Endomembrane Ca²⁺-ATPases play a significant role in virus-induced adaptation to oxidative stress

Abstract

Although the role of Ca²⁺ influx channels in oxidative stress signaling and cross-tolerance in plants is well established, little is known about the role of active Ca²⁺ efflux systems in this process. In our recent paper,¹⁷ we reported Potato Virus X (PVX)-induced acquired resistance to oxidative stress in Nicotiana benthamiana and showed the critical role of plasma membrane Ca²⁺/H⁺ exchangers in this process. The current study continues this research. Using biochemical and electrophysiological approaches, we reveal that both endomembrane P_2A and P_2B Ca²⁺-ATPases play significant roles in adaptive responses to oxidative stress by removing excessive Ca²⁺ from the cytosol, and that their functional expression is significantly altered in PVX-inoculated plants. These findings highlight the crucial role of Ca²⁺ efflux systems in acquired tolerance to oxidative stress and open up prospects for practical applications in agriculture, after in-depth comprehension of the fundamental mechanisms involved in common responses to environmental factors at the genomic, cellular and organismal levels.

The phenomenon of cross-tolerance to a variety of biotic and abiotic stresses is well-known.^1,2 Some of the demonstrated examples include the correlation between oxidative stress tolerance and pathogen resistance.^3–5 At the mechanistic level, changes in cytosolic Ca²⁺ levels [Ca²⁺]_cyt, have long been implicated as a quintessential component of this process.⁶ The rise in [Ca²⁺]_cyt is proven to be essential for the development of the oxidative burst required for triggering the activation of several plant defense reactions.^7,8 The observed elevation in H₂O₂ level is believed to result from Ca²⁺-dependent activation of the NADPH oxidase,⁸ which then causes a further increase in [Ca²⁺]_cyt via a positive feedback mechanism. This process is further accomplished by defense gene activation, phytoalexin synthesis and eventual cell death.⁹ Downstream from the stimulus-induced [Ca²⁺]_cyt elevation, cells possess an array of proteins that can respond to a message. Such proteins include calmodulin (CaM),¹⁰ Ca²⁺-dependent protein kinases¹¹ and CaM binding proteins.¹² Of note is that when Ca²⁺ channels are blocked, biosynthesis of ROS is prevented.¹³

While the role of Ca²⁺ influx channels in oxidative stress signaling and cross-tolerance in plants is well established, little is known about the involvement of active Ca²⁺ efflux systems in this process. In contrast, in animal systems the essential role of re-establishing [Ca²⁺]_cyt to resting levels is widely reported. A sustained increase in [Ca²⁺]_cyt in the alveolar macrophage is thought to be the consequence of membrane Ca²⁺-ATPase dysfunction.¹⁴ In endothelial cells, inhibition of the Ca²⁺/Na⁺ electroneutral exchanger of the mitochondria was named as one of the reasons for [Ca²⁺]_cyt increases.¹⁵ A significant loss of the plasma membrane Ca²⁺-ATPase (PMCA) activity was reported in brain synapses in response to oxidative stress,¹⁶ suggesting that PMCA may be a downstream target of oxidative stress.

In our recently published paper¹⁷ we reported the phenomenon of Potato Virus X (PVX)-induced acquired resistance to oxidative stress in Nicotiana benthamiana plants and showed the critical role of plasma membrane Ca²⁺/H⁺ exchangers in this process. Nonetheless, questions remain, is this transporter the only active Ca²⁺ efflux system involved in this process?

In addition to Ca²⁺/H⁺ exchangers, active Ca²⁺ extrusion could also be achieved by Ca²⁺-ATPases. Two major types of Ca²⁺-ATPases that differ substantially in their pharmacology and sensitivity to CaM are known.¹⁸ Type P_2A pumps (also called ER-type or ECA^19,20) are predominantly ER-localized,¹⁹ although they are also present at other endomembranes (e.g., tonoplast and Golgi). Four members of this group have been identified in the Arabidopsis genome (named AtECAs 1 to 4).^18,21 These pumps lack an N-terminal autoregulatory domain, are insensitive to CaM and suppressed by cyclopropiazonic acid (CPA).¹⁹ P_2B (or ACA) pumps contain an autoinhibitory N-terminal domain that possesses a binding site for Ca²⁺-CaM.¹⁸ Ten members are known in Arabidopsis (termed AtACA1, 2, 4 and 7 to 13).²¹ Plant P_2B pumps are located at the plasma membrane²⁰ as well as in inner membranes such as tonoplast (e.g., ACA4), ER (e.g., ACA2) and plastids.^18,19 These pumps probably constitute the basis for precise cytosolic Ca²⁺ regulation; as the Ca²⁺ concentration increases, CaM is activated and binds to the autoinhibitory domain of the Ca²⁺ pump. This results in the activation of the pump.

In our recent study,¹⁷ we found no significant difference between the purified plasma membranes fractions isolated from control and UV-treated tobacco plants (with or without PVX inoculation) either in the Ca²⁺-ATPase activity or in the Ca²⁺-ATPase expression level and its ability to bind CaM. This suggests that the plasma membrane P_2B type pumps (the only pump type known to be expressed at the plasma membrane) play no major role in removing excess Ca²⁺ from the cytosol under oxidative stress conditions. This led to an obvious question: what about endomembrane Ca²⁺-ATPases?

