Figure 3 . Schematic of the main processes involved in the “escape” strategy (pink boxes) characterized by aerenchyma formation, adventitious root development, and shoot elongation, and the “quiescence” strategy (purple boxes) characterized by the induction ofSUB1A that enhances carbohydrate and energy conservation and fermentation process in anaerobic metabolism. Depending on the species and ecotype, plants use either of these two strategies.
However, adventitious root development and shoot elongation require input of energy and carbohydrates (Voesenek & Bailey-Serres, 2015). Therefore, for the low-land rice ecotype that are often exposed to flash flooding and prolonged submergence, energy and carbohydrate reserves of plants can hardly afford to “escape” the floodwater, and thus the “quiescence” strategy is preferred (Ismail, 2018). In this ecotype theSUBMERGENCE 1 (SUB 1) locus encodes a group VII ERF,SUB 1A, expressed upon the induction of accumulated shoot endogenous ethylene, and which represses energy and carbohydrate consumption, including the “escape” processes via gibberellin signaling (Xu et al., 2006). Meanwhile SUB 1A enhances ethanolic fermentation, further promoting the conservation of energy and carbohydrates (Kuroha & Ashikari, 2020). The induction of “quiescence” strategy and its relationship with plant metabolism and “escape” strategy are also displayed in figure 3.
3.4 Eco-physiological feedbacks during flooding
Finally, we consider how feedbacks between the various processes involved in flooding responses result in adaptive behavior of flooding-tolerant species. Under soil waterlogging condition (figure 4), the root suffers from oxygen deficit whereas the shoot is fully aerated. Still, photosynthesis is reduced as root hypoxia causes reduced root water uptake and further downregulates stomatal conductance (Bradford & Hsiao, 1982; Toral-Juárez et al., 2021). The resulting reduced photosynthesis limits carbohydrate production, which is required for plant metabolism. Due to full aeration, shoot oxygen level does not significantly drop compared to non-stressed condition, but shoot-root oxygen diffusion is low due to high resistance to gas diffusion and especially in the absence of aerenchyma, shoot derived oxygen can hardly rescue the plant from carbon starvation and energy exhaustion (Pedersen & Colmer, 2014). To save energy and carbohydrate reserves under such oxygen deficit, plant aerobic metabolism is limited and may be even shifted to anaerobic metabolism (Parent et al., 2008). These generic waterlogging related processes are displayed with black arrows in figure 4.