FIGURE 3.

Schematic view of the factors involved in the response of meristematic cells to the alteration of the environmental gravity sensing in conditions of microgravity and the possible counteracting effect of light irradiation, particularly red light. Under normal gravity conditions, the gravity vector is sensed in statocytes of the root cap (“professional” cells), and the signal is transduced through the regulation of the auxin polar transport in the root. In meristematic cells, auxin regulates meristematic competence through its interaction with a number of growth coordinators appearing in the figure (nucleolin is a regulator of ribosome biogenesis which interacts, in turn, with cell cycle regulators; E2FB is a member of a family of transcription factors, regulated by auxin, which induces cell proliferation and growth; cyclins and cyclin-dependent kinases (CDKs) are major elements of cell cycle checkpoints controlling DNA replication and entry into mitosis). In addition, other mechanisms of gravity sensing and transduction must exist in “non-professional” cells, i.e., cells not specialized in gravity sensing, and, eventually, promote a similar response in meristematic/proliferating cells, as inferred from the results obtained in biological systems devoid of statocytes and statoliths. These mechanisms are poorly known at present. In microgravity conditions, auxin polar transport is inhibited, at least partially, auxin levels are high at meristems and growth coordinators are, in general, down regulated. The response is the disruption of meristematic competence in proliferating cells. Photoactivation by light (especially by red light), sensed and mediated by phytochromes (especially PHYB), is known to produce down regulation of auxin responsive genes and upregulation of many growth coordinators in meristematic cells, by means of direct and/or indirect mechanisms. These effects may counteract the gravitational stress in root meristematic cells in conditions of microgravity. Red arrows indicate down regulation, black arrows indicate up regulation and blue arrow indicates unknown effect.