Shall we talk? New details in crosstalk between copper and iron homeostasis uncovered in Arabidopsis thaliana

This article is a Commentary on Cai et al. (2024), 242: 1206–1217.

The carboxy terminal transmembrane domain of SPL7 mediates interaction with RAN1 at the endoplasmic reticulum to regulate ethylene signalling in Arabidopsis

In Arabidopsis, copper (Cu) transport to the ethylene receptor ETR1 mediated using RAN1, a Cu transporter located at the endoplasmic reticulum (ER), and Cu homeostasis mediated using SPL7, the key Cu-responsive transcription factor, are two deeply conserved vital processes. However, whether and how the two processes interact to regulate plant development remain elusive. We found that its C-terminal transmembrane domain (TMD) anchors SPL7 to the ER, resulting in dual compartmentalisation of the transcription factor. Immunoprecipitation coupled mass spectrometry, yeast-two-hybrid assay, luciferase complementation imaging and subcellular co-localisation analyses indicate that SPL7 interacts with RAN1 at the ER via the TMD. Genetic analysis revealed that the ethylene-induced triple response was significantly compromised in the spl7 mutant, a phenotype rescuable by RAN1 overexpression but not by SPL7 without the TMD. The genetic interaction was corroborated by molecular analysis showing that SPL7 modulates RAN1 abundance in a TMD-dependent manner. Moreover, SPL7 is feedback regulated by ethylene signalling via EIN3, which binds the SPL7 promoter and represses its transcription. These results demonstrate that ER-anchored SPL7 constitutes a cellular mechanism to regulate RAN1 in ethylene signalling and lay the foundation for investigating how Cu homeostasis conditions ethylene sensitivity in the developmental context.

SQUAMOSA promoter-binding protein-like 7 mediates copper deficiency response in the presence of high nitrogen in Arabidopsis thaliana

SQUAMOSA promoter-binding protein-like 7 mediates copper deficiency response in the presence of high nitrogen even with the sufficient level of copper in Arabidopsis thaliana. Under copper (Cu) deficiency, accumulation of mRNA encoding two Cu/Zn superoxide dismutases, CSD1 and CSD2, is downregulated to save Cu for plastocyanin. This downregulation depends on miR398 and is under the control of SQUAMOSA promoter-binding protein-like7 (SPL7). Arabidopsis seedlings are routinely cultured on Murashige and Skoog medium. However, the high nitrogen (N) content of the medium (60 mM) has been shown to induce a similar response to Cu deficiency. The mRNA and protein levels of CSD1 and CSD2 are reduced under high N conditions, even if the Cu concentration in the medium is sufficient (0.1-0.5 µM). In this study, we show that this symptom, similar to the Cu deficiency, occurred in the presence of high N largely depending on SPL7, suggesting that plants actually sensed Cu deficiency. However, a change in N concentration in the medium did not influence the total Cu concentration in shoots or roots. High N did not increase the protein content in leaves but facilitated rapid seedling growth. We speculate that this rapid growth causes a continuous Cu deficiency mainly because of high Cu uptake by mesophyll cells in the leaves. This idea was supported by the observation that plastocyanin did not overaccumulate at the range of 0.1-0.5 µM Cu with 30 mM N. In contrast, in the presence of 5 µM Cu with 30 mM N, plants accumulate more Cu in plastocyanin in the thylakoid lumen, resulting in a slight Cu deficiency in the chloroplast stroma. This process is independent of SPL7.

SPL7 locally regulates copper-homeostasis-related genes in Arabidopsis

In Arabidopsis, a central regulator of copper (Cu) homeostasis is the transcription factor SQUAMOSA promoter binding protein-like7 (SPL7). Under Cu deficiency, SPL7 induces the expression of miR398, which suppresses the expression of the genes CSD1 and CSD2, which encode cytosolic and chloroplastic isoforms of Cu/Zn superoxide dismutase, respectively. Consequently, the limited Cu is preferentially assigned to plastocyanin, which is essential for photosynthetic electron transport. Consistent with this function of miR398 related to photosynthesis, its expression is strongly induced in leaves. In this study, however, we showed that SPL7 was transcribed mainly around the vasculature in roots, where Cu levels were likely sensed. To test the possible long-distance signaling of Cu availability from roots to shoots, we conducted a series of grafting experiments using spl7 mutant and wild-type (WT) plants. Expression of Cu-responsive microRNAs and the resulting suppression of CSD1 and CSD2 mRNAs were observed in leaves only when the aerial part was from WT plants, in which a low level of SPL7 was transcribed also in the vascular tissues. Although local sensing of Cu was disturbed in the spl7 mutant, the Cu level was not affected in the shoots. SPL7 is expressed in specific cell layers in both roots and shoots and locally senses Cu availability, transmitting the information to surrounding cells.

