Germination of lettuce induced by inhibitors of protein synthesis

Induction of cocklebur seed germination by anaerobiosis: A question about the "inhibitor hypothesis" of seed dormancy

Germination of non-dormant small cocklebur (Xanthium pennsylvanicum Wallr.) seeds was improved by immersing them in water, suggesting that during their germination endogenous germination inhibitors are leached out. However, the same effect could be obtained by the quasi-anaerobic pre-incubation of the seeds. When seeds were fully imbibed, moreover, water immersion could no longer potentiate them to germinate, and only anaerobiosis increased the germination potential, thus raising a question against the "inhibitor hypothesis" of seed dormancy.

An enzyme to degrade lettuce endosperm cell walls. Appearance of a mannanase following phytochrome- and gibberellin-induced germination

Lettuce seeds (Lactuca sativa L. cv. Grand Rapids) stimulated to germinate by gibberellin and red light produce large amounts of endo-β-mannanase. This enzyme increases markedly following radicle emergence and is capable of degrading mannose-containing polysaccharides, which are the major components of the endosperm cell wall. Non-germinated seeds contain little enzyme and under conditions where gibberellin- or red light-stimulated germination is prevented (eg. by abscisic acid or prolonged far red light) enzyme levels remain low. Cycloheximide inhibits the increase in enzyme levels when supplied to germinating seeds, but the enzyme once produced is stable in vivo in the presence of this inhibitor for at least 24h. The majority of the extractable mannanase activity is located in the endosperm and we propose that the function of this enzyme is to mobilise the endosperm cell wall polysaccharides as a nutrient source for the growing embryo.

RNA synthesis and the germination of light-sensitive lettuce seeds

The germination of lettuce seeds is inhibited by the nucleotide base analogue 6-methylpurine. RNA synthesis has been measured during imbibition and germination as (32)P-phosphate incorporation into RNA species as fractionated by polyacrylamide gel electrophoresis. Seeds were surface sterilized and imbibed in the presence of various antibiotics. RNA preparations from lettuce seeds were coelectrophoresed with (3)H-RNA prepared from bacteria to check for bacterial contamination of the seeds. There is a much higher rate of RNA synthesis in illuminated, germinating seeds as compared to dark, non-germinating seeds. This difference does not develop until after 12 hours of imbibition at 27°, which is the time of onset of germination and radicle growth.

Temperature and other factors affecting chloramphenicol stimulation of the germination of light-sensitive lettuce seeds

D-threo-chloramphenicol at concentrations ranging from 1000 to 3000 μg/ml stimulated the germination of the light-sensitive seeds of the lettuce (Lactuca sativa L.) varieties Attractie and Grand Rapids. This stimulatory effect of chloramphenicol was markedly temperature dependent, increasing with decrereasing temperature. Seeds showed little response to chloramphenicol at temperatures of 28°C and above except in the case of light treated Attractie seed. The failure of one batch of Grand Rapids seed to respond to chloramphenicol was associated with the low degree of dormancy in this batch.When the germination of half-seeds or intact excised embryos of Attractie seed was inhibited osmotically with 0.15 M NaCl a stimulatory response to chloramphenicol was obtained suggesting that the site of action was in the embryo itself.Other inhibitors of protein synthesis, cycloheximide, puromycin and p-fluorophenylalanine, did not stimulate germination. Cycloheximide at concentrations of 10 μg/ml and above inhibited germination whereas puromycin and p-fluorophenylalanine were relatively ineffective as germination inhibitors.