The constitutive expression of Arabidopsis plasmodesmal-associated class 1 reversibly glycosylated polypeptide impairs plant development and virus spread

Abstract

Arabidopsis class 1 reversibly glycosylated polypeptides (C1RGPs) were shown to be plasmodesmal-associated proteins. Transgenic tobacco (Nicotiana tabacum) plants constitutively expressing GFP tagged AtRGP2 under the control of the CaMV 35S promoter are stunted, have a rosette-like growth pattern, and in source leaves exhibit strong chlorosis, increased photoassimilate retention and starch accumulation that results in elevated leaf specific fresh and dry weights. Basal callose levels around plasmodesmata (Pd) of leaf epidermal cells in transgenic plants are higher than in WT. Such a phenotype is characteristic of virus-infected plants and some transgenic plants expressing Pd-associated viral movement proteins (MP). The local spread of Tobacco mosaic virus (TMV) is inhibited in AtRGP2:GFP transgenics compared to WT. Taken together these observations suggest that overexpression of the AtRGP2:GFP leads to a reduction in Pd permeability to photoassimilate, thus lowering the normal rate of translocation from source leaves to sink organs. Such a reduction may also inhibit the local cell-to-cell spread of viruses in transgenic plants. The observed reduction in Pd permeability could be due to a partial Pd occlusion caused either by the accumulation of AtRGP2:GFP fusion in Pd, and/or by constriction of Pd by the excessive callose accumulation.

Uncoupling High Light Responses from Singlet Oxygen Retrograde Signaling and Spatial-Temporal Systemic Acquired Acclimation

“…As a result, adjacent sieve elements, and therefore adjacent leaves, are more likely to receive a signal from the phloem. Arabidopsis is an apoplastic loader, in which Suc produced during photosynthesis in BSC and phloem parenchyma cells, for example, is first exported into the apoplast, actively loaded into the sieve element-companion cell complex, and finally transported via Pd from the companion cell to the sieve element (Lalonde et al, 2004;Zavaliev et al, 2010). Pd have been speculated to be important for cell-to-cell signaling models (Mullineaux and Baker, 2010;Karpinski et al, 2011), and signals such as siRNAs are known to move through these channels from cell-to-cell over a short range and through the phloem over longer distances (Kalantidis et al, 2008).…”

Organelle–nucleus cross-talk regulates plant intercellular communication via plasmodesmata

“…Genes regulating callose synthesis (e.g., CHORUS/GLUCAN SYNTHASE-LIKE 8) or callose degradation [e.g., ATBETA-1,3-GLUCANASE_PUTATIVE PLASMODESMATAL ASSOCIATED PROTEIN (ATBG_PPAP)] are well represented in our dataset. Unexpectedly, genes predicted to decrease PD size exclusion limits, including REVERSIBLY GLYCOSYLATED POLYPEPTIDE (RGP3) (31) and PDLP5, PDLP6, and PDLP8 (12), exhibit increased expression in ise1 and ise2 mutants (Table 1). Similarly, expression of ATBG_PPAP is expected to increase PD size exclusion limits through callose removal (6), but ATBG_PPAP expression is down-regulated in ise1 and ise2 mutants (Table 1).…”

Reduced levels of class 1 reversibly glycosylated polypeptide increase intercellular transport via plasmodesmata

“…Thus loss of NbRGPs does not grossly affect plant development, in contrast to overexpression of AtRGP2-GFP. 16 Viral movement proteins (MPs) are commonly used as probes for intercellular transport via PD. TMV MP, P30, localizes to PD and moves from cell to cell via PD.…”

“…Taken together, our findings are complementary to the data where over-production of RGPs reduces intercellular transport via PD. 16 RGPs were first isolated during attempts to purify enzymes with b-1,4-glucan synthase (GS1) activity from pea. 15 RGPs subsequently have been isolated from several other plant species including maize 11 Arabidopsis 12,22 wheat 23 and rice.…”

“…12 Overexpression of AtRGP2-GFP in stable tobacco transgenic plants causes stunting and chlorosis of source leaves due to increased starch accumulation and reduced chlorophyll content. 16 Interestingly, AtRGP2-transgenic plants display much higher levels of callose at PD than in their control counterparts and exhibit reduced cell-to-cell spread of Tobacco mosaic virus (TMV). The authors suggest that increased callose deposition at PD may lead to reduced PDdependent viral movement.…”