Perturbation of auxin homeostasis and signaling by overexpression of PINOID in Arabidopsis thaliana : A leaf growth and stress response study
7 September 2016
UAntwerpen, Campus Groenenborger, U0.24 - Groenenborgerlaan 171 - 2020 Antwerpen (route: UAntwerpen, Campus Groenenborger
PhD defence Kumud Saini - Faculty of Science, Department of Biology
The plant hormone auxin is unequivocally proven to be essential for embryonic and postembryonic growth in plants, including the model plant Arabidopsis thaliana. Any alteration in auxin homeostasis leads to a plethora of growth and developmental defects. To expand our understanding of auxin-regulated leaf growth we used several mutants of PINOID (PID) gene. PID, a serine-threonine kinase belonging to AGC kinase family, is known to affect the subcellular distribution of PIN-FORMED auxin efflux carriers and thus helps to regulate the directionality of the auxin flux. Leaves of single knock out pid mutants showed small pleiotropic leaf growth defects and occasionally three cotyledons were initiated together with pin-like inflorescences. Contrary to pid mutants, PID overexpression lines displayed severely reduced leaf growth and less number of leaves in a single rosette.
Hormone analysis in the PID overexpression lines showed that the phenotype was due to perturbations of auxin homeostasis. Visualization of the DR5 auxin-sensor suggested changes in auxin distribution and signaling in the leaves overexpressing PID. Kinematic analysis of leaf epidermal cells showed that PID above certain threshold inhibits cell division and expansion processes and thus affects the final leaf size. Transcriptome data further supported the inhibitory effects of PID overexpression on growth at organ and cellular level.
The severity of the phenotype and inducibility of the PID gene in different stress conditions (based on public microarray data from Genevestigator studies) led us to the hypothesis that PID might be involved in stress responses. Our transcriptome data strengthened this finding by showing upregulation of many stress-responsive markers genes including hormone-specific genes. We tested this hypothesis by analyzing reactive oxygen species (ROS) levels and other metabolic stress markers, such as malondialdehyde (MDA), ROS generating and scavenging enzymes. Our findings suggest that ectopic expression of PID induces stress responses in the leaves and modulates whole plant responses towards environmental conditions such as drought and osmotic stress. This involves a complex but intriguing interaction among different phytohormones, ROS and downstream signaling components.
This study provides the first evidence of the cellular basis of a leaf phenotype due to PID overexpression and of the finding that PID above a certain threshold might adversely affect leaf growth by inhibiting cell division, endoreduplication and expansion processes through alterations in auxin homeostasis. To our knowledge we are also first to suggest a role of PID in stress responses.