Endogenous Serotonin Accumulation Coincides with Reorganization of Auxin Efflux Protein (PIN1) and Actin (ACT8) Accompanying Primary Root Growth Inhibition in NaCl-Stress-Induced Etiolated Sunflower ( Helianthus annuus ; cv. KBSH 44) Seedlings

Interaction between the serotonin and auxin signaling routes is crucial for the regulation of root system architecture in plants under normal and challenging environments. Current investigation deciphers the interrelations among serotonin accumulation, auxin transport, and actin distribution accompanying root growth inhibition in NaCl-stressed sunflower seedlings. Application of NaCl stress (120 mM) or 1-napthylphthalamic acid (NPA-5 µM; auxin transport inhibitor) leads to increased serotonin accumulation in the endodermis and pericycle cells in the differentiating zone of primary roots. Analysis by high-performance liquid chromatography shows significant increase in serotonin content in NaCl-stressed and NPA-treated seedling roots. Immunolocalization of auxin efflux proteins (PIN1) shows that NaCl stress results in disruption of the acropetal PIN 1 gradient in the vascular cells of the primary roots, thereby possibly altering auxin levels in the differentiating region of roots. Serotonin accumulation in the endodermal and pericycle cells of NaCl-treated primary roots also coincides with possible auxin deficiency caused by a reduction in acropetal auxin transport. Actin localization in the vascular cells of primary roots shows a disruption of ACT 8 (an isoform of vegetative actin) assembly in response to salt stress. The disruption of PIN 1 gradient in the vascular cells of primary roots, thus, accompanies the disorganization of ACT 8 isoforms, coinciding with the inhibition of primary root elongation and reduction in lateral root branching. Since reactive oxygen species (ROS) are key players of auxin–serotonin signaling, NaCl stress (present work) presumably causes changes in the ROS levels which might regulate auxin transport and serotonin accumulation in the vascular region of roots. Thus, a probable crosstalk between serotonin, auxin, and actin is evident to accompany NaCl-stress-induced restructuration of root system architecture in sunflower seedlings.