Carbohydrate Content and Enzyme Metabolism in Developing Canola Siliques

Abstract

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...

Probing in Vivo Metabolism by Stable Isotope Labeling of Storage Lipids and Proteins in Developing Brassica napusEmbryos

“…As development proceeds, the concentration of Glc and Fru decreases severalfold, whereas that of Suc increases about 10-fold (Table I) such that for seeds in the late cotyledon stage (Ͼ3 mg fresh weight, late cotyledon, 26 DAF), Suc dominates over Glc and Fru. The change in the ratio of hexoses to Suc is similar to that observed earlier by King et al (1997) in whole seeds of B. napus as well as by Hill and Rawsthorne (2000) in endosperm liquid. After 26 DAF, the growth of the embryo comes to an end and the embryo takes up most of the volume inside the seed coat.…”

“…This is suggested by the fact that during embryo development of B. napus embryos, the Suc to hexose ratio in the endosperm liquid increases (see Table I; King et al, 1997). With the increase of Suc concentration in the endosperm liquid and the onset of storage product accumulation, induction of Suc synthase is reported in seeds of B. napus (King et al, 1997) and induction of Suc synthase mRNA and a Suc transporter mRNA are observed in seeds of Arabidopsis (Ruuska et al, 2002). Thus, the relative ratio of uptake of [U-13 C 6 ]Glc and Suc may change during embryo development.…”

“…After the mid-cotyledon stage, rapid accumulation of storage lipid and proteins occurs (Norton and Harris, 1975;Pomeroy et al, 1991), resulting in mature seeds with 40% to 50% (w/w) oil and 30% (w/w) protein (Murphy and Cummis, 1989). During storage product accumulation, Suc at high concentration is the predominant sugar in the seed and has been considered the main carbon source for the embryo (Norton and Harris, 1975;King et al, 1997). However, in addition to sugars, considerable amounts of amino acids and organic acids have been reported to be present in the endosperm liquid of Phaseolus vulgaris (Smith, 1973) and, as reported in this paper, are also present in the endosperm liquid of B. napus.…”

Differential expression analysis of transcripts related to oil metabolism in maturing seeds of Jatropha curcas L.

“…They are also critical for plant growth and development in addition to being responsible for the production of accessible energy and the creation of primary building blocks of other metabolisms . Up regulation of both the putative sucrose synthase transcripts identified in this study during early maturation (JC1 stage) suggests the importance of sucrose synthase (King et al 1997;Li et al 2006) in providing carbon for oil biosynthesis. Higher transcript level for the putative UDP-glycosyl transferase only in JC1 also indicates a key role for the corresponding enzyme in transferring (Bowles et al 2005) sugar moieties from activated sugars to acceptor molecules like proteins, lipids and secondary metabolites, particularly during early seed maturation stages.…”

Enhanced Seed Oil Production in Canola by Conditional Expression of Brassica napus LEAFY COTYLEDON1 and LEC1-LIKE in Developing Seeds

“…In rapeseed, Suc is mainly produced in the silique wall during the seed-filling stage, which is transported into developing seeds, and is then converted into hexose UDP-Glc and Fru. In the cytosol and the plastid of the embryo cells, hexose is further converted into acetyl-CoA, a precursor of FA biosynthesis (King et al, 1997;Baud et al, 2002;Hill et al, 2003;Schwender et al, 2003). Using isolated plastids from developing rapeseed seeds, it has been found that key cytosolic metabolites, including Glc-6-P, malate, phosphoenol pyruvate, and pyruvate, are imported into the plastid and then converted into acetyl-CoA through the glycolytic pathway to initiate de novo synthesis of FAs, of which pyruvate and Glc-6-P cause the highest rate of FA biosynthesis (Kang and Rawsthorne, 1994;Rawsthorne, 2002).…”