4.1 Photosynthesis as a hub for crosstalk and feedback responses
Pathogen infection has been shown to decrease photosynthesis and water use efficiency, as well as affect stomatal patterning, all of which decrease plant productivity and tolerance to abiotic stress (Bilgin et al. 2010; Grimmer et al., 2012; Kissoudis et al., 2014). Downregulation of photosynthesis-related genes is a core response of plants to abiotic stress, as well as during damage due to biotic agents, including arthropods, fungi, bacterial or viral pathogens (Bilgin et al., 2010; Cohen & Leach, 2020). Overexpression of a master-regulator gene of photosynthesis, HYR (HIGHER RICE YIELD), enhanced drought tolerance in rice (Ambavaram et al., 2014). Mutants of protein phosphatases, localized to the chloroplast and involved in photosynthetic pathways, showed reduced lesion development and pathogen multiplication, indicating regulatory genes involved in both photosynthesis and plant immune suppression could be key targets to understand plant growth and defense trade-offs (Akimoto-Tomiyama et al., 2018). Such downregulation of photosynthesis when exposed to stress along with upregulation of genes involved in defense marks the transition from growth/reproduction to defense, as has been explained in the growth-differentiation hypothesis (Herms & Mattson, 1992). Meta-analytic studies involving transcriptome surveys from several different plant species and biotic stress factors indicated slow turnover of various photosynthesis-related proteins and supported the hypothesis that plants invest resources in immediate defense needs but without long-term losses in photosynthetic capability and productivity (Bilgin et al., 2010; Akimoto-Tomiyama et al., 2018). In addition to photosynthesis being a hub of crosstalk, downregulation of these genes is likely a protective mechanism against photooxidative damage during abiotic stress (Dalal & Tripathy, 2018).