*MS Channels in the Plastid Envelope

Our studies on the plastid-localized Arabidopsis MscS homologs MSL2 and MSL3 have revealed how organelles use these proteins to respond to membrane stretch, how their activity is integrated into cellular signaling networks, and how plants adjust to their absence. These results suggest that plant MscS homologs are not merely safety valves but regulated channels with multiple roles at the organellar, cellular, and organismal level.

Plastid osmotic stress linked to callus production at the shoot apical meristem.

Older msl2 msl3 mutants show a striking and rare phenotype, the production of callus--or a undifferentiated mass of cells--at the shoot apex. We found greatly enlarged proplastids in the shoot apical meristem of msl2 msl3 mutants, and provide evidence that these osmotically stressed plastids trigger the production of apical callus through two non-redundant pathways, a signaling loop that regulates growth/differentiation decisions in the shoot apical meristem, and a plastid-to-nucleus retrograde signaling pathway.

  • M. E. Wilson, Matt Mixdorf, R. H. Berg and E. S. Haswell. (2016). Plastid Osmotic Shock Influences Dedifferentiation at the Plant Shoot Apex. Development, in press.

RNA-seq analysis of msl2 msl3 and other variegated mutants.

In an attempt to understand the molecular basis of the leaf variegation and other developmental defects that morphology defects observed in a number of plastid dysfunction mutants, we performed RNA-sequencing on msl2 msl3, ggps1-1, and crl mutants. We were unable to find any commonalities between the genes differentially expressed in these mutants and in other mutant transcriptomes, and conclude that either there is no common plastid-to-nucleus signaling pathway in these mutants, or it is subtle and cannot be detected through transcriptomic methods.

  • D. R. Luesse, M. E. Wilson and E. S. Haswell. (2015). RNA-Sequencing Analysis of the msl2msl3, crl, and ggps1 Mutants Indicates that Diverse Sources of Plastid Dysfunction do not Alter Leaf Morphology Through a Common Signaling Pathway. Frontiers in Plant Science 6:1148.
  • Raw data from the analyses presented here have been deposited into the NCBI SRA under the accession SRP065605.

Plastid osmotic stress activates cellular stress responses.

We used the msl2 msl3 double mutant as a tool to study the cellular response to organellar osmotic stress. We found that organellar osmotic stress produced  several hallmarks of drought or environmental osmotic stress, including hyperaccumulation of osmolytes and the drought stress hormone ABA. We propose that plastids can serve as intracellular osmosensors, initiating a cellular response to osmotic imbalance across the plastid envelope.

  • M. E. Wilson, M. R. Basu, G. B. Bhaskara, P. E. Verslues and E. S Haswell. (2014). Plastid Osmotic Stress Activates Cellular Stress Responses in Arabidopsis thaliana. Plant Physiology 165:119-128.

MSL2 and MSL3 protect leaf epidermal plastids from hypoosmotic stress during normal plant growth.

We used genetic, physiological, and media manipulations to show that under normal growth conditions MscS homologs located in the plastid envelope serve as membrane tension-sensitive osmotic conduits, allowing the plastid to constantly adjust to moderate changes in stromal and/or cytoplasmic osmolyte concentrations that occur during normal plant growth.

  • K. M. Veley, S. Marshburn, C. Clure and E. S. Haswell. (2012). Mechanosensitive Channels Protect Plastids from Hypoosmotic Shock During Normal Plant Growth. Current Biology 22:408-413.
  • K. M. Veley and E. S. Haswell. (2012). Plastids and Pathogens: Mechanosensitive Channels and Survival in a Hypoosmotic World. Plant Signaling and Behavior 7: 668-671.

MscS-like channels influence division site selection in bacteria and chloroplasts.

We identified two new components of the division machinery (MscS-Like 2 and 3) that provide a mechanistic link between tension in the chloroplast envelope and the progression of chloroplast division. We also show that this link is evolutionarily conserved, as the well-characterized mechanosensitive channels of E. coli—MscS, MscL and MscK—also contribute to FtsZ ring placement during bacterial cell division.

  • M. E. Wilson, G. S. Jensen, and E. S. Haswell. (2011). Two Mechanosensitive Channel Homologs Influence FtsZ Ring Placement in Arabidopsis. The Plant Cell 23: 2939-2949. Featured on the cover of the August issue.
  • M. E. Wilson and E. S. Haswell. (2012). A Role for Mechanosensitive Channels in Chloroplast and Bacterial Fission. Plant Signaling and Behavior 7:1-4.