of its organs and so helps define the architecture of both the root and shoot systems. In fact, diverse aspects of plant development can be altered by gravity; for example, the weight of a branch leads to stress within the plant and the production of additional supporting materials called reaction wood.7 The past decade has seen significant advances in our understanding of how gravity is sensed and translated into changes in plant growth and development; it has also highlighted critical gaps in our knowledge of how these events are induced.
Perception of gravity in plants is now generally accepted to involve specialized cells containing mobile starch-filled organelles, called amyloplasts, that likely act as the gravity-responsive masses triggering cellular responses.8 In the aerial parts of the plant, these sensors lie in the vasculature, a sheath of endodermal cells surrounding tissues specialized for water and nutrient transport, whereas in the root, they are localized to the extreme tip in the root cap. Evidence also points to an unidentified second root gravity sensory system outside of the root cap.9,10 A range of molecular components that are linked to gravity perception have been identified,11,12 but currently still unknown are the precise molecular identities of the receptors that translate the physical force of gravity to cellular signal(s). Similarly, the identity of the immediate signals generated by this sensory system and the associated response components that encode the directional information remain to be defined. As stated in a previous National Research Council (NRC) report on space biology13 our ignorance of the cellular gravity perception machinery remains a fundamental gap in our understanding of how gravity can affect plant growth and development. A NASA research thrust into these fundamental control mechanisms underlying plant growth and development would provide knowledge needed to design plant-based systems as an integral component of bioregenerative life support systems for extended human spaceflight, as well as provide a better understanding of plant growth control mechanisms on Earth.
The mechanisms underlying the control of subsequent plant growth responses have received intensive study, with directional transport of the plant hormone auxin emerging as a significant regulatory element. For example, proteins of the AUX/LAX, PIN, and ABCB families are now known to represent the major transporters that direct the flow of this growth-regulating hormone.14 However, the mechanisms linking gravity perception to the correct placement and relocalization of these transporters, and to the systems that regulate their activities, still remain to be defined.15,16 Other hormones and signals such as cytokinins, ethylene, and reactive oxygen species have also been proposed to be integral regulators of plant gravity-responsive growth,17 and there remains a significant open question as to the interrelationships between these regulatory systems. Advancing and integrating our knowledge of plant growth control are further critical components of research for NASA to pursue. Such analyses will contribute fundamental knowledge of the controls of plant form, with potentially widespread application on Earth, where features such as growth habit (which underpinned the green revolution18) and even responsiveness to gravity (crop recovery after lodging,19 where the weather has bent a crop flat to the ground) have important impacts on crop yields and harvesting. This insight into plant development and physiological responses would also be critical to our ability to design bioregenerative life support systems that incorporate plants to provide sustained replenishment of water and air and to provide food for extended crewed missions into space.
The past decade has also seen an increasing realization that the responses of plants to gravity are inextricably intermingled with those to other stimuli. Thus, perceptions of light, touch, and water gradients have all been shown to modulate the gravity response20 and vice versa. For example, touch stimulation of a plant causes decreased gravity response,21 whereas gravitropism may suppress the growth response of roots toward water sources.22,23 In the spaceflight environment, plants are exposed to many stimuli other than reduced gravity. Light levels and quality, atmospheric composition, nutrient levels, and water availability are all critical elements shaping plant growth in space. Profiling the alterations in gene expression seen upon changes in gravity has also exposed a complex response network that shares common elements with reactions to other stimuli such as touch and light.24,25 Illuminating the integration of multiple stimuli is key to understanding the effects of gravity and spaceflight. A program focused on such mechanistic understanding would present opportunities for collaboration with programs at the National Science Foundation (NSF), the U.S. Department of Agriculture, and the National Institutes of Health (NIH).
Considering the presence of a multiplicity of stimuli and the often extreme environments of spaceflight, these interactions of the gravity perception machinery with other signaling systems may have important and likely unexpected effects on plant growth in space. Robust transcriptional profiling, coupled to proteomic and metabolic