Plants are aerobic organisms that have evolved to maintain specific requirements for oxygen (O2) leading to a correct respiratory energy supply during growth and development. There are certain plant developmental cues and biotic or abiotic stress responses where O2 levels are scarce. This O2 deprivation known as hypoxia may occur in hypoxic niches of plant specific tissues and during adverse environmental cues like pathogen attack and flooding. In general, plants respond to hypoxia through a complex reprogramming of their molecular activities with the aim of reducing the impact of stress on their physiological and cellular homeostasis. This review focuses on the fine-tuned regulation of hypoxia triggered by a network of gaseous compounds that includes O2, ethylene (ET) and nitric oxide (NO). In view of recent scientific advances, we aim to compile those molecular mechanisms mediated by phytoglobins (PGBs) and by the N-degron proteolytic pathway, with special emphasis on embryogenesis, seed imbibition and germination, and also specific structures, most notably root apical (RAM) and shoot apical (SAM) meristems. In addition, those biotic and abiotic stresses that comprise hypoxia are also highlighted.

Plant root growth and developmental capacities reside in a few stem cells of the root apical meristem (RAM). Maintenance of these stem cells requires regenerative divisions of the initial stem cell niche (SCN) cells, self-maintenance, and proliferative divisions of the daughter cells. This ensures sufficient cell diversity to guarantee the development of complex root tissues in the plant. Damage in the root during growth involves the formation of a new post-embryonic root, a process known as regeneration. Post-embryonic root development and organogenesis processes include primary root (PR) development and SCN maintenance, plant regeneration and the development of adventitious and lateral roots. These developmental processes require a fine-tuned balance between cell proliferation and maintenance. An important regulator during root development and regeneration is the gasotransmitter nitric oxide (NO). In this review we have sought to compile how NO regulates cell rate proliferation, cell differentiation, and quiescence of SCNs, usually through interaction with phytohormones, or other molecular mechanisms involved in cellular redox homeostasis. NO exerts a role on molecular components of the auxin (Aux) and cytokinin (CK) signaling pathways in PR that affects cell proliferation and maintenance of the RAM. During root regeneration, a peak of Aux and CKs triggers specific molecular programs. Moreover, NO participates in adventitious root formation through its interaction with players of the brassinosteroids (BRs) and CKs signaling cascade. Lately, NO has been implicated in root regeneration under hypoxia conditions by regulating stem cell specification through phytoglobins.

Florida's strawberry (Fragaria × ananassa Duch) production system is mainly dependent on short-day cultivars produced as bare-root (BR) transplants, which are high-yielding and low-cost options for Florida growers. The strawberry industry in Florida is greatly dependent on early yield (mid-November, early December). Therefore, Florida growers must secure rapid establishment of the BR transplants and for that reason, high volumes of irrigation water are applied to reduce air temperature around plant crowns and mitigate desiccation. This practice accounts for nearly 14.7 million m 3 of irrigation water between mid-September and early October. Several alternatives are available to growers to reduce irrigation water for establishment. One of those alternatives suggests replacing BR transplants for actively growing strawberry plugs (SPs). However, the higher price of SP transplants seems to be the main limitation for their implementation. Alternately, growers could explore the possibility of introducing intermittent irrigation or low-volume sprinklers into their system to establish BR transplants. An inexpensive option, based on a large body of research, would be the application of crop protectants against excessive sun radiation, which could reduce irrigation water for establishment by up to 30%. Despite the suggested alternatives, there is still a great deal of work needed to increase grower's confidence in these technologies.