To address this issue, microsomal membrane fractions were isolated from tobacco leaves in a manner previously described for plasma membrane fractions¹⁷ (Fig. 1A). Western blot and CaM overlay assays were then made to investigate the role of endomembrane P_2B Ca²⁺-ATPases in our reported phenomena of acquired resistance. The results show that the expression of the P_2B Ca²⁺ pumps in PVX-inoculated plants is significantly higher than in control plants (Fig. 1B), correlating well with the CaM overlay assay (Fig. 1C). As no difference was observed for the P_2B Ca²⁺-ATPase expression levels in the plasma membranes,¹⁷ the observed difference in the microsomal fractions of PVX-infected plants must be due to an increased expression of endomembrane P_2B Ca²⁺-ATPases. Given the fact that Ca²⁺ pumps have a high affinity for calcium, the observed increase in endomembrane P_2B-type Ca²⁺-ATPases expression in PVX-inoculated plants may be advantageous for more efficient Ca²⁺ removal from the cytosol into internal organelles.

Figure 1.

Expression of P_2B Ca²⁺ in purified microsomal fractions from tobacco leaves. Measurements were undertaken C = mock controls; C-UV = mock controls treated with UV-light; PVX = PVX infected plants; PVX-UV = PVX inoculated plants treated with UV-light. (A) Coomassie Brilliant Blue-stained gel; (B) Protein blot immunostained with a non isoform-specific polyclonal antibody for P_2B Ca²⁺-ATPases; (C) CaM overlay assay.

To decipher the possible role of P_2A Ca²⁺-ATPases in acquired resistance, a series of electrophysiological experiments were conducted using inhibitors of P_2A-type Ca²⁺-ATPases, such as thapsigargin (TG)²² and cyclopiazonic acid (CPA).²³ Ion-selective Ca²⁺ microelectrodes were prepared as described elsewhere in reference ²⁴ and ²⁵, and net Ca²⁺ fluxes were measured from tobacco mesophyll tissue following previously described protocols.¹⁷ Leaf pre-treatment for 2 h in either of these inhibitors dramatically suppressed the net Ca²⁺ efflux measured from tobacco mesophyll cells 2 h after UV light exposure (Fig. 2). Given the specificity of TG and CPA inhibitors for P_2A-type Ca²⁺-ATPases, these results strongly support a hypothesis that both endomembrane P_2A and P_2B Ca²⁺-ATPases play significant roles in plant adaptive responses to oxidative stress. This is achieved by removing excess Ca²⁺ from the cytosol.

Figure 2.

Effect of known Ca²⁺-ATPase blockers on light-induced Ca²⁺ flux kinetics after 20 min of UV-C treatment. Leaf mesophyll segments were pre-treated in either 5 µM TG (thapsigargin) or 50 µM CPA (cyclopiazonic acid) for 1–1.5 h prior to exposure to UV-C light. Net Ca²⁺ fluxes were measured 2 h after the end of UV treatment. These were compared with two controls: (1) no pre-treatment/no UV exposure (closed circles) and (2) no pre-treatment/20 min UV exposure (open squares). Mean ± SE (n = 4 to 7).

Combining these results with our previously reported observations in reference ¹⁷, the following model is proposed (Fig. 3). Oxidative stress (such as UV) causes increased ROS production in leaf chloroplasts, leading to the elevated [Ca²⁺]_cyt. Several Ca²⁺ efflux systems are involved in restoring basal cytosolic Ca²⁺ levels. Two of these, the plasma membrane Ca²⁺/H⁺ exchanger¹⁷ and endomembrane P_2A and P_2B Ca²⁺-ATPases (as reported in this study) are upregulated in PVX inoculated plants and contribute to the improved tolerance to oxidative stress. Overall, these findings highlight the potential role of Ca²⁺ efflux systems in virus-induced tolerance to oxidative stress in plants. This is consistent with our previous reports on the important role of Ca²⁺ efflux systems in biotic stress tolerance²⁶ and brings forth possibilities for genetic engineering of more tolerant plants by targeting expression and regulation of active Ca²⁺ efflux systems at either the plasma or endomembranes.

Figure 3.

The proposed model of oxidative stress signaling and the role of Ca²⁺-efflux systems in acquired resistance and plant adaptation to oxidative stress.

Overall, a better adaptation of virus-infected plants to a short wave UV irradiation as compared to uninfected controls may suggest that infection triggers common defense mechanisms that could be efficient against secondary unrelated stresses. This observation may lead to the development of novel strategies to protect plants against complex environmental stress conditions.

Addendum to: Shabala S, Baekgaard L, Shabala L, Fuglsang A, Babourina O, Palmgren MG, et al. Plasma membrane Ca2+ transporters mediate virus-induced acquired resistance to oxidative stress. Plant Cell Environ. 2011;34:406–417. doi: 10.1111/j.1365-3040.