Cd-induced Cu deficiency responses in Arabidopsis thaliana: are phytochelatins involved?

Cadmium (Cd) exposure can disturb the homeostasis of essential elements. In Arabidopsis thaliana, Cd induces a squamosa promoter binding protein-like 7 (SPL7)-dependent Cu deficiency response. We investigated how Cd induces a Cu deficiency response. The Cu deficiency response consists of the active SPL7 transcription factor binding to GTAC motifs in promoters of among others several Cu transporters, a Cu chaperone, and cupro-miRNAs to regulate Cu homeostasis. We demonstrated that the addition of supplemental Cu to Cd-exposed A. thaliana plants diminished the Cu deficiency response in roots, while it even disappeared in leaves. Exposure of plants to Cd in combination with extra Cu reduced Cd levels in both roots and leaves resulting in an improved cellular oxidative state. Furthermore, we demonstrated a role for phytochelatins (PCs) in the Cd-induced Cu deficiency response, because it was reduced in roots of cad1-3 mutant plants exposed to Cd. In conclusion, a working mechanism is provided in which it is suggested that Cd increases PC levels that can complex both Cd and Cu. This results in cellular Cu deficiency and subsequently the activation of SPL7 and hence the induction of the Cu deficiency response.

Toxicity responses of Cu and Cd: the involvement of miRNAs and the transcription factor SPL7

BACKGROUND

MicroRNAs are important posttranscriptional regulators of gene expression playing a role in developmental processes as well as in stress responses, including metal stress responses. Despite the identification of several metal-responsive miRNAs, the regulation and the role of these miRNAs and their targets remain to be explored. In this study, miRNAs involved in the response to Cd and Cu excess in Arabidopsis thaliana are identified. In addition, the involvement of the transcription factor SPL7, namely the key regulator of Cu homeostasis, in these metal stress responses is demonstrated by the use of an spl7 knockout mutant. Furthermore, more insight is given in the Cd-induced Cu deficiency response through determining the effects of adding supplemental Cu to Cd-exposed plants.

RESULTS

Thirteen miRNAs were identified in response to Cu and Cd excess in A. thaliana. Several of these miRNAs (miR397a, miR398b/c and miR857) were oppositely affected under Cu and Cd exposure. The induced expression of these miRNAs after Cd exposure was totally abolished in the spl7 mutant (SQUAMOSA promoter binding protein like7), indicating a major role for SPL7 in the Cd response. Plants exposed to Cd showed a higher Cu content in the roots, whereas the Cu content in the leaves of the spl7 mutant was reduced. Furthermore, the Cd-induced Cu deficiency response disappeared when supplemental Cu was added.

CONCLUSIONS

Copper- and Cd-responsive miRNAs were identified and several of them are SPL7-dependently regulated. SPL7 seems to be a shared component between both the Cu toxicity and the Cd toxicity response, yet oppositely regulated, that is inactivated after Cu exposure and activated after Cd exposure. Since SPL7 is the key regulator of Cu homeostasis, and Cd affects the Cu homeostasis, we hypothesize that SPL7 is activated in response to Cd possibly due to a Cd-induced Cu deficiency. Since adding additional Cu to Cd-exposed plants resulted in the disappearance of the Cu deficiency response, Cd possibly provokes Cu deficiency, thereby activating SPL7 and inducing subsequently the Cu deficiency response.

Regulation of Cu delivery to chloroplast proteins

Plastocyanin is a copper (Cu)-requiring protein that functions in photosynthetic electron transport in the thylakoid lumen of plants. To allow plastocyanin maturation, Cu must first be transported into the chloroplast stroma by means of the PAA1/HMA6 transporter and then into the thylakoid lumen by the PAA2/HMA8 transporter. Recent evidence indicated that the chloroplast regulates Cu transport into the thylakoids via Clp protease-mediated turnover of PAA2/HMA8. Here we present further genetic evidence that this regulatory mechanism for the adjustment of intra-cellular Cu distribution depends on stromal Cu levels. A key transcription factor mediating Cu homeostasis in plants is SQUAMOSA promoter binding protein-like7 (SPL7). SPL7 transcriptionally regulates Cu homeostasis when the nutrient becomes limiting by up-regulating expression of Cu importers at the cell membrane, and down-regulating expression of seemingly non-essential cuproproteins. It was proposed that this latter mechanism favors Cu delivery to the chloroplast. We propose a 2-tiered system which functions to control plant leaf Cu homeostasis: SPL7 dependent transcriptional regulation of cuproproteins, and PAA2/HMA8 turnover by the Clp system, which is independent on SPL7.