Microgravity effects on different stages of higher plant life cycle and completion of the seed-to-seed cycle.

Human inhabitation of Space requires the efficient realisation of crop cultivation in Bioregenerative Life Support Systems (BLSSs). It is well-known that plants can grow under Space conditions. However, perturbations of many biological phenomena have been highlighted due to the effect of altered gravity and its possible interactions with other factors. The mechanisms priming plant responses to Space factors, as well as the consequences of such alterations on crop productivity, have not completely unravelled. These perturbations can occur at different stages of plant life and are potentially responsible for the failure of the completion of the seed-to-seed cycle. After brief considerations on the main constraints found in the most recent experiments aimed to produce seeds in Space, we focus on two developmental phases in which the plant life cycle can be interrupted more easily than in others also on Earth. The first regards gametogenesis and pollination. We focus on male gamete formation also identifying potential constraints to subsequent fertilisation. The second stage comprises seedling development and establishment. We discuss the reason for the slowness of development at seedling stage which often occurs under microgravity conditions and can reduce the success of establishment. We finally highlight how similar alterations at cytological level not only can be common to different processes occurring at different life stages, but can be primed by different stress factors. We conclude claiming that a systematic analysis of all growth and reproductive phases during plant life cycle is needed to optimize resource utilization in plant-based BLSSs.