Gene expression prior to radicle emergence in imbibed tomato seeds

In tomato (Lycopersicon esculentum Mill.) seeds, the primary control of germination resides in the tissues of the radicle tip and the endosperm cap enclosing it. It is a reasonable assumption that germination is regulated ultimately by specific gene expression in these tissues. We are therefore interested in identifying the genes that are expressed in these tissues during imbibition and prior to radicle emergence and in understanding how their expression is influenced by environmental and hormonal factors that also modify the germination rate or percentage. Two approaches were utilized to identify genes specifically associated with the events Initiated by seed imbibition and culminating in radicle emergence. The genes for proteins or enzymes that were expected to play a role in germination have been cloned using polymerase chain reaction (PCR) or library-screening methods, including genes for several cell wall hydrolases (endo-beta-mannanase, cellulase, polygalacturonase, xyloglucan endotransglycosylase, beta-1,3-glucanase, and chitinase), expansins and galactinol synthase. In addition, mRNA populations from both radicle tips and endosperm caps of the gibberellin-deficient (gib-1) tomato mutant were compared by differential cDNA display following imbibition in the presence or absence of gibberellin (GA). Additional mRNAs identified by this approach that are either expressed or repressed in these tissues in response to GA include ones coding for an arabinosidase, a vacuolar H+-translocating ATPase subunit, a GA-responsive transcript and the activating subunit of a protein kinase involved in sugar sensing. These studies are beginning to reveal the wide array of genes and biochemical processes that are involved in the transition from seed development to germination.