Conclusions and Future Directions
Although SO receives far less attention in the field of plant stress
biology than other ROS such as hydrogen peroxide
(H2O2) and superoxide
(O2-), a growing body of evidence
indicates that it plays a role in plant responses to diverse challenges,
including biotic as well as abiotic stresses. Singlet oxygen can be
generated in gaseous form by photosensitive phytoalexins at the plant
surface, where it is able to act as a direct defense against insect
herbivores and other biotic stressors. In addition, SO accumulates
within plant cells in the cytosol, peroxisomes, nucleus, mitochondria,
and especially in the chloroplasts. Numerous abiotic and biotic stresses
perturb the cell’s photosynthetic machinery, promoting SO generation,
and this positions SO well to sound the alarm and activate adaptive
responses (Lu and Yao, 2018). SO can also be generated as the result of
the enzymatic activities of lipoxygenases or other stress-responsive
peroxidases (Chen et al., 2021; Dmitrieva et al., 2020). This highly
reactive ROS is an important player in retrograde signaling and is known
to reprogram nuclear gene expression through more than one distinct
pathway, including sensing by EX1 at the grana margins and signaling via
the β-carotene derivative β-CC in the grana core. SO also modulates
phytohormone signaling and, via EX1, can activate programmed cell death.
While SO signaling is known in some cases to promote plant adaptation to
stress, certain necrotrophic plant pathogens appear to have co-opted
this response to facilitate the infection, secreting SO-inducing
phytotoxins that form lesions and create infection courts for the
pathogen. In short, SO plays diverse and important roles in biotic
stress responses, and its adaptive significance for the host plant and
for the attacker varies among different interaction pairs.
Unfortunately, SO accumulation, signaling, and outcomes for resistance
or susceptibility have been characterized in very few biotic
interactions so far. To better understand the roles of SO in plant
biotic interactions, we propose that the biotic stress community has the
following research needs:
- Improved methods for direct detection and quantification of SO
in vivo . Several reviews cover the current options for SO
detection in destructive assays and in situ detection
(Dmitrieva et al. 2020; Prasad et al. 2018; You et al., 2018). While
fluorescent sensors such as Singlet Oxygen Sensor Green (SOSG) are
useful tools, uneven penetration of these exogenous sensors represent
a significant challenge. SO-responsive reporter gene constructs
overcome this issue, but have separate concerns about specificity.
Therefore, there is a need for improved SO-specific in vivosensors, to characterize accumulation of this molecule in plant
interactions with biotic stressors.
- Comprehensive identification of SO-responsive signaling
pathways, and elucidation of the relationships among them . The
relationships among the known components of SO signaling (e.g.
EX1/EX2, β-CC, SAFE1, GUN4, and multiple transcription factors) are
not yet well-understood. Moreover, studies suggest the existence of
other yet-to-be-identified nodes in SO signaling (e.g. D’Alessandro et
al., 2018). As a more comprehensive understanding of the SO response
network emerges, it will enable a better understanding of how this
network impacts biotic interactions.
- Characterization of the effects of different biotic stresses on
SO accumulation and the different branches of SO signaling, including
comparisons of compatible and incompatible biotic interactions . As
yet, the role of SO has been examined in relatively few biotic
interactions, and often in an indirect fashion. As a greater number of
biotic interactions are considered from the perspective of SO, we
anticipate that important commonalities and differences will emerge
among interaction types. Within a particular combination of host plant
and attacker species, it can be particularly informative to compare
compatible and incompatible interactions, which are governed by known
determinants of virulence/avirulence in the attacker and/or resistance
or susceptibility factors in the host. These comparisons could help
identify correlations between SO responses and the outcomes of the
interaction.
- More precise tools to manipulate SO accumulation in vivo,
including control of the timing, dosage, and localization of its
generation . Because the consequences of SO accumulation likely vary
depending upon the timing, dosage, and location of accumulation, tools
are needed to manipulate these variables and assess their
consequences. Mammalian cell lines have recently been engineered to
express a genetically encoded photosensitizer targeted to specific
subcellular compartments that can deliver different doses of SO
depending upon light exposure (Liang et al. 2020). The development of
similar sensors for use in plants could dramatically advance our
understanding of SO signaling, and in particular, the role of
different organelles in SO-mediated responses.
- Evidence on how promotion or attenuation of SO accumulation or
SO signaling pathways, singly and in combination, impact the outcome
of plant biotic interactions . For definitive evidence on the adaptive
significance of particular signaling events, we ultimately rely on the
ability to manipulate these events and test the phenotypic effects of
enhancing or abrogating them. For example, null mutations in EX1 and
EX2 have been invaluable in identifying the roles of these proteins in
SO signaling. Therefore, identifying a more comprehensive set of
signaling nodes (#2 above) and developing methods to enhance or
inhibit them, in combination with methods to manipulate accumulation
of SO itself (#4), are essential steps towards understanding the
roles of SO in biotic stress.
In closing, SO signaling is an important emerging area of study in the
field of plant stress biology, and advances in this area will likely
identify novel mechanisms of plant adaptation to biotic attack in
addition to environmental stress. Due to its close relationship with the
photosynthetic machinery, SO also is an important linker between primary
metabolism and defense. Understanding this linkage is critical to in
order to leverage plant defense mechanisms for the protection of crop
health and productivity of the face of increasing environmental stresses
and changing pest pressures.