4.2 ⎪ Differences between SW and IT in the extent of response to HLC
4.2.1 ⎪ Stronger enrichment in SW versus IT under HLC
Growth under HLC conditions prompted stronger enrichment in SW compared to IT in genes of the ontology categories of acclimation to cold and drought (responses to water deprivation and to abscisic acid) that have overlapping features (Heino, Sandman, Lång, Nordin & Palva 1990). This pattern is consistent with the greater freezing tolerance and upregulation of photosynthetic capacity in SW compared to IT (see also Cohu et al. 2013b; Stewart et al. 2016) as well as the lesser excitation pressure in the chloroplast (more oxidized QA reduction state) of HLC-grown SW compared to IT under experimental high-light exposure. The stronger downregulation of genes involved in light-harvesting in HLC-grown IT suggests that IT limits excitation pressure by lowering light-collection capacity, which is consistent with the lower QA reduction state under very low light (when thermal dissipation is not triggered) in HLC-grown IT compared to SW as well as IT’s lower chlorophyll a + bcontent and higher chlorophyll a/b ratio that are indicative of a smaller antenna size (due to preferential degradation of the outer, chlorophyll b -containing light-harvesting complexes). This is consistent with previous studies in which SW increased, rather than decreased, light absorption during cold acclimation and apparently limited excitation pressure by greater utilization of excitation energy in photosynthetic electron transport (Cohu et al. 2013b), as well as greater photoprotective thermal dissipation (Oakley et al. 2018). Our present findings in HLC growth conditions indicate that the acclimatory adjustments in SW are more conducive to productivity maintenance, while adjustments in IT still mitigate oxidative stress.
Two examples of genes with expression patterns that match those of the greater photosynthetic acclimation in SW compared to IT are SUS1(AT5G20830) and EGR2 (AT5G27930). SUS1 is a sucrose synthase strongly induced under abiotic stress but not required for sucrose accumulation under conditions favorable for growth (Kilianet al. 2007; Barratt et al. 2009). High foliar sucrose levels are, furthermore, linked to increased palisade cell height in leaves grown under high light (Katagiri et al. 2016; Hoshinoet al. 2019). EGR2 is a negative regulator of growth (Bhaskara, Wen, Nguyen & Verslues 2017). Over-expression of EGR2caused a reduction of cell elongation and rosette size, whereasegr2 null mutation enhanced both processes (Bhaskara et al. 2017).