60
The deposition of callose in the leaves of Arabidospsis throughout the leaf vascular system suggested due to secretion of siderophores resulting the cell wall rigidity. The deposition callose differs from the well-described pattern induced by
pathogens and MAMP, which appear in the inter vein zone. One of the most widely
accepted hypothesis is that the siderophores mediated transient changes in iron status in the vascular system, causing ROS generation, followed by accumulation of
callose along the veins.
4.5 Mechanisms of Siderophores Action on Plant Immune
Response
As highlighted in this paper, siderophores primarily possess two opposing potential
roles during infection processes: pathogenesis effectors involved in host invasion
and defense elicitors. This dual property is also found in some protein effectors
secreted by fungal pathogens (Bent and Mackey 2007).
4.5.1 Siderophore Regulation via Their Iron Scavenging
Effect
Iron scavenging by siderophores may alter the stoichiometric metal balance and this
may result in siderophore-mediated activation of plant immune responses. For example, if any metalloproteins bind with a particular metal, it can lead to disturbance in
maetal balance (e.g., Zn
2+
replaces Fe
2+
) thereby resulting in a danger signal. This
alarming signal could be detected through pathogenic effectors that are injected into
plant cells to promote infection (Block et al. 2008). Surprisingly, these effectors
strongly activate immune responses and the hypersensitive response, leading to cell
death. In the case of siderophores, no cell death has ever been reported. Aznar and his
coworkers (2014) reported that deferrioxamine (DFO) triggers the defense at a concentration that coincides with the concentration of iron in the leaf.
The immune responses are not activated below this level suggesting that the plant
can tolerate the presence of strong iron scavengers at a certain level, corresponding
to their own iron content. Above this level of level, the “danger signal” is inactivated. Choi et al. (2007) demonstrated that the activation of immunity by the elimination of iron is effective in mammal cells as well as in plants. As discussed above,
in Arabidopsis, immunity mediated by siderophores requires transporter IRT1 indicating that, siderophores mediated defense caused not only due to the strong iron
deficiency but also due to the uptake of metals. Thus, exceptionally in this case, the
effect of iron starvation caused by the scavenging only triggers immunity. In contrast, the disturbance in metal homeostasis due to iron scavengers is probably the
signal that activates defensive responses in both animals and plants (Aznar and
Dellagi 2015).
S. Ashraf et al.
The deposition of callose in the leaves of Arabidospsis throughout the leaf vascular system suggested due to secretion of siderophores resulting the cell wall rigidity. The deposition callose differs from the well-described pattern induced by
pathogens and MAMP, which appear in the inter vein zone. One of the most widely
accepted hypothesis is that the siderophores mediated transient changes in iron status in the vascular system, causing ROS generation, followed by accumulation of
callose along the veins.
4.5 Mechanisms of Siderophores Action on Plant Immune
Response
As highlighted in this paper, siderophores primarily possess two opposing potential
roles during infection processes: pathogenesis effectors involved in host invasion
and defense elicitors. This dual property is also found in some protein effectors
secreted by fungal pathogens (Bent and Mackey 2007).
4.5.1 Siderophore Regulation via Their Iron Scavenging
Effect
Iron scavenging by siderophores may alter the stoichiometric metal balance and this
may result in siderophore-mediated activation of plant immune responses. For example, if any metalloproteins bind with a particular metal, it can lead to disturbance in
maetal balance (e.g., Zn
2+
replaces Fe
2+
) thereby resulting in a danger signal. This
alarming signal could be detected through pathogenic effectors that are injected into
plant cells to promote infection (Block et al. 2008). Surprisingly, these effectors
strongly activate immune responses and the hypersensitive response, leading to cell
death. In the case of siderophores, no cell death has ever been reported. Aznar and his
coworkers (2014) reported that deferrioxamine (DFO) triggers the defense at a concentration that coincides with the concentration of iron in the leaf.
The immune responses are not activated below this level suggesting that the plant
can tolerate the presence of strong iron scavengers at a certain level, corresponding
to their own iron content. Above this level of level, the “danger signal” is inactivated. Choi et al. (2007) demonstrated that the activation of immunity by the elimination of iron is effective in mammal cells as well as in plants. As discussed above,
in Arabidopsis, immunity mediated by siderophores requires transporter IRT1 indicating that, siderophores mediated defense caused not only due to the strong iron
deficiency but also due to the uptake of metals. Thus, exceptionally in this case, the
effect of iron starvation caused by the scavenging only triggers immunity. In contrast, the disturbance in metal homeostasis due to iron scavengers is probably the
signal that activates defensive responses in both animals and plants (Aznar and
Dellagi 2015).
S. Ashraf et al.