substantial reduction in tryphine lipids was shown to compromise pollen germination and, consequently, resulted in male sterility. We investigated the role of two long-chain acyl-CoA synthetases (LACSs) in Arabidopsis with respect to their contribution to the production of tryphine lipids. LACS was shown to provide CoA-activated very long chain fatty acids (VLCFA-CoAs) to the pathways of wax biosynthesis. The allocation of sufficient quantities of VLCFA-CoA precursors should therefore be relevant to the generation of tryphine lipids. Here, we report on the identification of lacs1 lacs4 double knock-out mutant lines that were conditionally sterile and showed significant reductions in pollen coat lipids. Whereas the contributions of both LACS proteins to surface wax levels were roughly additive, their co-operation in tryphine lipid biosynthesis was clearly more complex. The inactivation of LACS4 resulted in increased levels of tryphine lipids accompanied by morphological anomalies of the pollen grains. The additional inactivation of LACS1 neutralized the morphological defects, decreased the tryphine lipids far below wild-type levels and resulted in conditionally sterile pollen.
Regulatory systems often evolve by duplication of ancestral systems and subsequent specialization of the components of the novel signal transduction systems. In the Gram-positive soil bacterium Bacillus subtilis, four homologous antitermination systems control the expression of genes involved in the metabolism of glucose, sucrose and β-glucosides. Each of these systems is made up of a sensory sugar permease that does also act as phosphotransferase, an antitermination protein, and a RNA switch that is composed of two mutually exclusive structures, a RNA antiterminator (RAT) and a transcriptional terminator. We have studied the contributions of sugar specificity of the permeases, carbon catabolite repression, and protein–RAT recognition for the straightness of the signalling chains. We found that the β-glucoside permease BglP does also have a minor activity in glucose transport. However, this activity is irrelevant under physiological conditions since carbon catabolite repression in the presence of glucose prevents the synthesis of the β-glucoside permease. Reporter gene studies, in vitro RNA–protein interaction analyzes and northern blot transcript analyzes revealed that the interactions between the antiterminator proteins and their RNA targets are the major factor contributing to regulatory specificity. Both structural features in the RATs and individual bases are important specificity determinants. Our study revealed that the specificity of protein–RNA interactions, substrate specificity of the permeases as well as the general mechanism of carbon catabolite repression together allow to keep the signalling chains straight and to avoid excessive cross-talk between the systems.
The nodule enzymes glutamate synthase (GOGAT) and phosphoenolpyruvate carboxylase (PEPC) are important in the assimilation of symbiotically fixed N in alfalfa (Medicago sativa L.). The objectives of this study were: to evaluate several alfalfa germplasm sources for variability in nodule GOGAT and PEPC activities, and to determine the relationships between N2 fixation and plant yield traits with nodule GOGAT and PEPC enzyme activities. Approximately 160 plants from each of six alfalfa germplasms were grown in sand culture in the glasshouse and assayed for nodule GOGAT and PEPC enzyme activities. Twenty‐four genotypes (four from each germplasm) were selected to represent a range in GOGAT and PEPC activities. These genotypes were evaluated in glasshouse and field clonal studies for GOGAT and PEPC activities per gram nodule fresh weight, acetylene reduction activity (ARA), nodule mass, nodule color, nodule soluble protein concentration, and plant yield. Variability for nodule GOGAT and PEPC activities within each of the six germplasms was large and normally distributed. The 12 high GOGAT genotypes (2 from each germplasm) were 44% greater GOGAT activity than the 12 low genotypes. The 12 high PEPC genotypes were 27% greater in PEPC activity than the 12 low genotypes. Both groups of high enzyme activity genotypes were significantly greater than the low genotypes in ARA, nodule soluble protein concentration, and field shoot yield. Both GOGAT and PEPC activities were positively correlated (r=0.61**, significant at the 0.01 probability level). Also, GOGAT and PEPC were each correlated with ARA (r=0.43*, significant at the 0.05 probability level, and 0.45*, respectively), nodule color score (r=0.46** and 0.73**) soluble protein concentration (r= 0.77** and 0.90**). Plant yield and N2 fixation measured in the field were not correlated with nodule GOGAT and PEPC activities measured in the glasshouse. The relationships between GOGAT and PEPC with other plant traits indicated that these enzymes are associated with the host‐Rhizobium meliloti Dang. symbiosis and should be explored as possible selection criteria to include in an alfalfa breeding program.
Smooth bromegrass (Bromus inermis Leyss.) is an important cool‐season forage grass in the northern half of the USA and in Canada. The objective of this study was to determine the response to selection for high and low forage yield, increased seed weight, high fertility index, and greater disease resistance in a population of smooth bromegrass selected previously for increased seed weight. Six synthetics derived from fourth‐cycle selections for increased seed weight were evaluated for seed and forage traits and compared to the third‐cycle population (SBC3) from which parent clones were selected. Selection criteria for parents of each synthetic were: SB‐1, high general combining ability (GCA) for forage yield with emphasis on regrowth after early June harvest; SB‐2, high GCA for total forage yield; SB‐3, high clonal seed weight; SB‐4, high clonal fertility index; SB‐5, high GCA for resistance to brown leaf spot [Pyrenophora bromi (Died.) Drechsl.]; andS B‐6, low GCA for forage yield. Synthetics SB‐1 and 2 were similar to SB‐C3 in total forage yield. SB‐1 was 13% greater than SB‐C3 in second‐harvest forage yield. SB‐3 was 8% heavier than SB‐C3 in 100‐seed weight. SB‐4, the only synthetic satisfactory in seed yield, was 14 percentage units higher than SB‐C3 in fertility index. SB‐5 appeared more resistant to brown leaf spot than SB‐C3 and all other synthetics. SB‐6 yielded 8% less forage than SB‐C3. Significant positive correlations were obtained between Syn 2 performance of synthetics and mean topcross progeny performance of their parental clones for total and second harvest forage yield, and for brown leaf spot reaction. Relatively intense selection pressure for fertility during selection for seed weight would be necessary to maintain seed yields at satisfactory levels. Forage yield was maintained at a high level in spite of intense selection for 100‐seed weight with relatively few plants selected in each cycle.
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