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Analysis of increased size exclusion limit 1 Reveals Organelle Redox State Regulates Intercellular Transport Via Plasmodesmata in Plants


Plasmodesmata are cytoplasmic channels crossing plant cell walls allowing cell-to-cell transport of macromolecules. Plasmodesmata (PD) have critical functions in plant growth and development, allowing the transport of nutrients, signaling molecules, and developmentally important macromolecules such as transcription factors and RNA. The ability of PD to transport large molecules is tightly regulated throughout plant growth and development and in response to changing conditions. Despite extensive research, the regulation of PD transport remains poorly understood.

The Arabidopsis thaliana embryo lethal mutant increased size exclusion limit 1 (ise1) was characterized. ise1 embryos exhibit increased PD mediated transport of fluorescent tracers and have a higher frequency of branched and twinned PD than wild type embryos. ISE1 encodes a plant-specific DEAD-box RNA helicase that localizes specifically to mitochondria. Mitochondrial metabolism is severely compromised in ise1 mutant embryos as their mitochondrial proton gradient is disrupted and reactive oxygen species (ROS) production is increased. Deep sequencing of mitochondrial RNA from ise1 suspension cells reveals that mitochondrial intron RNA is more abundant in ise1. Northern analysis confirms that ISE1 functions in the processing of mitochondrial intron RNA.

Recent studies suggest that intercellular transport via PD is regulated by cellular redox state. However increased production of ROS has been associated with both increased and decreased intercellular transport via PD. Here we show that PD transport is positively regulated by ROS production in mitochondria following treatment with salicyhydroxamic acid and negatively regulated by ROS production in chloroplasts following treatment with paraquat. Additionally, silencing of two genes, ISE1 and ISE2, that both increase transport via PD, increases ROS production in mitochondria and decreases ROS production by chloroplasts respectively. The data together support a consistent model where PD transport is positively regulated by oxidation of the mitochondrial redox state and negatively regulated by oxidation of the chloroplast redox state.

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