Inhibition of Polyamine Homeostasis Facilitates Root Extension by Modulating IAA and PIN1 Distribution in Etiolated Salt-Stressed Sunflower Seedlings

Plant root growth and development undergo significant alterations as an adaptation to environmental stressful conditions. Remodeling of roots exposed to salinity is coordinated by complex interactions among various signaling pathways involving phytohormones, nitric oxide (NO), and reactive oxygen species (ROS). Polyamines (PAs), small, cationic amine molecules with diverse roles in plant growth and stress responses, are also known to influence root morphology. Studies reported that treatment of sunflower seedlings with PA biosynthesis inhibitors [DFMA (DL-α-difluoromethylarginine) or DFMO (DL-α-difluoromethylornithine)], promotes extension growth of primary roots both in the absence or presence of NaCl. This work explores the possible role of PAs and their crosstalk with auxin signaling in the modeling of the root morphology in etiolated sunflower seedlings. We observed that inhibition of root growth by salinity is possibly governed by a disruption of the polar localization of PIN1 (auxin efflux protein) leading to IAA deficiency in the root apex. Application of PA inhibitors (DFMA or DFMO), in contrast, brings about an enhancement in IAA accumulation in the root apices of seedlings relative to control (−NaCl), albeit to a lesser degree in seedlings also exposed simultaneously to 120 mM NaCl. These alterations in IAA accumulation coincide with changes in primary root extension previously reported in sunflower seedlings in response to treatment with PA inhibitors, both in the absence or presence of NaCl. We found that the enhancement in root extension observed in seedlings subjected to a combined treatment of PA biosynthesis inhibitors and NaCl possibly involves the maintenance of polar distribution of PIN1 in the root cells which, in turn, may be responsible for the restoration of IAA distribution in the root apex to further support the observed extension growth of primary root. On the other hand, the role of nitric oxide and hydrogen peroxide in the observed PA inhibitor-triggered response on root extension remains uncertain at present. Therefore, a possible role of PAs and their crosstalk with auxin is evident in root architecture remodeling in sunflower seedlings exposed to salinity.