ERAD-related E2 and E3 enzymes modulate the drought response by regulating the stability of PIP2 aquaporins

Abstract

Endoplasmic reticulum-associated degradation (ERAD) is known to regulate plant responses to diverse stresses, yet its underlying molecular mechanisms and links to various stress signaling pathways are poorly understood. Here, we show that the ERAD component ubiquitin-conjugating enzyme UBC32 positively regulates drought tolerance in Arabidopsis thaliana by targeting the aquaporins PIP2;1 and PIP2;2 for degradation. Furthermore, we demonstrate that the RING-type ligase Rma1 acts together with UBC32 and that the E2 activity of UBC32 is essential for the ubiquitination of Rma1. This complex ubiquitinates a phosphorylated form of PIP2;1 at Lys276 to promote its degradation, thereby enhancing plant drought tolerance. Extending these molecular insights into crops, we show that overexpression of Arabidopsis UBC32 also improves drought tolerance in rice (Oryza sativa). Thus, beyond uncovering the molecular basis of an ERAD-regulated stress response, our study suggests multiple potential strategies for engineering crops with improved drought tolerance.

Root Membrane Ubiquitinome under Short-Term Osmotic Stress

“…Increased ubiquitination of K3 and K276 was observed in PIP2;1 upon short-term mannitol treatment (Table 1). K276 ubiquitination was recently shown to mediate PIP2;1 degradation upon long-term drought [17]. However, since we observed that PIP2;1 cellular abundance was unchanged (Table 1), the increased ubiquitination of K276 observed when plants are subjected to a 1 h mannitol treatment cannot be involved in PIP2;1 degradation.…”

“…Nine of them exhibited ubiquitinated residues in their N-and/or C-terminus (Table S3, Figure S5). Since only three lysine residues are described in the literature as being ubiquitinated [11,17], the present work greatly increases our knowledge regarding the ubiquitination of PIPs. Increased ubiquitination of K3 and K276 was observed in PIP2;1 upon short-term mannitol treatment (Table 1).…”

“…This osmotic treatment was shown to induce maximal root hydraulic conductivity inhibition by 60%, which can be accounted for by a decrease in aquaporin function [43] and not PIP degradation since the abundance of all PIPs was unchanged (Table S2). Thus, even though ubiquitination at K3 and K276 can be involved in PIP2;1 degradation (Figure 5, [17]), we assume that ubiquitination induces different consequences upon short-term mannitol treatment. Indeed, short-term osmotic treatment induces PIP2;1 selective endocytosis [44].…”

Ubiquitination in the rice blast fungus Magnaporthe oryzae: from development and pathogenicity to stress responses

“…For example, PHO2, a ubiquitinconjugating enzyme in Arabidopsis, directly associates with the phosphate transporter PHO1 and negatively regulates its stability (Liu et al 2012). Furthermore, UBC27 and UBC32 directly interact with their target proteins ABI1 and PIP2;1/PIP2;2 respectively, and promote the ubiquitination and thereby degradation of target proteins (Pan et al 2020;Chen et al 2021). These findings shed light on at least some of the ubiquitin-conjugating enzymes that participate in target recognition directly (Fig.…”

“…For example, phosphorylation can directly modify the stability of proteins or regulate the activity of E3 liagese. Phosphorylated ABA transporter NRT1.2/NPF4.6 and aquaporin proteins PIP2;1/PIP2;2 exhibit a faster degradation rate, whereas the degradation of their non-phosphorylated forms is repressed (Chen et al 2021;Zhang et al 2021). Like ubiquitination, SUMOylation also occurs at certain lysine residues; however, it often acts in competition with ubiquitination to enhance the stability of its substrates.…”