H. Karchi et al., THE LYSINE-DEPENDENT STIMULATION OF LYSINE CATABOLISM IN TOBACCO SEEDREQUIRES CALCIUM AND PROTEIN-PHOSPHORYLATION, The Plant cell, 7(11), 1995, pp. 1963-1970
The accumulation of free lysine in tobacco seed triggers the stimulati
on of lysine-ketoglutarate reductase, an enzyme that acts; in lysine c
atabolism. The mechanism of lysine-ketoglutarate reductase stimulation
was studied in two different systems: (1) developing seeds of wild-ty
pe plants in which the low basal lysine-ketoglutarate reductase activi
ty can be stimulated by the exogenous addition of lysine; and (2) deve
loping seeds of transgenic tobacco plants expressing a bacterial dihyd
rodipicolinate synthase in which lysine-ketoglutarate reductase activi
ty is stimulated by endogenous lysine overproduction. In both systems,
the stimulation of lysine-ketoglutarate reductase activity was signif
icantly reduced when treated with the Ca2+ chelator EGTA, Moreover, th
e inhibitory effect of EGTA was overcome by the addition of Ca2+ but n
ot Mg2+, suggesting that the lysine-dependent activation of lysine-ket
oglutarate reductase requires Ca2+. This was further confirmed by a si
gnificant stimulation of lysine-ketoglutarate reductase activity follo
wing the treatment of wild-type seeds with ionomycin (an ionophore tha
t increases Ca2+ flow into the cytoplasm), In addition, treatment of w
ild-type seeds with the protein phosphatase inhibitor okadaic acid tri
ggered a significant induction in lysine-ketoglutarate reductase activ
ity, whereas treatment of the transgenic seeds with the protein kinase
inhibitor K-252a caused a significant reduction in its activity. Thus
, we conclude that the stimulation of lysine-ketoglutarate reductase a
ctivity by lysine in tobacco seed operates through an intracellular si
gnaling cascade mediated by Ca2+ and protein phosphorylation.