Specialised plant pathogens are in many ways adapted to exploit their host plants. Infective prop... more Specialised plant pathogens are in many ways adapted to exploit their host plants. Infective propagules should reach the appropriate plant tissue, gain access to the tissue and negate or suppress various kinds of constitutive and inducible resistance mechanisms. The resistance type most frequently deployed in plant breeding is the race-specific resistance, where a hypersensitive response of plant tissue is elicited by an avirulence factor produced by the pathogen. The great disadvantage of this type of resistance is, that it is often ephemeral. Detailed screening of germplasm may result in the discovery of alternative defence mechanisms not associated with hypersensitivity, that may be durable. Avoidance mechanisms may reduce the chance of infection. Upright plant habit has been reported to decrease spore deposition in cereals. Crop architecture may also affect aspects such as humidity and aeration in the crop, and hence the chances for successful infection by pathogen propagules. Other examples of avoidance are leaf surface properties that interfere with leaf wettability, germ tube orientation and finding of stomata to enter the leaf. Stomata in some accessions of Hordeum chilense are excessively covered by cuticular wax that prevent rust fungal germ tubes from perceiving the stomata, resulting in failure of penetration of the pathogen into the leaf. There is evidence that incompatible host species, biotrophic pathogenic fungi induce basic compatibility by suppressing defence mechanisms. Failure of this induction results in abortion of the infection attempt. Several cases of apparently durable resistance are discussed that are based on failure of haustorium formation, and are not associated with hypersensitivity. They may represent cases where the pathogen has problems in establishing basic compatibility.
The capacity of the triose-phosphate shuttle and various combinations of glycolytic intermediates... more The capacity of the triose-phosphate shuttle and various combinations of glycolytic intermediates to substitute for the ATP requirement for fatty-acid and glycerolipid biosynthesis in pea (Pisum sativum L.) root plastids was assessed. In all cases, ATP gave the greatest rates of fatty-acid and glycerolipid biosynthesis. Rates of up to 66 and 27 nmol·(mg protein)−1·h−1 were observed for the incorporation of acetate and glycerol-3-phosphate into lipids in the presence of ATP. In the absence of exogenously supplied ATP, the triose-phosphate shuttle gave up to 44 and 33% of the ATP-control activity in promoting fatty-acid and glycerolipid biosynthesis from acetate and glycerol-3-phosphate, respectively. The optimum shuttle components were 2 mM dihydroxyacetonephosphate (DHAP), 2 mM oxaloacetic acid and 4 mM inorganic phosphate (referred to as the DHAP shuttle). Glyceraldehyde-3-phosphate, as a shuttle triose, was approximately 82% as effective as DHAP in promoting fatty-acid synthesis while 2-phosphoglycerate, 3-phosphoglycerate, and phosphoenolpyruvate were only 27–37% as effective as DHAP. When glycolytic intermediates were used as energy sources for fatty-acid synthesis, in the absence of both exogenously supplied ATP and the triose-phosphate shuttle, phosphoenolpyruvate, 2-phosphoglycerate, fructose-6-phosphate and glucose-6-phosphate each gave 48%, 17%, 23% and 17%, respectively, of the ATP-control activity. Other triose phosphates tested were much less effective in promoting fatty-acid synthesis. When exogenously supplied ATP was supplemented with the DHAP shuttle or glycolytic intermediates, the complete shuttle increased fatty-acid biosynthesis by 37% while DHAP alone resulted in 24% stimulation. Glucose-6-phosphate, fructose-6-phosphate and glycerol-3-phosphate similarly all improved the rates of fatty-acid synthesis by 20–30%. In contrast, 3-phosphoglycerate, 2-phosphoglycerate and phosphoenolpyruvate all inhibited fatty-acid synthesis by approximately 10% each. The addition of the DHAP shuttle and glycolytic intermediates with or without exogenously supplied ATP caused an increase in the proportion of radioactive oleate and a decrease in the proportion of radioactive palmitate synthesized. The use of these alternative energy sources resulted in higher amounts of free fatty acids and triacylglycerol, and lower amounts of diacylglycerol and phosphatidic acid. The data presented here indicate that ATP is superior in promoting in-vitro fatty-acid biosynthesis in pea root plastids; however, both the triose-phosphate shuttle and glycolytic metabolism can produce some of the ATP required for fatty-acid biosynthesis in these plastids.
