Programmed Cell Death as a Response to High Light, UV and Drought Stress in Plants - Abiotic Stress: Plant Responses and Applications in Agriculture - page 210

Chloroplasts are strongly suggested to be key players during cell death responses as they
constitute an important source of defense signaling molecules such as ROS, reactive nitro‐
gen species (RNS) and defense hormones like salicylic acid (SA) and jasmonic acid (JA). The
oxidative burst is one of the earliest and most common plant response to abiotic and biotic
stimuli [20]. The application of chloroplast-targeted, ROS-generating herbicides such as
methyl viologen (paraquat) induces cell death with the typical apoptotic traits [21].
Some of key proteins controlling animal cell death such as the Bcl-2 family and caspases
have been proven to be not conserved in plants. It suggests that plants have developed some
unique mechanisms of PCD [15]. Although orthologs of caspases have not been found in
plants based on the sequence similarity, several studies using caspase-speci fi c peptide inhib‐
itors suggested the presence of caspase-like proteases (metacaspases) [22]. These caspase in‐
hibitors have been demonstrated to prevent chemically-, UV- or HR-induced PCD [23-25]
indicating that caspase-like proteins are indeed involved in the regulation of PCD in plants.
Metacaspases (MC) differ from animal caspases in their substrate specificity as they cleave
proteins after arginine or a lysine residues. Nine predicted metacaspase-encoding genes
have been found in Arabidopsis thaliana and divided into two classes, depending on the pres‐
ence (type I) or absence (type II) of the N-terminal zinc- fi nger domain that has the homology
to the LESION SIMULATING DISEASE 1 (LSD1) protein (see later) [26,27]. This domain is
known to participate in protein-protein interactions and could indicate that oligomerization
is important for MCs type I activation. The catalytic activities of AtMC4, AtMC5 and AtMC8
have been found to be Ca 2+ -dependent while AtMC9 is active under mildly acidic condi‐
tions. Thus, alterations in cellular Ca 2+ concentration and pH, that are common during vari‐
ous stresses, may help to control MCs activation. The sequence of AtMC4 has also revealed
potential self-cleavage sites that may facilitate additional regulation of protease activity to
achieve sensitive control of PCD [28]. Additionally, metacaspase ATMC4 (AtMCP2) has
been proven to play a positive regulatory role in biotic and abiotic stress-induced PCD [29].
AtMC1 and AtMC2, belonging to type I metacaspases, have opposite roles in the cell death
control. There is a genetic evidence that AtMC2 acts as a negative regulator of AtMC1-in‐
duced PCD. Therefore, it is hypothesized that proteins belonging to MC family execute ei‐
ther anti- or pro-apoptotic functions and compete with each other in making the cell life-
death decisions [30].
Although no orthologues of Bcl-2 family genes ( Bcl-2 or Bax ) have been found in plants,
there are some studies demonstrating that the expression of these genes in plants can regu‐
late programmed cell death pathway [31,32]. Transgenic plants overexpressing animal antia‐
poptosis genes such as Bcl-2 have been proven to exhibit enhanced tolerance to both biotic
and abiotic stress conditions [33,34]. Moreover, the homologue for animal Bax Inhibitor (BI)
has been identified in Arabidopsis [35] and proven to inhibit cell death in plants expressing
mammalian Bax [36]. Arabidopsis BI-1 (AtBI-1) has been reported to localize in the endoplas‐
mic reticulum (ER) and to contain predicted transmembrane α-helices in the sequence, that
are conserved in two other AtBI-1-related proteins: BI-2 and BI-3. These proteins are hy‐
pothesized to function in a similar fashion to the Bcl-2 family - as regulators of pro-death or
survival pathways [10]. In plants, mRNA level of AtBI-1 increases during leaf senescence