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).