ENDO-BETA-MANNANASE ACTIVITY PRESENT IN CELL-WALL EXTRACTS OF LETTUCEENDOSPERM PRIOR TO RADICLE EMERGENCE

Lettuce (Lactuca sativa L.) endosperm cell walls isolated prior to rad icle emergence underwent autohydrolysis, the rate of which was correla ted with whether radicle emergence would subsequently occur. Extractio n of endosperm cell walls with 6 M LiCl suppressed autohydrolysis, and the desalted extract possessed activity that was capable of hydrolyzi ng purified locust bean galactomannan but not arabinogalactan, carboxy methylcellulose, glucomannan, polygalacturonic acid, tomato galactoman nan, or native lettuce endosperm cell walls. Some hydrolytic activity was detected on endosperm cell walls if they were modified by partial trifluoroacetic acid hydrolysis or pretreatment with guanidinium thioc yanate. In extended incubations the cell wall enzyme extract released only large molecular mass fragments from locust bean galactomannan, in dicating primarily endo-activity. Galactomannan-hydrolyzing activity i n the cell wall extract increased as a function of imbibition time and was greatest just prior to radicle emergence. Thermoinhibition (imbib ition at 32 degrees C) or treatment with abscisic acid at a temperatur e optimal for germination (25 degrees C) suppressed both germination a nd endosperm cell wall mannanase activity, whereas alleviation of ther moinhibition with gibberellic acid was accompanied by significant enha ncement of mannanase activity. We conclude that a cell wall-hound endo -beta-mannanase is expressed in lettuce endosperm prior to radicle eme rgence and is regulated by the same conditions that govern germination .