Ca^2 . Further continuation of this work in maize suspension cultures led the authors to suggest that a mi-
tochondrial Ca^2 store contributes to anoxia-induced [Ca^2 ]cyt[131]. Measurement of [Ca^2 ]cytin intact
Arabidopsisseedlings expressing aequorin also provided direct evidence for the elevated [Ca^2 ]cytlevels
in response to anoxia treatment [132].
The most abundant metal in the earth’s crust is the trivalent cation aluminum, which is highly toxic
to plant metabolism and growth [133,134]. Aluminum induces transient elevations of [Ca^2 ]cytlevels in
wheat protoplasts [135] and in Arabidopsisseedlings [136]. In contrast to these results, Jones et al. [137]
provided evidence for reduced levels of [Ca^2 ]cytin response to aluminum in tobacco BY2 cell cultures.
However, a study using aequorin-expressing Arabidopsisplants has confirmed that roots treated with
higher levels of aluminum showed decreased levels of [Ca^2 ]cyt. These results suggested that aluminum
inhibits the Ca^2 -permeable channels, thereby creating aluminum toxicity [138]. However, the severity
of aluminum toxicity depends on the concentration of Ca^2 , indicating that the higher levels of Ca^2 in
the soil could prevent the aluminum toxicity.
The levels of ethylene and ABA are known to change in response to stress factors [55,139]. Us-
ing Ca^2 -binding fluorescent dyes, it has been demonstrated that ABA, gibberellic acid, and auxin in-
crease [Ca^2 ]cyt[24,25,52–55,62,140,141]. However, transgenic plants with reconstituted aequorin did
not show changes in [Ca^2 ]cytin response to hormones and heat shock [26]. This could be due to var-
ious factors such as sensitivity of the method, localized nature of the response because of restricted tar-
get cells, and stability of aequorin at high temperature. ABA-induced changes in [Ca^2 ]cythave been
attributed to both Ca^2 release from internal stores and Ca^2 influx from external stores [53,141].
Wounding of plants by herbivores and other mechanical stress factors induce jasmonic acid (JA) [142].
To understand the involvement of Ca^2 in JA-induced gene expression, Leon et al. [58] analyzed the
JA- and wound-induced gene expression and Ca^2 levels in wound- and jasmonic acid–treated Ara-
bidopsis. The JA-responsive genes, JR1toJR3, are inducible by JA and wounding, whereas wound-re-
sponsive genes, WR3andacyl CoA oxidase(ACO), are inducible by wound only [58], indicating the
presence of JA-independent and -dependent wound-induced signal transduction pathways in Arabidop-
sis. These authors have shown that the increased levels of [Ca^2 ]cytexerted two opposing actions on
the expression of WR3andACOgenes (up-regulated) and JR genes (down-regulated) in response to the
wound signal. Therefore, Ca^2 has opposing actions in these two-signal transduction pathways [58].
However, the mechanism(s) by which stress and mechanical signals such as cold, salt, and wind cause
changes in [Ca^2 ]cytis not clear.
B. Biotic Stress Signals
Fungal elicitors that induce defense response in plants also elevate [Ca^2 ]cyt[26,66,68,69]. However, the
magnitude and kinetics of Ca^2 transients induced by touch, cold, and fungal elicitors were found to be
different [26,28,37]. In fact, artificial elevation of [Ca^2 ]cytby Ca^2 ionophore A23187 influences the
production of phytoalexins in soybean and carrot cell cultures. Fungal elicitor treatment of carrot proto-
plasts resulted in elevated levels of [Ca^2 ]cytas measured by the influx of^45 Ca^2 [143]. Using CaM an-
tagonists, phenothiazines and naphthalenesulfonamides, Vogeli et al. [144] showed the inhibition of
sesquiterpene phytoalexin biosynthesis in tobacco suspension cultures. These authors suggested a re-
quirement for elevated [Ca^2 ]cytin the biosynthesis of defense compounds in response to pathogens.
Apoptosis is a process that occurs in plants and animals in which certain cells undergo pro-
grammed cell death. The characteristic features of apoptosis include cell shrinkage, plasma membrane
blebbing and bleeding, and nuclear condensation [66,145]. Avirulent Pseudomonas syringaecaused a
hypersensitive reaction (HR) and programmed cell death in soybean cells and leaf tissue (pv. glycinea)
and in Arabidopsisleaves (pv. tomato). Apoptosis is also observed in tobacco cells treated with the fun-
gal peptide cryptogein and soybean cells treated with H 2 O 2 [66]. At the early stages of HR, levels of
AOSs are produced at the site of infection [68]. The AOSs stimulate rapid influx of Ca^2 into the site
of infection and initiate the HR in order to develop resistance against invading pathogens [66,68]. These
findings provided evidence for the role of Ca^2 in the apoptosis signal transduction pathway [66]. Pars-
ley protoplasts treated with an oligopeptide elicitor, derived from the cell wall of Phytophthora sojae,
also induced rapid increase in [Ca^2 ]cytlevels [69]. These authors also identified a plasma mem-
brane–located Ca^2 channel named LEAC (large conductance elicitor-activated ion channel) using
patch clamp analysis with parsley protoplasts [69]. A study by Xu and Heath [146] provided direct ev-
702 REDDY AND REDDY