INCREASED SIZE EXCLUSION LIMIT2Encodes a Putative DEVH Box RNA Helicase Involved in Plasmodesmata Function duringArabidopsisEmbryogenesis

Abstract

Here, we characterize the Arabidopsis thaliana embryo-defective mutant increased size exclusion limit2 (ise2). In contrast with wild-type embryos, ise2 mutants continue to traffic 10-kD fluorescent dextran in the mid-torpedo stage of development. ise2 embryos contain branched as well as simple plasmodesmata (PD) compared with wild-type embryos, which only contain simple PD. Positional cloning reveals that the ISE2 gene encodes a putative DEVH box RNA helicase that shares sequence homology with RNA helicases involved in RNA degradation pathways in other organisms. ISE2 localizes to granule-like structures in the cytoplasm. These granules increase in number when plant cells are stressed. These features are characteristic of stress granules (SGs) in mammalian cells, suggesting that ISE2 granules represent plant-specific SGs. Genetic data demonstrate that the ISE2 helicase is involved in posttranscriptional gene silencing and the determination of cell fate. These data together suggest that ISE2 function affects PD structure and function through the regulation of RNA metabolism and consequent gene expression.

Plasmodesmata formation and cell-to-cell transport are reduced in decreased size exclusion limit 1 during embryogenesis in Arabidopsis

“…All five of these mutants produce severe phenotypes during embryo or seedling development; however, except GSL8 none encodes proteins that localize to PD. ISE1 encodes a mitochondrial DEAD-box RNA helicase (22), ISE2 encodes a chloroplast DEVH-type RNA helicase (23,30), GAT1 encodes a chloroplast thioredoxin (26), and DSE1 encodes a WDR protein that localizes to the nucleus and cytoplasm. Only, GSL8 encodes a putative glucan synthase, that may localize close to PD, as it is involved in callose deposition in cell walls at the cell plate (31).…”

“…Down-regulation of PD aperture in embryo development occurs at the midtorpedo stage (21). This transition in embryonic PD aperture provided the basis for a genetic screen that identified two mutants, increased size exclusion limit1 (ise1) and ise2, that fail to down-regulate PD permeability (22,23). These mutants also have increased proportions of branched (X-and Y-shaped) PD and twinned PD (15).…”

An Arabidopsis ATP-Dependent, DEAD-Box RNA Helicase Loses Activity upon IsoAsp Formation but Is Restored by PROTEIN ISOASPARTYL METHYLTRANSFERASE

“…Despite the fact that there are at least 58 DEAD-box helicases present in the Arabidopsis genome (Mingam et al, 2004), apparently none shared sufficient redundancy with PRH75 given the lethality of the prh75 mutation. There have been 800 emb genes identified to date in this model plant, of which seven were D/RNA helicases (At1G01040, At1G08840, At1G12770, At1G20960, At1G32490, At3G53110, and At5G26742) (Tzafrir et al, 2004), and an additional four D/RNA helicases, other than PRH75, have since been identified as belonging to the emb category (At1G70070 [Kobayashi et al, 2007], At2G35340 [Pagnussat et al, 2005], At5G08610 [PIGMENT DEFECTIVE 340…”

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

“…We have mapped and cloned two genes, INCREASED SIZE EXCLU-SION LIMIT (ISE)1 and ISE2, that, when mutated, result in increased intercellular transport of fluorescent 10-kDa dextrans at the midtorpedo stage of embryogenesis. In addition to increased PD trafficking, ise1 and ise2 mutants display increased proportions of twinned and branched PD compared with WT embryos (16)(17)(18); this increase is likely attributable to more de novo production of secondary PD. In support, reduction of ISE1 and/or ISE2 transcript levels in mature leaves by gene silencing causes specific increases in secondary PD concomitant with increased PD transport (18).…”

“…ISE2 encodes a predicted DEVHtype RNA helicase (16), and microscopic examination of transgenic plants constitutively expressing ISE2 tagged at its C terminus with GFP (ISE2-GFP) suggested that ISE2-GFP localized to granules that were often associated with chloroplasts (figure 8A of ref. 16) or, less frequently, with a marker for plant RNA processing bodies, mRNA decapping enzyme 2 (DCP2)-RFP (figure 9 of ref. 16).…”