Arabidopsis response to low-phosphate conditions includes active changes in actin filaments and PIN2 polarization and is dependent on strigolactone signalling.

Strigolactones (SLs) are plant hormones that regulate the plant response to phosphate (Pi) growth conditions. At least part of SL-signalling execution in roots involves MAX2-dependent effects on PIN2 polar localization in the plasma membrane (PM) and actin bundling and dynamics. We examined PIN2 expression, PIN2 PM localization, endosome trafficking, and actin bundling under low-Pi conditions: a MAX2-dependent reduction in PIN2 trafficking and polarization in the PM, reduced endosome trafficking, and increased actin-filament bundling were detected in root cells. The intracellular protein trafficking that is related to PIN proteins but unassociated with AUX1 PM localization was selectively inhibited. Exogenous supplementation of the synthetic SL GR24 to a SL-deficient mutant (max4) led to depletion of PIN2 from the PM under low-Pi conditions. Accordingly, roots of mutants in MAX2, MAX4, PIN2, TIR3, and ACTIN2 showed a reduced low-Pi response compared with the wild type, which could be restored by auxin (for all mutants) or GR24 (for all mutants except max2-1). Changes in PIN2 polarity, actin bundling, and vesicle trafficking may be involved in the response to low Pi in roots, dependent on SL/MAX2 signalling.

Strigolactone analog GR24 triggers changes in PIN2 polarity, vesicle trafficking and actin filament architecture.

Strigolactones (SLs) are plant hormones that regulate shoot and root development in a MAX2-dependent manner. The mechanism underlying SLs' effects on roots is unclear. We used root hair elongation to measure root response to SLs. We examined the effects of GR24 (a synthetic, biologically active SL analog) on localization of the auxin efflux transporter PIN2, endosomal trafficking, and F-actin architecture and dynamics in the plasma membrane (PM) of epidermal cells of the primary root elongation zone in wildtype (WT) Arabidopsis and the SL-insensitive mutant max2. We also recorded the response to GR24 of trafficking (tir3), actin (der1) and PIN2 (eir1) mutants. GR24 increased polar localization of PIN2 in the PM of epidermal cells and accumulation of PIN2-containing brefeldin A (BFA) bodies, increased ARA7-labeled endosomal trafficking, reduced F-actin bundling and enhanced actin dynamics, all in a MAX2-dependent manner. Most of the der1 and tir3 mutant lines also displayed reduced sensitivity to GR24 with respect to root hair elongation. We suggest that SLs increase PIN2 polar localization, PIN2 endocytosis, endosomal trafficking, actin debundling and actin dynamics in a MAX2-dependent fashion. This enhancement might underlie the WT root's response to SLs, and suggests noncell autonomous activity of SLs in roots.

Strigolactone signaling in root development and phosphate starvation.

Strigolactones (SLs), have recently been recognized as phytohormone involve in orchestrating shoot and root architecture. In, roots SLs positively regulate root hair length and density, suppress lateral root formation and promote primary root meristem cell number. The biosynthesis and exudation of SLs increases under low phosphate level to regulate root responses. This hormonal response suggests an adaptation strategy of plant to optimize growth and development under nutrient limitations. However, little is known on signal-transduction pathways associated with SL activities. In this review, we outline the current knowledge on SL biology by describing their role in the regulation of root development. Also, we discuss the recent findings on the non-cell autonomous signaling of SLs, that involve PIN polarization, vesicle trafficking, changes in actin architecture and dynamic in response to phosphate starvation.