Auxin confers protection against ER stress in Caenorhabditis elegans

Abstract

Auxins are plant growth regulators that influence most aspects of plant development through complex mechanisms. The development of an auxin-inducible degradation (AID) system has enabled rapid, conditional protein depletion in yeast and cultured cells. More recently, the system was successfully adapted to Caenorhabditiselegans to achieve auxin-dependent degradation of targets in all tissues and developmental stages. Whether auxin treatment alone has an impact on nematode physiology is an open question. Here we show that indole-3-acetic acid (IAA), the auxin most commonly used to trigger AID in worms, functions through the conserved IRE-1/XBP-1 branch of the Unfolded Protein Response (UPR) to promote resistance to endoplasmic reticulum (ER) stress. Because the UPR not only plays a central role in restoring ER homeostasis, but also promotes lipid biosynthesis and regulates lifespan, we suggest that extreme caution should be exercised when using the AID system to study these and related processes.

Keywords: Auxin; ER stress; Tunicamycin; Unfolded Protein Response.

Fig. 1.

Auxin protects developing worms against ER stress. (A) Proportion of wild-type animals reaching the L4-adult stage after 4 days of development on plates containing tunicamycin (3 µg/ml) with or without auxin (1 mM). Error bars represent SEM from three independent experiments (Student's t-test, **P<0.01 auxin versus no auxin). (B) Proportion of wild-type animals reaching the L4-adult stage after 4 days on plates containing tunicamycin (3 µg/ml) combined with increasing concentrations of auxin (0.1–1 mM). Error bars show SEM from three independent experiments (one-way ANOVA with Tukey's multiple comparison test, *P<0.05, ***P=0.0005, ****P<0.0001 auxin versus no auxin).

Fig. 2.

Auxin promotes ER stress resistance in adult worms. Survival of adult wild-type animals on plates containing either tunicamycin (40 µg/ml) alone (N=147), or tunicamycin and auxin (1 mM) (N=144). (P<0.0001). Worms were exposed to tunicamycin with or without auxin starting from the first day of adulthood (day 0). The P-value was calculated using the log-rank (Mantel-Cox) method. Mean and maximum lifespan were 7.8 and 10, and 8.9 and 11, for tunicamycin and tunicamycin+auxin, respectively. A replicate experiment is shown in Fig. S1.

Fig. 3.

The XBP-1/IRE-1 pathway of the UPR is required for auxin to induce ER stress resistance. (A) Proportion of xbp-1(tm2457), xbp-1(zc12), and ire-1(ok799) mutants, and wild-type animals reaching the L4-adult stage on plates containing either tunicamycin (2 µg/ml) only, or both tunicamycin and auxin (1 mM). (B) Proportion of atf-6(ok551), pek-1(ok275), and wild-type animals reaching the L4-adult stage in the presence of tunicamycin (3 µg/ml) and auxin (1 mM) or tunicamycin alone. For both (A) and (B) error bars show SEM from three independent experiments (multiple t-test with Holm-Sidak correction, **P<0.005, ***P<0.0005 auxin versus no auxin).