Insight into plant annexin function: from shoot to root signaling

Abstract

The multifunctionality of plant annexins and their importance for coordinating development and responses to biotic and abiotic environment have been largely reviewed. We recently described a tobacco annexin, named Ntann12, which is mainly localized in the nucleus of root cells when the plant is grown under light conditions. We also found that auxin and polar auxin transport are essential for Ntann12 accumulation in root cells. Under dark condition, Ntann12 is no longer detected in the root system. In the present addendum, light, regulating auxin signaling, is evidenced as an essential determinant for the synchronization of growth and development between the shoot and the root during light/dark cycle. A speculative model for Ntann12 is described and discussed with regards to relevant literature data.

Figure 1. Ntann12 immunolocalization in a tobacco longitudinal root section visualized by fluorescence microscopy. 2D maximum projection of a 32-confocal image stack, corresponding to a root volume of 238.1 µm × 238.1 µm × 57.88 µm. Analysis was done on a Leica SP2 confocal microscope with a 63X objective (n.a. = 1.4); bar = 30 µm. (A) Phase contrast; (B) immunostaining with affinity-purified antiserum against Ntann12; (C) superposition of (A and B) (for Materials and Methods, see ref. 6).

Figure 2. Ntann12 function in tobacco root: a speculative model. (1) Light induces auxin synthesis in young tissues of the aerial part. (2) Auxin moves from shoot to root system via the PAT. (3) Auxin accumulation in root cells induces Ntann12 expression. (4) The increase of the level of cytosolic Ca²⁺ induced by auxin accumulation triggers recruitment of Ntann12 to negatively charged phospholipids. (5) Ntann12 interacts with other proteins (X1–X3) possibly via a phosphorylation-dependent regulation. (6) The protein complex reaches the cell nucleus. (7) The nuclear Ntann12 protein-complex may acts as a transcription factor to induce the expression of target genes that are required for the functioning of root system in daylight condition.