Occurrence and Biosynthesis of Glyoxysomal Enzymes in Ripening Cucumber Seeds

Abstract

Glyoxysomal enzymes, being necessary during seed germination, are already synthesized at the stage of seed maturation. Two stages of embryogenesis of cucumber seeds (Cucumis sativus) were investigated. One was characterized by the presence of microbodies showing catalase and enoyl-CoAJiydratase activities. Microbodies at a later stage contained, in addition, malate synthase and isocitrate lyase. The biosynthesis of three microbody components was followed in a pulse chase-labelling experiment which demonstrated that the biosynthesis of cytosolic species of malate synthase, isocitrate lyase and enoyl-CoA hydratase preceded the appearance of these proteins in microbodies. Vorkommen und Biosynthese von glyoxysomalen Enzymen in reifenden Samen von Cucumis sativusZusammenfassung: Die Biosynthese glyoxysomaler Enzyme, die während der Samenkeimung für die Fettmobilisierung in den Glyoxysomen benötigt werden, erfolgt bereits während der Samenbildung. Zwei Stadien der Embryogenese wurden -am Beispiel der Gurkensamen (Cucumis sativus) -genauer untersucht: Ein Mikrokörper-Stadium war charakterisiert durch das Vorkommen von Katalase und Enoyl-CoAHydratase, in einem späteren Stadium verhielten sich die Mikrokörper -mit einem vollen Satz von Glyoxylat-Zyklus-Enzymen -ähnlich den Glyoxysomen. In diesem Stadium der Samenentwicklung wurde auch die Biosynthese der glyoxysomalen Komponenten durch ein Pulsechase-Experiment verfolgt. In drei untersuchten Fällen trat die radioaktive Markierung zuerst in den cytoplasmatischen Pools und dann erst in den Glyoxysomen auf.

Enzyme Development and Glyoxysome Characterization in Cotyledons of Cotton Seeds

“…This is consistent with Ihle and Dure's proposal (11) that mRNAs for these "germination" enzymes in cotton seeds are transcribed during embryogenesis, but are not translated until needed for postgerminative growth. Studies in our laboratory (2,18,19,26) and by others (6,12,13) indicate that many of the enzymes involved in gluconeogenesis from reserve lipid already are active in maturing embryos and are incorporated into microbodies prior to desiccation. Table I shows that mature, dry cotton seeds possess activities for all the gluconeogenesis-related enzymes examined (except isocitrate lyase).…”

Cottonseed Malate Synthase

“…MS2 (EC 4.1.3.2), one of the key glyorxylate cycle enzymes, is especially interesting in that it appears and increases during seed maturation following the development of catalase and ,B-oxidation enzymes, but precedes the initial appearance ofisocitrate lyase (4,7). The rise in enzyme activities following seed germination is accompanied by an ultrastructurally obvious (29) and quantitatively measurable (21) increase in glyoxysome volume, whereas changes in glyoxysome volume are not perceptible during late seed maturation.…”

“…Kindl's group (12,13,19,20), however, reported that the 5S monomer was the precursor to the 19S octamer, and that the MS aggregate was outside the ER simply exhibiting sedimentation behavior on gradients similar to ER vesicles. Therefore, ER in their view, did not have a bearing on the biosynthesis of glyoxysomal MS. Only one study (with cucumber seeds) has been conducted on the radiolabeling of MS relative to its biogenesis during seed maturation (7). Immature seeds removed from the fruit, i.e.…”

“…Bowden MS was in the ER (23). Gonzalez (10) (7,16). However, the immature seeds were radiolabeled in the absence of ABA, thus the data reflected events that occurred during a germination mode, rather than one where precocious germination was blocked by the presence of ABA.…”

“…However, the immature seeds were radiolabeled in the absence of ABA, thus the data reflected events that occurred during a germination mode, rather than one where precocious germination was blocked by the presence of ABA. They (7) concluded that MS was synthesized in the cytosol prior to import into glyoxysomes. MS (Table I).…”