While diverse microbe‐ or damage‐associated molecular patterns (MAMPs/DAMPs) typically trigger a common set of intracellular signalling events, comparative analysis between the MAMPs flg22 and elf18 revealed MAMP‐specific differences in Ca2+ signalling, defence gene expression and MAMP‐mediated growth arrest in Arabidopsis thaliana. Such MAMP‐specific differences are, in part, controlled by BAK1, a kinase associated with several receptors. Whereas defence gene expression and growth inhibition mediated by flg22 were reduced in bak1 mutants, BAK1 had no or minor effects on the same responses elicited by elf18. As the residual Ca2+ elevations induced by diverse MAMPs/DAMPs (flg22, elf18 and Pep1) were virtually identical in bak1 mutants, a differential BAK1‐mediated signal amplification to attain MAMP/DAMP‐specific Ca2+ amplitudes in wild‐type plants may be hypothesized. Furthermore, abrogation of reactive oxygen species (ROS) accumulation, either in the rbohD mutant or through inhibitor application, led to loss of a second Ca2+ peak, demonstrating a feedback effect of ROS on Ca2+ signalling. Conversely, mpk3 mutants showed a prolonged accumulation of ROS but this did not significantly impinge on the overall Ca2+ response. Thus, fine‐tuning of MAMP/DAMP responses involves interplay between diverse signalling elements functioning both up‐ or downstream of Ca2+ signalling.

Previous studies have demonstrated that auxin (indole‐3‐acetic acid) and nitric oxide (NO) are plant growth regulators that coordinate several plant physiological responses determining root architecture. Nonetheless, the way in which these factors interact to affect these growth and developmental processes is not well understood. The Arabidopsis thaliana F‐box proteins TRANSPORT INHIBITOR RESPONSE 1/AUXIN SIGNALING F‐BOX (TIR1/AFB) are auxin receptors that mediate degradation of AUXIN/INDOLE‐3‐ACETIC ACID (Aux/IAA) repressors to induce auxin‐regulated responses. A broad spectrum of NO‐mediated protein modifications are known in eukaryotic cells. Here, we provide evidence that NO donors increase auxin‐dependent gene expression while NO depletion blocks Aux/IAA protein degradation. NO also enhances TIR1‐Aux/IAA interaction as evidenced by pull‐down and two‐hybrid assays. In addition, we provide evidence for NO‐mediated modulation of auxin signaling through S‐nitrosylation of the TIR1 auxin receptor. S‐nitrosylation of cysteine is a redox‐based post‐translational modification that contributes to the complexity of the cellular proteome. We show that TIR1 C140 is a critical residue for TIR1–Aux/IAA interaction and TIR1 function. These results suggest that TIR1 S‐nitrosylation enhances TIR1–Aux/IAA interaction, facilitating Aux/IAA degradation and subsequently promoting activation of gene expression. Our findings underline the importance of NO in phytohormone signaling pathways.

The COP1/SPA complex acts as an E3 ubiquitin ligase to repress photomorphogenesis by targeting activators of the light response for degradation. Genetic analysis has shown that the four members of the SPA gene family (SPA1–SPA4) have overlapping but distinct functions. In particular, SPA1 and SPA2 differ in that SPA1 encodes a potent repressor in light‐ and dark‐grown seedlings, but SPA2 fully loses its function when seedlings are exposed to light, indicating that SPA2 function is hyper‐inactivated by light. Here, we have used chimeric SPA1/SPA2 constructs to show that the distinct functions of SPA1 and SPA2 genes in light‐grown seedlings are due to the SPA protein sequences and independent of the SPA promoter sequences. Biochemical analysis of SPA1 and SPA2 protein levels shows that light exposure leads to rapid proteasomal degradation of SPA2, and, more weakly, of SPA1, but not of COP1. This suggests that light inactivates the COP1/SPA complex partly by reducing SPA protein levels. Although SPA2 was more strongly degraded than SPA1, this was not the sole reason for the lack of SPA2 function in the light. We found that the SPA2 protein is inherently incapable of repressing photomorphogenesis in light‐grown seedlings. The data therefore indicate that light inactivates the function of SPA2 through a post‐translational mechanism that eliminates the activity of the remaining SPA2 protein in the cell.