The Integrity of the Plant Golgi Apparatus Depends on Cell Growth-Controlled Activity of GNL1

ABSTRACTMembrane traffic and organelle integrity in the plant secretory pathway depend on ARF-GTPases, which are activated by guanine-nucleotide exchange factors (ARF-GEFs). While maintenance of conserved roles, evolution of unique functions as well as tissue-specific roles have been shown for a handful of plant ARF-GEFs, a fundamental yet unanswered question concerns the extent to which their function overlaps during cell growth. To address this, we have characterized pao, a novel allele of GNOM-like 1 (GNL1), a brefeldin A (BFA)-insensitive ARF-GEF, isolated through a confocal microscopy-based forward genetics screen of the Golgi in Arabidopsis thaliana. Specifically, we have analyzed the dependence of the integrity of trafficking routes and secretory organelles on GNL1 availability during expansion stages of cotyledon epidermal cells, an exquisite model system for vegetative cell growth analyses in intact tissues. We show that Golgi traffic is influenced largely by GNL1 availability at early stages of cotyledon cell expansion but by BFA-sensitive GEFs when cell growth terminates. These data reveal an unanticipated level of complexity in the biology of GNL1 by showing that its cellular roles are correlated with cell growth. These results also indicate that the cell growth stage is an important element weighting into functional analyses of the cellular roles of ARF-GEFs.SUMMARYMembrane traffic depends on ARF-GTPases whose activity is controlled by guanine-nucleotide exchange factors (ARF-GEFs). How the activity of ARFs and ARF-GEFs is correlated during cell growth is largely unknown. This work demonstrates that Golgi traffic is influenced largely by the known ARF-GEF GNL1 at early stages of cotyledon cell expansion but by brefeldin A-sensitive GEFs when cell growth terminates. These data reveal an unanticipated level of complexity in the biology of GNL1 by showing that its cellular roles are correlated with cell growth.