Figure 5. Signaling pathways involved in the crosstalk between
light and biotic stress defense.
Plant responses to pathogen or herbivore attack are affected by light.
During biotic stress defense, plants perceive and mediate light signals
via chloroplasts or photoreceptors. The chloroplast redox status,
especially the redox status of the plastoquinone (PQ) pool, influences
the plant defense to pathogens. The functionality of chloroplasts plays
an important role in mediating the plant hypersensitive response (HR).
The chloroplast-located calcium-sensing receptor CAS mediates calcium
signals affecting biotic stress defense. In addition, CAS controls the
accumulation of salicylic acid (SA). In Arabidopsis plants, SA
levels are regulated by blue light via a CRY2/PHOT2-mediated and by red
light via phytochrome-regulated pathways. CRY2 and PHOT2 negatively
regulate COP1 which in turn regulates the stability of the R protein HRT
(Hypersensitive Response to TCV), thereby influencing pathogen
resistance. The homologous, phytochrome-regulated TFs FHY3 and FAR1
influence SA-induced defense by controlling HEMB1 expression. The
plant SA content influences the protein kinase NPR1 which regulates the
transcription of SA-induced defense genes, such as PR1 ,
representing a marker for systemic acquired resistance (SAR). CRY1
promotes PR1 gene expression. During shade (low R:FR ratios),
phosphorylation of NPR1 is inhibited which affects PR1 expression
and SA-induced defense. Shade environments also attenuate jasmonic acid
(JA)-induced defense. The Arabidopsis sulfotransferase ST2a which
is regulated by PIFs and responsible for the formation of
HSO4-JA thereby decreasing levels of active JA is
upregulated under FR light conditions. The stability of JAZ proteins is
enhanced during low R:FR ratios in a phyB-dependent manner leading to
attenuated defense responses. In addition, gibberellin (GA) activity is
enhanced during shade resulting in decreased DELLA protein
functionality. DELLAs represent negative regulators of the shade
avoidance syndrome (SAS) and are involved in preventing interaction of
JAZ proteins with its targets. UV-B light perceived by the UVR8 receptor
enhances sinapate production involved in biotic stress defense. For more
information concerning the different pathways, please refer to section
5. Abbreviations: R, red light; FR, far-red light; B, blue light; UV,
ultraviolet light; TFs, transcription factors.