Tobacco plants respond to the constitutive expression of the tospovirus movement protein NS_M with a heat‐reversible sealing of plasmodesmata that impairs development

Abstract

SummaryViral infection often results in typical symptoms, the biological background of which has remained elusive. We show that constitutive expression of the NS M viral movement protein (MP) of tomato spotted wilt virus in Nicotiana tabacum is sufficient to induce severe, infection-like symptoms, including pronounced deficiencies in root and shoot development. Leaves failed to expand and were arranged in a rosette due to the absence of internode elongation. Following the sink-source transition they accumulated excessive amounts of starch and developed fusing chlorotic patches in the mesophyll, resembling virus-induced chlorotic lesions. Eventually, the leaves became entirely white and brittle. With a combination of techniques, including photosystem II quantum-yield measurements, iontophoresis of symplasmic tracers, bombardment with pPVX.GFP and double immunolabelling it was shown that these symptoms correlated with the obstruction of NS M -targeted mesophyll plasmodesmata (Pd) in source tissues by depositions of 1,3-b-D-glucan (GLU) or callose. Temperature-shift treatments (TST; 22 fi 32°C 1 ), known to abolish chlorotic local lesions, also abolished the chlorotic 'superlesions' of transgenic plants and rescued plant development, by restoring the transport capacity of Pd through the action of 1,3-b-D-glucanase (GLU-h) or callase. Return of these elongated, TSTrecovered plants to 22°C reintroduced superlesions and arrested shoot elongation, resulting in the formation of a rosette of clustered leaves at the shoot tip. Collectively, this indicates that the symptoms of NS M plants are self-inflicted and due to a basal defence response that counteracts prolonged interference of the MP with Pd functioning. This type of defence may also play a role in the formation of symptoms during viral infection.

CENL1Expression in the Rib Meristem Affects Stem Elongation and the Transition to Dormancy inPopulus

“…The differentiating cells of the RZ possessed more chlorophyll than the meristematic part of the apex ( Figure 7C), reflecting the fact that the meristematic part of the apex and the youngest leaves in general do not possess fully developed chloroplasts (Rinne et al, 2005). This made it possible to identify the approximate site of the RM, at the lower edge of the whitish zone in Figure 7C.…”

AnArabidopsisGPI-Anchor Plasmodesmal Neck Protein with Callose Binding Activity and Potential to Regulate Cell-to-Cell Trafficking

“…A key component in Pd flux control is callose deposited in the neighboring cell wall (Iglesias and Meins, 2000;Bucher et al, 2001;Rinne et al, 2005;Ueki and Citovsky, 2005). This is believed to exert a structural constriction around the neck of (A) Crude protein extracts from two independent 35Spro:PDCB1 overexpressing hemizygous transgenic Arabidopsis Col-0 lines (OE1 and OE2) were assessed for PDCB1 accumulation by immunoblot analysis using anti-PDCB1 antibody compared with wild type (wt) Columbia-0 (Col-0) (top).…”

“…Callose deposition at Pds is stimulated by physical and physiological stresses (Hughes and Gunning, 1980;Beffa and Meins, 1996;Iglesias and Meins, 2000;Sivaguru et al, 2000;Rinne et al, 2005;Ueki and Citovsky, 2005;Maeda et al, 2006) and is proposed to provide a mechanism for regulating Pd flux (Botha and Cross, 2000;Iglesias and Meins, 2000;Levy et al, 2007a). Hence, for example, plants with reduced accumulation of the glycolytic enzyme 1,3-b-glucanase had increased callose accumulation and a reduction in the experimental molecular size exclusion limit.…”

Chilling of Dormant Buds HyperinducesFLOWERING LOCUS Tand Recruits GA-Inducible 1,3-β-Glucanases to Reopen Signal Conduits and Release Dormancy inPopulus

“…Dormant buds were incubated for 5 d with and without GA 3 and GA 4 as described above (GA applications) and thereafter fixed overnight at 48C in 2% (v/v) glutaraldehyde and 3% (v/v) paraformaldehyde in 100 mM phosphate citrate buffer, pH 7.2, as described earlier (Rinne et al, 2005). The samples were infiltrated gradually into LR White Resin and kept 4 d in 100% before polymerization at 558C.…”

“…Net callose deposition is governed by the joint action of 1,3-b-glucansynthase (callose synthase) and 1,3-b-glucanase (glucan hydrolase family 17 [GH17]) (Rinne et al, 2005;Levy et al, 2007a;Simpson et al, 2009). Shifts in the balance of these enzymes are central to many developmental events and cellular defense responses (Rinne and van der Schoot, 2003).…”