The second step of the biphasic endosperm cap weakening that mediates tomato (Lycopersicon esculentum) seed germination is under control of ABA

Introduction The role of abscisic acid (ABA) in the weakening of the Endosperm weakening is a prerequisite for the completion endosperm cap prior to radicle protrusion in tomato of seed germination in a number of species. Several (Lycopersicon esculentum Mill. cv. Moneymaker) seeds studies have been undertaken to investigate the role of was studied. The endosperm cap weakened substan- cell wall enzymes in endosperm weakening. In a few tially in both water and ABA during the first 38 h of species enzyme activity has been identified and associated imbibition. After 38 h the force required for endosperm with cell wall hydrolysis and completion of germination cap puncturing was arrested at 0.35 N in ABA, whereas (Bewley, 1997). A class I b-1,3-glucanase was found in in water a further decrease occurred until the radicle the micropylar endosperm of Nicotiana tabacum L. seeds protruded. During the first 2 d of imbibition endo-b- and was shown to correlate with endosperm rupture. The mannanase activity was correlated with the decrease enzyme was induced by GA 4 in photodormant seeds and in required puncture force and with the appearance inhibited by ABA (Leubner-Metzger et al., 1996). In of ice-crystal-induced porosity in the cell walls as Datura ferox L. phytochrome-induced b-mannosidase and observed by scanning electron microscopy. Prolonged endo-b-mannanase activities were found to be associated incubation in ABA resulted in the loss of endo-b- with the weakening of the micropylar endosperm, mannanase activity and the loss of ice-crystal-induced although activity of these enzymes was not restricted to porosity, but not in a reversion of the required punc- the micropylar part of the endosperm (Sanchez and de ture force. ABA also had a distinct but minor effect on Miguel, 1997). Also phytochrome-induced cellulase activthe growth potential of the embryo. However, endo- ity, mainly located in the radicle half of the endosperm, sperm cap resistance played the limiting role in the correlated with endosperm weakening and germination completion of germination. It was concluded that (a) (Sanchez et al., 1986). In Lactuca sativa L. seeds cell endosperm cap weakening is a biphasic process and wall-bound endo-b-mannanase activity was found to cor(b) inhibition of germination by ABA is through the relate with germination (Dutta et al., 1997), although second step in the endosperm cap weakening process. this mannanase was not capable of hydrolysing native lettuce endosperm cell walls. In Capsicum annuum L.