The Pollen Receptor Kinase LePRK2 Mediates Growth-Promoting Signals and Positively Regulates Pollen Germination and Tube Growth  

Abstract

In flowering plants, the process of pollen germination and tube growth is required for successful fertilization. A pollen receptor kinase from tomato (Solanum lycopersicum), LePRK2, has been implicated in signaling during pollen germination and tube growth as well as in mediating pollen (tube)-pistil communication. Here we show that reduced expression of LePRK2 affects four aspects of pollen germination and tube growth. First, the percentage of pollen that germinates is reduced, and the time window for competence to germinate is also shorter. Second, the pollen tube growth rate is reduced both in vitro and in the pistil. Third, tip-localized superoxide production by pollen tubes cannot be increased by exogenous calcium ions. Fourth, pollen tubes have defects in responses to style extract component (STIL), an extracellular growth-promoting signal from the pistil. Pollen tubes transiently overexpressing LePRK2-fluorescent protein fusions had slightly wider tips, whereas pollen tubes coexpressing LePRK2 and its cytoplasmic partner protein KPP (a Rop-GEF) had much wider tips. Together these results show that LePRK2 positively regulates pollen germination and tube growth and is involved in transducing responses to extracellular growth-promoting signals.

RopGEF7Regulates PLETHORA-Dependent Maintenance of the Root Stem Cell Niche inArabidopsis

“…The aberrant expression patterns of the auxin-responsive reporters DR5rev:GFP and DR5:GUS and the dramatic reduction in PIN1 expression in RopGEF7 RNAi embryos and seedlings ( Figures 9D to 9N) suggest that these defects could be mediated by defective auxin distribution and the resulting auxin-regulated responses. The observations that RopGEFs activate RAC/ROPs and mediate RAC/ROP-induced cell polarization (Kaothien et al, 2005;Gu et al, 2006;Zhang et al, 2008) and that RopGEF7 interacts with At RAC1 and activates At RACs (Figure 4) suggest that RopGEF7 may activate At RACs, including At RAC1, to regulate auxin-induced expression of PLT genes. Since auxin activates RAC/ROPs, which in turn stimulate auxin-responsive gene expression via regulated proteolysis of repressor AUX/IAA proteins (Tao et al, 2002(Tao et al, , 2005, and inhibits endocytosis (Bloch et al, 2005), and the RAC/ROP effector ICR1 mediates PIN polarization (Hazak et al, 2010), RopGEF7 could regulate PIN accumulation and polarization directly through RAC/ROP signaling.…”

“…Thus, while RopGEF7 may contribute to mediating auxinactivated RAC/ROP signaling, the auxin-regulated RopGEF7 expression is apparently mediated by pathways different from those that regulate early auxin-responsive gene expression (Abel et al, 1995). Furthermore, upstream regulators of RopGEF7 signaling activity remain unknown, although receptor-like kinases, such as the pollen receptor kinases from tomato and Arabidopsis (Kaothien et al, 2005;Zhang and McCormick, 2007;Zhang et al, 2008) and, more recently, the FERONIA receptorlike kinase from Arabidopsis, have been shown to regulate RAC/ROP signaling. In particular, FERONIA has been shown to interact directly with RopGEFs, including RopGEF7 (Duan et al, 2010).…”

Shining Light on the Function of NPH3/RPT2-Like Proteins in Phototropin Signaling

“…1) function redundantly to regulate auxin movements required for organogenesis and root gravitropism in concert with AGCVIII kinases other than the phots (Cheng et al, 2008;Li et al, 2011). NRL8 (otherwise known as SETH6) is involved in pollen tube growth (Lalanne et al, 2004), which also requires AGC kinases activity (Zhang et al, 2008). Consequently, NRL proteins are proposed to function as AGC kinase modules that regulate various aspects of auxin trafficking and signaling.…”

Signals fly when kinases meet Rho-of-plants (ROP) small G-proteins

“…If the tomato PRK2 expression is suppressed in pollen tubes, the Ca 2+ -induced apical ROS production is compromised [61], suggesting that the ANXUR1/2-and PRK-regulated pathways are interrelated [27].…”