We report an improved method for white clover (Trifolium repens) transformation usingAgrobacteriu... more We report an improved method for white clover (Trifolium repens) transformation usingAgrobacterium tumefaciens. High efficiencies of transgenic plant production were achieved using cotyledons of imbibed mature seed. Transgenic plants were recovered routinely from over 50% of treated cotyledons. Thebar gene and phosphinothricin selection was shown to be a more effective selection system thannptII (kanamycin selection) oraadA (spectinomycin selection). White clover was transformed with the soybean auxin responsive promoter, GH3, fused to the GUS gene (β-glucuronidase) to study the involvement of auxin in root development. Analysis of 12 independent transgenic plants showed that the location and pattern of GUS expression was consistent but the levels of expression varied. The level of GH3:GUS expression in untreated plants was enhanced specifically by auxin-treatment but the pattern of expression was not altered. Expression of the GH3:GUS fusion was not enhanced by other phytohormones. A consistent GUS expression pattern was evident in untreated plants presumably in response to endogenous auxin or to differences in auxin sensitivity in various clover tissues. In untreated plants, the pattern of GH3:GUS expression was consistent with physiological responses which are regarded as being auxin-mediated. For the first time it is shown that localised spots of GH3:GUS activity occurred in root cortical tissue opposite the sites where lateral roots subsequently were initiated. Newly formed lateral roots grew towards and through these islands of GH3:GUS expression, implying the importance of auxin in controlling lateral root development. Similarly, it is demonstrated for the first time that gravistimulated roots developed a rapid (within 1 h) induction of GH3:GUS activity in tissues on the non-elongating side of the responding root and this induction occurred concurrently with root curvature. These transgenic plants could be useful tools in determining the physiological and biochemical changes that occur during auxin-mediated responses.
Maintaining or improving soil phosphorus (P) fertility is challenging for dryland organic produce... more Maintaining or improving soil phosphorus (P) fertility is challenging for dryland organic producers in the northern Great Plains (NGP) of North America because of the region’s high pH, calcareous soils combined with limited supplies of P-rich organic materials, such as manure. Certain inorganic P sources, such as phosphate rock (PR), are allowed on organic farms yet are sparingly soluble in neutral to alkaline soils. Green manure (GM) crops may improve availability of PR, yet little is known on their ability to dissolve PR in NGP soils. A 2-year cropping sequence study was conducted on a Montana farm that had been managed organically for 20 year. First-year crop treatments included spring pea (Pisum sativum L.), buckwheat (Fagopyrum esculentum L.), yellow mustard (Sinapis alba L.), and tilled fallow, fertilized with 0, 3.1, and 7.7 kg available P ha−1 as PR, and terminated at early pod stage with tillage. Following GM incorporation, winter wheat (Triticum aestivum L.) was sown in the fall and harvested at maturity. Aboveground P uptake of winter wheat was not affected by previous GM crop or PR rate, despite differences in GM P uptake amounts among crops and PR rates. Labile P fractions in the surface soil (0–0.15 m) were also not increased over pre-fertilization levels by PR rate or previous crop. The results suggest GMs have a limited ability to increase P availability of PR for a subsequent crop in this region.
Specialised plant pathogens are in many ways adapted to exploit their host plants. Infective prop... more Specialised plant pathogens are in many ways adapted to exploit their host plants. Infective propagules should reach the appropriate plant tissue, gain access to the tissue and negate or suppress various kinds of constitutive and inducible resistance mechanisms. The resistance type most frequently deployed in plant breeding is the race-specific resistance, where a hypersensitive response of plant tissue is elicited by an avirulence factor produced by the pathogen. The great disadvantage of this type of resistance is, that it is often ephemeral. Detailed screening of germplasm may result in the discovery of alternative defence mechanisms not associated with hypersensitivity, that may be durable. Avoidance mechanisms may reduce the chance of infection. Upright plant habit has been reported to decrease spore deposition in cereals. Crop architecture may also affect aspects such as humidity and aeration in the crop, and hence the chances for successful infection by pathogen propagules. Other examples of avoidance are leaf surface properties that interfere with leaf wettability, germ tube orientation and finding of stomata to enter the leaf. Stomata in some accessions of Hordeum chilense are excessively covered by cuticular wax that prevent rust fungal germ tubes from perceiving the stomata, resulting in failure of penetration of the pathogen into the leaf. There is evidence that incompatible host species, biotrophic pathogenic fungi induce basic compatibility by suppressing defence mechanisms. Failure of this induction results in abortion of the infection attempt. Several cases of apparently durable resistance are discussed that are based on failure of haustorium formation, and are not associated with hypersensitivity. They may represent cases where the pathogen has problems in establishing basic compatibility.
