Oscillatory ROP GTPase Activation Leads the Oscillatory Polarized Growth of Pollen Tubes

Abstract

Oscillation regulates a wide variety of processes ranging from chemotaxis in Dictyostelium through segmentation in vertebrate development to circadian rhythms. Most studies on the molecular mechanisms underlying oscillation have focused on processes requiring a rhythmic change in gene expression, which usually exhibit a periodicity of >10 min. Mechanisms that control oscillation with shorter periods (<10 min), presumably independent of gene expression changes, are poorly understood. Oscillatory pollen tube tip growth provides an excellent model to investigate such mechanisms. It is well established that ROP1, a Rho-like GTPase from plants, plays an essential role in polarized tip growth in pollen tubes. In this article, we demonstrate that tip-localized ROP1 GTPase activity oscillates in the same frequency with growth oscillation, and leads growth both spatially and temporally. Tip growth requires the coordinate action of two ROP1 downstream pathways that promote the accumulation of tip-localized Ca2+ and actin microfilaments (F-actin), respectively. We show that the ROP1 activity oscillates in a similar phase with the apical F-actin but apparently ahead of tip-localized Ca2+. Furthermore, our observations support the hypothesis that the oscillation of tip-localized ROP activity and ROP-dependent tip growth in pollen tubes is modulated by the two temporally coordinated downstream pathways, an early F-actin assembly pathway and a delayed Ca2+ gradient-forming pathway. To our knowledge, our report is the first to demonstrate the oscillation of Rho GTPase signaling, which may be a common mechanism underlying the oscillation of actin-dependent processes such as polar growth, cell movement, and chemotaxis.

Imaging of the Yellow Cameleon 3.6 Indicator Reveals That Elevations in Cytosolic Ca2+ Follow Oscillating Increases in Growth in Root Hairs of Arabidopsis

“…While this has been confirmed for different species of pollen tubes by independent groups (Pierson et al, 1996;Messerli and Robinson, 1997;Watahiki et al, 2004;Hwang et al, 2005), it has not been repeated for tip growth in fungal hyphae (LopezFranco et al, 1994;Sampson et al, 2003). Oscillating growth in root hairs has only recently been reported (Monshausen et al, 2007) and is under further investigation in this report.…”

Exocytosis Precedes and Predicts the Increase in Growth in Oscillating Pollen Tubes

“…Second, the alkaline band near the apex becomes more alkaline in anticipation of growth (;-808) (Lovy-Wheeler et al, 2006), presumably due to a H + -ATPase at the plasma membrane (Certal et al, 2008), whose activity oscillates. Third, Rops oscillate in activity, and recent work by Hwang et al (2005) indicates that they lead growth (;-908). Finally, NAD (P)H oscillates with NAD(P) + , increasing in anticipation of an increase in the growth rate (;-808) (Cá rdenas et al, 2006).…”

Ca2+-Dependent Protein Kinase11 and 24 Modulate the Activity of the Inward Rectifying K+ Channels inArabidopsisPollen Tubes

“…A number of Ca 2+ -regulated cellular activities, such as cytoskeleton dynamics, exocytosis, and endocytosis, are directly or indirectly involved in the regulation of pollen tube growth (Roy et al, 1999;Hwang et al, 2005;Helling et al, 2006;Konrad et al, 2011). Based on the observation that Ca 2+ is known to regulate the activity of ion channels in many other physiological processes, Ca 2+ and its downstream components are assumed to play crucial roles in modulating ion channel activity during pollen tube growth.…”