Canberra, ACT 2601,Little biochemical information is available on carbohydrate metabolism in developing canola (Brassica napus L.) silique (pod) wall and seed tissues. This research examines the carbohydrate contents and sucrose (Suc) metabolic enzyme activities in different aged silique wall and seed tissues during oil filling. l h e silique wall partitioned photosynthate into Suc over starch and predominantly accumulated hexose. l h e silique wall hexose content and soluble acid invertase activity rapidly fel1 as embryos progressed from the early-to late-cotyledon developmental stages. A similar trend was not evident for alkaline invertase, Suc synthase (SuSy), and Suc-phosphate synthase. Silique wall SuSy activities were much higher than source leaves at all times and may serve to supply the substrate for secondary cell wall thickening. In young seeds starch was the predominant accumulated carbohydrate over the sampled developmental range. Seed hexose levels dropped as embryos developed from the early-to midcotyledon stage. Hexose and starch were localized to the testa or liquid endosperm, whereas Suc was evenly distributed among seed components. With the switch to oil accumulation, seed SuSy activity increased by 3.6-fold and soluble acid invertase activity decreased by 76%. These data provide valuable baseline knowledge for the genetic manipulation of canola seed carbon partitioning.The sources of assimilate for developing seeds of canola (Brassica napus L.; Brassica rapa L.) have not been clearly elucidated. During the life of a plant there is a clear sequence of developmental phases that proceed from leaf to stem to silique (pod) to seed (Mendham and Salisbury, 1995). Leaf photosynthesis provides assimilate for the growth of shoot and root meristems. At the initiation of reproductive growth, there is a rapid increase in flowerbearing branches from the shoot apical meristem. The photosynthetic leaf area then quickly declines because of senescence (Pechan and Morgan, 1985), thereby removing one source of assimilate at a time when seeds have a great import demand. At this time, only the oldest seeds at the base of a plant would have begun storage product synthesis. In the absence of leaves, silique wall photosynthesis is the main source of assimilates during this growth phase and may contribute up to 50 to 60% of final plant dry matter (Lewis and Thurling, 1994). Australia (S.P.K.)Like other dicotyledonous plants, canola produces seed storage products in the embryo (Murphy and Cummins, 1989). Early in development, the embryo is very small and the main seed constituents are the testa and liquid endosperm (Fowler and Downey, 1970). During these initial stages embryo cells are rapidly dividing. At the early-to midcotyledon stages (Pomeroy et al., 1991), embryo cells begin to rapidly expand, the resulting growth consumes the liquid endosperm, and the embryo fills the seed's interna1 space (Fowler and Downey, 1970). Coincident with rapid embryo growth, storage oil accumulates and peaks at maximum fresh weight (Ra...
The potential for developing canola (Brassica napus L.) seeds and the interior silique (pod) wall to refix respired CO2 has been investigated. From ribulose-1,5-bisphosphate carboxylase–oxygenase (Rubisco) and phosphoenolpyruvate carboxylase (PEPC) activities, seeds were estimated to have a greater CO2 fixation capacity than silique wall endocarp during oil filling. The major component of seed fixation capacity was embryo Rubisco, which had a total activity of 6.3 nmol min-1 embryo-1 (3.7 µmol min-1 mg chlorophyll-1) at 28 days after anthesis (DAA) with smaller contributions from seed coat and embryo PEPC. Rubisco activities were probably maximal in vivo because of high silique cavity CO2 concentrations (0.8 to 2.5%). Seed chlorophyll content rapidly increased over 10-fold from 20 to 30 DAA and, with 20% of incident light transmitted through the silique wall, embryos demonstrated appreciable photosynthetic electron transport rates and most energy produced appeared to be used for Rubisco-catalysed CO2 fixation. Endocarp refixation capacity was less than seeds because chlorophyll content was not enriched and PEPC activities were relatively small. These data indicate that developing seeds and also endocarp refix respired CO2 and that embryo chlorophyll plays a critical role in this refixation.
Transgenic Nicotiana tabacum L. (tobacco) plants expressing an unregulated gene for pyrophosphate-dependent fructose-6-phosphate 1-phosphotransferase (PFP) from the fermentor protist Giardia lamblia were produced. Independently transformed lines revealed a high level of Giardia PFP activity but unaltered activities of native plant PFP, phosphofructokinase, and fructose-1,6-bisphosphatase. Transgenic plants exhibited a decrease in total biomass but no dramatic physiological or morphological alterations or significant reduction of seed yield. Both source and sink tissues demonstrated altered partitioning: leaf starch was significantly lower at the beginning and end of the daily light period and young seeds had lower starch but higher lipid levels, and these changes were correlated with PFP activity. Transgenic seeds had significantly higher percentages of germination, and microscopic examination of these seeds showed a temporal enhancement in growth of the young embryo. The significance of these results as they relate to our current understanding of PFP is discussed.Key words: pyrophosphate-dependent fructose-6-phosphate 1-phosphotransferase (PFP), transgenic, Nicotiana tabacum, glycolysis, photosynthate partitioning.
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