Autophagy triggered by iron‐mediated ER stress is an important stress response to the early phase of Pi starvation in plants

Abstract

SUMMARY Inorganic phosphate (Pi) is essential for plant growth. However, Pi is often limiting in soil. Hence, plants have established several mechanisms of response to Pi starvation. One of the important mechanisms is Pi recycling, which includes membrane lipid remodeling and plastid DNA degradation via catabolic enzymes. However, the involvement of other degradation systems in Pi recycling remains unclear. Autophagy, a system for degradation of intracellular components, contributes to recycling of some nutrients, such as nitrogen, carbon, and zinc, under starvation. In the present study, we found that autophagy‐deficient mutants depleted Pi early and exhibited severe leaf growth defects under Pi starvation. The main cargo of autophagy induced by early Pi depleted conditions was the endoplasmic reticulum (ER), indicating that ER‐phagy, a type of autophagy that selectively degrades the ER, is involved in the response to the early phase of Pi starvation for contribution to Pi recycling. This ER‐phagy was suppressed in an INOSITOL‐REQUIRING ENZYME 1 double mutant, ire1a ire1b, in which ER stress responses are defective, suggesting that the early Pi starvation induced ER‐phagy is induced by ER stress. Furthermore, iron limitation and inhibition of lipid‐reactive oxygen species accumulation suppressed the ER‐phagy. Interestingly, membrane lipid remodeling, a response to late Pi starvation, was accelerated in the ire1a ire1b under early Pi‐depleted conditions. Our findings reveal the existence of two different phases of responses to Pi starvation (i.e. early and late) and indicate that ER stress‐mediated ER‐phagy is involved in Pi recycling in the early phase to suppress acceleration of the late phase.

Intracellular phosphate recycling systems for survival during phosphate starvation in plants

“…For example, while Pi starvation induced the expression of ENDO1 in Petunia, this did not occur in A. thaliana grown on Pi-depleted media for 14 days (Peŕez-Amador et al, 2000;Jones et al, 2021). Also, recent studies using A. thaliana showed that ER-phagy only contributes to Pi recycling in the early phase of Pi starvation (2-3 days) and that RCB-mediated chlorophagy salvages Pi under high nitrate/Pi starvation conditions (Yoshitake et al, 2021;Yoshitake et al, 2022). Hence, AtENDO1 may be involved in Pi recycling in early phase of Pi starvation or under certain conditions.…”

“…Also, recent studies using A . thaliana showed that ER-phagy only contributes to Pi recycling in the early phase of Pi starvation (2-3 days) and that RCB-mediated chlorophagy salvages Pi under high nitrate/Pi starvation conditions ( Yoshitake et al., 2021 ; Yoshitake et al., 2022 ). Hence, AtENDO1 may be involved in Pi recycling in early phase of Pi starvation or under certain conditions.…”

“…Recently, it was also reported that under Pi starvation AtPAP17 localizes in the lytic vacuole, (del Pozo et al, 1999;Veljanovski et al, 2006). Since it is known that Pi starvation enhances accumulation of reactive oxygen species (ROS), which leads to programmed cell death (Van Camp et al, 1998;Overmyer et al, 2000;Tyburski et al, 2009;Yoshitake et al, 2022), that proteins may contribute to plant growth not only through the resupply of Pi from intracellular components but also because increase ROS degradation under Pi starvation. It is known that a phosphohydrolase for inositol pyrophosphates (PP-InsPs), one of the phosphoesters, has a role in Pi starvation response, but it is unclear whether this enzyme is involved in Pi recycling (Gaugler et al, 2022).…”

“…Plant autophagy is induced by nutrient starvation, such as carbon, nitrogen, and zinc deficiency ( Yoshimoto et al., 2004 ; Izumi et al., 2010 ; Shinozaki et al., 2020 ). Pi starvation also induces autophagy in tobacco BY-2 cells and A. thaliana ( Tasaki et al., 2014 ; Naumann et al., 2019 ; Yoshitake et al., 2022 ). Also, experiments using an ER stress sensor mutant have shown that Pi starvation-mediated autophagy is regulated by ER stress response ( Naumann et al., 2019 ).…”

“…ER stress induces ER-phagy, a type of autophagy that degrades ER specificity ( Liu et al., 2012 ). Recently, it has also been reported that ER stress is induced by oxidative lipid accumulation and that ER-phagy contributes to Pi recycling in leaf cells ( Yoshitake et al., 2022 ). Furthermore, the timing of inducing the ER-phagy mediated Pi recycling system is earlier than that of PHOSPHATE STARVATION RESPONSE 1 transcription factor regulated Pi starvation responses, such as membrane lipid remodeling ( Pant et al., 2015 ), suggesting that plants have two phases in their Pi starvation response, to effectively adapt to natural fluctuations in Pi starvation.…”