Nuclear membranes control symbiotic calcium signaling of legumes

Abstract

Nuclear-associated oscillations in calcium act as a secondary messenger in the symbiotic signaling pathway of legumes. These are decoded by a nuclear-localized calcium and calmodulin-dependent protein kinase, the activation of which is sufficient to drive downstream responses. This implies that the calcium oscillations within the nucleus are the predominant signals for legume symbiosis. However, the mechanisms that allow targeted release of calcium in the nuclear region have not been defined. Here we show that symbiosis-induced calcium changes occur in both the nucleoplasm and the perinuclear cytoplasm and seem to originate from the nuclear membranes. Reaction diffusion simulations suggest that spike generation within the nucleoplasm is not possible through transmission of a calcium wave from the cytoplasm alone and that calcium is likely to be released across the inner nuclear membrane to allow nuclear calcium changes. In agreement with this, we found that the cation channel DMI1, which is essential for symbiotic calcium oscillations, is preferentially located on the inner nuclear membrane, implying an essential function for the inner nuclear membrane in symbiotic calcium signaling. Furthermore, a sarco/endoplasmic reticulum calcium ATPase (SERCA) essential for symbiotic calcium oscillations is targeted to the inner nuclear membrane, as well as the outer nuclear membrane and endoplasmic reticulum (ER). We propose that release of calcium across the inner nuclear membrane allows targeted release of the ER calcium store, and efficient reloading of this calcium store necessitates the capture of calcium from the nucleoplasm and nuclear-associated cytoplasm.L egumes form mutualistic symbiotic interactions with mycorrhizal fungi and with rhizobial bacteria that aid in the uptake of nutrients (1, 2). Establishment of both symbioses requires the common symbiosis (Sym) signaling pathway (1, 2) that involves calcium oscillations after perception of diffusible signals from the symbionts (3, 4): Nod factors from rhizobia and Myc factors from mycorrhizal fungi (4-7). The calcium oscillations are concentrated in the perinuclear region (3), and a nuclear-targeted calcium reporter showed that part of these oscillations occurs in the nucleoplasm (8). The decoder of the calcium oscillations, a calcium and calmodulin-dependent protein kinase (CCaMK), is localized to the nucleoplasm (9, 10), implying that intranuclear calcium changes are paramount. Furthermore, some of the components of the Sym pathway required for the induction of calcium oscillations are localized to the nuclear envelope: two cation channels and three components of the nuclear pore (11-14). All of this points to the nuclear membrane as playing a central role in symbiotic calcium oscillations.The question of whether calcium changes can derive from the nucleus has been a contentious point for many years (15, 16). In animals, it is widely accepted that calcium events in and around the nucleus have significant effects on signaling pathways in the nucleus (15). Depend...

Buffering Capacity Explains Signal Variation in Symbiotic Calcium Oscillations

“…In animal systems, some Ca 2+ -gated K + channels are voltage gated, but the lack of voltage sensor motifs in the transmembrane domain of DMI1 suggests that changes in voltage would have no effect on the conductance of DMI1. The Ca 2+ pump is modeled to represent MCA8, a SERCA-type Ca 2+ ATPase (Capoen et al, 2011), and it resequesters the released Ca 2+ into the Ca 2+ store against the concentration gradient. In this model, DMI1 is presumed to be able to change the membrane voltage to provide a counterion current to the Ca 2+ movement.…”

“…The pumps are needed to resequester Ca 2+ after each release event, actively transporting Ca 2+ against the concentration gradient using ATP. A recent study found a SERCA-type Ca 2+ ATPase, MCA8, that is located on the inner and outer nuclear envelope of M. truncatula and is required for the symbiotic Ca 2+ oscillations (Capoen et al, 2011). Such SERCA pumps are widely distributed on plant membranes, and the variation in their structure points to them being differentially regulated (Sze et al, 2000).…”

“…In particular, both the symbioses show characteristic perinuclear and nucleoplasmic localized calcium (Ca 2+ ) oscillations, so-called Ca 2+ spiking Sieberer et al, 2009). It has been suggested that Ca 2+ is released from an internal store, most likely the nuclear lumen and associated endoplasmic reticulum (ER; Matzke et al, 2009), with targeted release in the nuclear region (Capoen et al, 2011).…”

“…In Capoen et al (2011), we investigated the establishment and transmission of spatial waves across the nuclear envelope and demonstrated that the key components for Ca 2+ spiking reside on the inner and outer surface of the nuclear membrane. The computational framework we employed for this analysis makes a number of approximations in order to provide the computational efficiency required to perform spatiotemporal simulations.…”

“…DMI2 codes for a plasma membrane receptor-like kinase (Endre et al, 2002;Stracke et al, 2002) that is required to facilitate further signal transduction in the cell (Bersoult et al, 2005). DMI1 is a cation channel located preferentially on the inner nuclear envelope Edwards et al, 2007;Riely et al, 2007;Charpentier et al, 2008;Capoen et al, 2011;Venkateshwaran et al, 2012). DMI3 encodes a calcium calmodulin-dependent protein kinase that interacts with downstream components and is thought to be the decoder of the Ca 2+ oscillations Mitra et al, 2004;Hayashi et al, 2010).…”