Signals and Targets Triggered by Self-Incompatibility in Plants: Recognition of “Self ” Can Be Deadly - Communication in Plants: Neuronal Aspects of Plant Life - page 75

6 Signals and Targets Triggered

by Self-Incompatibility in Plants:
Recognition of “Self ” Can Be Deadly

S.G.Thomas,S.Huang,C.J.Staiger,V.E.Franklin-Tong

Abstract Plants often respond to stimuli with dramatic rearrangements of their actin
cytoskeleton. Identification of the signals and transduction machinery that lead to changes in
actin dynamics and organization is therefore of considerable interest. The self-incompatibil-
ity(SI)responseinthefieldpoppy, Papaver rhoeas L., involves signal-mediated inhibition
of pollen tube growth in response to interaction with incompatible S proteins. This triggers
larges increases in [Ca 2+ ] i and downstream a number of signalling components and targets
are modified in incompatible pollen. We have observed dramatic alterations to the actin
cytoskeleton in response to SI and established that this involves massive and sustained
actin depolymerization. We have begun to identify and characterize several actin-binding
proteins, including profilin and gelsolin, that may cooperate to transduce the signal from
aCa 2+ wave into destruction of the cytoskeletal network that is essential for tip growth.
Recently, we identified a role for programmed cell death (PCD) signalling cascades being
triggered in SI and that a caspase-like activity is involved in mediating irreversible pollen
tube inhibition. Our data suggest that there is evidence for crosstalk between the SI-induced
signalling cascades. We are currently investigating whether the signalling cascades for actin
alterations and PCD are linked and whether the actin cytoskeleton functions as a sensor
of cellular stress and can initiate PCD. Our current knowledge of the signalling cascade in
P. r h o e a s pollen therefore involves both early and late responses that work in concert to
ensure that pollen does not effect fertilization. Early cessation of tip growth is mediated by
destruction of the actin cytoskeleton, and this appears to cross-talk with a subsequent PCD
cascade that commits the pollen to die.

6.1
Introduction

Cell signalling mechanisms are vital for plants to adapt and survive in
a harsh and challenging environment (Delledonne et al. 2005; Nürnberger
et al. 2005; Rhodes et al. 2005 in this volume). Plants respond to various
stimuli with dramatic rearrangements of their cytoplasm and these are
often mediated by a dynamic actin cytoskeleton. Thus, a major focus of
research is the identification of the signals and transduction machinery
that lead to changes in actin dynamics and organization.
The self-incompatibility (SI) response in the field poppy, Papaver rhoeas
L., provides a good example of a genetically controlled system that employs
signal-mediated inhibition of pollen tube growth in response to a specific
stimulus. The ability to reproduce the SI response in Papaver pollen in