The capacity of the triose-phosphate shuttle and various combinations of glycolytic intermediates... more The capacity of the triose-phosphate shuttle and various combinations of glycolytic intermediates to substitute for the ATP requirement for fatty-acid and glycerolipid biosynthesis in pea (Pisum sativum L.) root plastids was assessed. In all cases, ATP gave the greatest rates of fatty-acid and glycerolipid biosynthesis. Rates of up to 66 and 27 nmol·(mg protein)−1·h−1 were observed for the incorporation of acetate and glycerol-3-phosphate into lipids in the presence of ATP. In the absence of exogenously supplied ATP, the triose-phosphate shuttle gave up to 44 and 33% of the ATP-control activity in promoting fatty-acid and glycerolipid biosynthesis from acetate and glycerol-3-phosphate, respectively. The optimum shuttle components were 2 mM dihydroxyacetonephosphate (DHAP), 2 mM oxaloacetic acid and 4 mM inorganic phosphate (referred to as the DHAP shuttle). Glyceraldehyde-3-phosphate, as a shuttle triose, was approximately 82% as effective as DHAP in promoting fatty-acid synthesis while 2-phosphoglycerate, 3-phosphoglycerate, and phosphoenolpyruvate were only 27–37% as effective as DHAP. When glycolytic intermediates were used as energy sources for fatty-acid synthesis, in the absence of both exogenously supplied ATP and the triose-phosphate shuttle, phosphoenolpyruvate, 2-phosphoglycerate, fructose-6-phosphate and glucose-6-phosphate each gave 48%, 17%, 23% and 17%, respectively, of the ATP-control activity. Other triose phosphates tested were much less effective in promoting fatty-acid synthesis. When exogenously supplied ATP was supplemented with the DHAP shuttle or glycolytic intermediates, the complete shuttle increased fatty-acid biosynthesis by 37% while DHAP alone resulted in 24% stimulation. Glucose-6-phosphate, fructose-6-phosphate and glycerol-3-phosphate similarly all improved the rates of fatty-acid synthesis by 20–30%. In contrast, 3-phosphoglycerate, 2-phosphoglycerate and phosphoenolpyruvate all inhibited fatty-acid synthesis by approximately 10% each. The addition of the DHAP shuttle and glycolytic intermediates with or without exogenously supplied ATP caused an increase in the proportion of radioactive oleate and a decrease in the proportion of radioactive palmitate synthesized. The use of these alternative energy sources resulted in higher amounts of free fatty acids and triacylglycerol, and lower amounts of diacylglycerol and phosphatidic acid. The data presented here indicate that ATP is superior in promoting in-vitro fatty-acid biosynthesis in pea root plastids; however, both the triose-phosphate shuttle and glycolytic metabolism can produce some of the ATP required for fatty-acid biosynthesis in these plastids.
We report an improved method for white clover (Trifolium repens) transformation usingAgrobacteriu... more We report an improved method for white clover (Trifolium repens) transformation usingAgrobacterium tumefaciens. High efficiencies of transgenic plant production were achieved using cotyledons of imbibed mature seed. Transgenic plants were recovered routinely from over 50% of treated cotyledons. Thebar gene and phosphinothricin selection was shown to be a more effective selection system thannptII (kanamycin selection) oraadA (spectinomycin selection). White clover was transformed with the soybean auxin responsive promoter, GH3, fused to the GUS gene (β-glucuronidase) to study the involvement of auxin in root development. Analysis of 12 independent transgenic plants showed that the location and pattern of GUS expression was consistent but the levels of expression varied. The level of GH3:GUS expression in untreated plants was enhanced specifically by auxin-treatment but the pattern of expression was not altered. Expression of the GH3:GUS fusion was not enhanced by other phytohormones. A consistent GUS expression pattern was evident in untreated plants presumably in response to endogenous auxin or to differences in auxin sensitivity in various clover tissues. In untreated plants, the pattern of GH3:GUS expression was consistent with physiological responses which are regarded as being auxin-mediated. For the first time it is shown that localised spots of GH3:GUS activity occurred in root cortical tissue opposite the sites where lateral roots subsequently were initiated. Newly formed lateral roots grew towards and through these islands of GH3:GUS expression, implying the importance of auxin in controlling lateral root development. Similarly, it is demonstrated for the first time that gravistimulated roots developed a rapid (within 1 h) induction of GH3:GUS activity in tissues on the non-elongating side of the responding root and this induction occurred concurrently with root curvature. These transgenic plants could be useful tools in determining the physiological and biochemical changes that occur during auxin-mediated responses.
Maintaining or improving soil phosphorus (P) fertility is challenging for dryland organic produce... more Maintaining or improving soil phosphorus (P) fertility is challenging for dryland organic producers in the northern Great Plains (NGP) of North America because of the region’s high pH, calcareous soils combined with limited supplies of P-rich organic materials, such as manure. Certain inorganic P sources, such as phosphate rock (PR), are allowed on organic farms yet are sparingly soluble in neutral to alkaline soils. Green manure (GM) crops may improve availability of PR, yet little is known on their ability to dissolve PR in NGP soils. A 2-year cropping sequence study was conducted on a Montana farm that had been managed organically for 20 year. First-year crop treatments included spring pea (Pisum sativum L.), buckwheat (Fagopyrum esculentum L.), yellow mustard (Sinapis alba L.), and tilled fallow, fertilized with 0, 3.1, and 7.7 kg available P ha−1 as PR, and terminated at early pod stage with tillage. Following GM incorporation, winter wheat (Triticum aestivum L.) was sown in the fall and harvested at maturity. Aboveground P uptake of winter wheat was not affected by previous GM crop or PR rate, despite differences in GM P uptake amounts among crops and PR rates. Labile P fractions in the surface soil (0–0.15 m) were also not increased over pre-fertilization levels by PR rate or previous crop. The results suggest GMs have a limited ability to increase P availability of PR for a subsequent crop in this region.
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