A phosphatidic acid-based retrograde signaling pathway responsive to abiotic stress in Arabidopsis

Due to their sessile nature, plants are constantly exposed to the environment and must cope with sometimes extreme changes in conditions during the day or growing season. Plants depend on photosynthesis as their primary means to generate energy and building blocks, and adverse environmental conditions can stress the photosynthetic apparatus leading to the production of toxic byproducts. In the long term, stress experienced by the chloroplast must be communicated to the nucleus to adjust the expression of genes providing robust abiotic stress resilience in a process called retrograde signaling. Here, we propose a retrograde signaling mechanism that starts with the accumulation of phosphatidic acid at the outer chloroplast membrane. A mutant of Arabidopsis thaliana, lppγ lppɛ1, disrupted in two chloroplast envelope membrane-located phosphatidic acid phosphatases, shows reduced growth and activation of abscisic acid-mediated abiotic stress-response pathways, among other changes, as determined by RNA-Seq analysis. The mutant is more resistant to freezing and osmotic stress. To identify components of the proposed retrograde signaling pathway, we conducted a suppressor screen in the lppγ lppɛ1 mutant and identified a mutation that causes the loss of MED16, which is a component of Mediator, a transcriptional complex in the nucleus affecting the expression of genes involved in abiotic stress tolerance, among others. Based on these findings, we are proposing a lipid-based, retrograde signaling mechanism in response to abiotic stresses such as freezing.