Microgravity environment uncouples cell growth and cell proliferation in root meristematic cells: the mediator role of auxin

Abstract

Experiments performed in space have evidenced that, in root meristematic cells, the absence of gravity results in the uncoupling of cell growth and cell proliferation, two essential cellular functions that support plant growth and development, which are strictly coordinated under normal ground gravity conditions. In space, cell proliferation appears enhanced whereas cell growth is depleted. Since coordination of cell growth and proliferation is a major feature of meristematic cells, the observed uncoupling is a serious stress condition for these cells producing important alterations in the developmental pattern of the plant. Auxin plays a major role in these processes both by assuring the coupling of cell growth and proliferation under normal conditions and by exerting a decisive influence in the uncoupling under altered gravity conditions. Auxin is a mediator of the transduction of the gravitropic signal and its distribution in the root is altered subsequent to a change in the gravity conditions. This altered distribution may produce changes in the expression of specific growth coordinators leading to the alteration of cell cycle and protein synthesis. Therefore, available data indicate that the effects of altered gravity on cell growth and proliferation are the consequence of the transduction of the gravitropic signal perceived by columella cells, in the root tip.

Figure 1

Model of the actions and interactions leading to the maintenance of the meristematic competence (involving the coordination between cell growth and cell proliferation) specifically showing the players involved in the transduction of the gravitropic signal. Under normal ground gravity conditions, this gravitropic signal is a growth promoter, since it stimulates the growth of the primary root according to the gravity vector. The mechanic stimulus is transduced by auxin, which is the mediator that, in the particular case of meristematic cells, modulates the expression of a variety of molecules (cyclins, CD Ks, nucleolar proteins, transcription factors such as E2Fs, secondary mediators such as ANT , etc.,) that coordinate the regulation of cell growth and cell proliferation and finally keep coupled the processes of protein synthesis (cell growth) and cell cycle progression (cell proliferation). A change in the gravity conditions disrupts this equilibrium leading to the dis-coordination or uncoupling of the two essential cellular processes, and to the severe alteration of the meristematic competence. Scheme adapted and modified from Mizukami.