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.