Evidence for reduced plasmodesmata callose accumulation in Nicotiana benthamiana leaves with increased symplastic cell-to-cell communication caused by RNA processing defects of chloroplasts

RNA processing defects in chloroplasts were previously associated with increased plasmodesmata (PD) permeability. However, the underlying mechanisms for such association are still unknown. To provide insight into this, we silenced the expression of chloroplast-located INCREASED SIZE EXCLUSION LIMIT 2 (ISE2) RNA helicase in Nicotiana benthamiana leaves and determined an increase in PD permeability which is caused by a reduction of PD callose deposition. Moreover, the silencing of two other nuclear genes encoding chloroplastic enzymes involved in RNA processing, RH3, and CLPR2, also increased PD permeability accompanied by reduced callose accumulation at PD. In addition, we quantified the plastidic hydrogen peroxide levels using the chloroplast-targeted fluorescent sensor, HyPer, in ISE2, RH3, and CLPR2 silenced N. benthamiana leaves. The levels of chloroplastic hydrogen peroxide were not correlated with the increased cell-to-cell movement of the marker protein GFP2X. We, therefore, propose that defects in chloroplast RNA metabolism mediate PD gating by suppressing PD callose deposition, and hydrogen peroxide levels in the organelles are not directly linked to this process.

Interplay between potato virus X and RNA granules in Nicotiana benthamiana

Cytoplasmic RNA granules consist of microscopic agglomerates of mRNAs and proteins and occur when the translation is reversibly and temporally halted (stress granules, SGs) or mRNAs are targeted for decapping (processing bodies, PBs). The induction of RNA granules formation by virus infection is a common feature of mammalian cells. However, plant-virus systems still remain poorly characterized. In this work, the SG marker AtUBP1b was expressed in Nicotiana benthamiana plants to decipher how the virus infection of plant cells affects SG dynamics. We found that the hypoxia-induced SG assembly was substantially inhibited in Potato virus X (PVX)-infected cells. Furthermore, we determined that the expression of PVX movement protein TGBp1 by itself, mimics the inhibitory effect of PVX on SG formation under hypoxia. Importantly, overexpression of AtUBP1b showed inhibition of the PVX spreading, whereas the overexpression of the dominant negative AtUBP1brrm enhanced PVX spreding, indicating that AtUBP1b negatively affects PVX infection. Notably, PVX infection did not inhibit the formation of processing bodies (PBs), indicating PVX has distinct effects depending on the type of RNA granule. Our results suggest that SG inhibition could be part of the virus strategy to infect the plant.