FIGURE 1.

Nitric oxide participates in morphogenesis and development of plants through the interaction with hormones, reactive oxygen species, calcium, and protein post-translational modifications. (1) Dormancy and germination. NO efficiently breaks the dormancy and/or promote germination of several orthodox seeds and could play a pivotal role in sensing environmental conditions appropriate for seed germination (Kopyra and Gwóźdź, 2003; Krasuska et al., 2015). Ethylene and NO, counteract ABA action in seeds improving dormancy release and germination (Arc et al., 2013). Nitrite is the substrate for NO synthesis under hypoxia, and is proposed an auto-regulatory mechanism since NO reversible inhibits seed O₂ consumption (Borisjuk et al., 2007). (2) NO regulates lateral root formation (Correa-Aragunde et al., 2008), primary root growth (Fernández-Marcos et al., 2011), adventitious roots formation (Pagnussat et al., 2004) and root hair development (Lombardo et al., 2006). NO affects chlorophyll level (Liu and Guo, 2013), vegetative growth (Beligni and Lamattina, 2001; Lozano-Juste and León, 2011), symbiosis nodule formation (Hichri et al., 2015), stomatal movement (García-Mata and Lamattina, 2001; Chen et al., 2013b), and Fe homeostasis (Buet and Simontacchi, 2015). (3) Interaction between NO and hormonal responses has been reviewed in Sanz et al. (2015). (4) For description of NO participation in selected abiotic stress conditions see the text. NO is also an important component of the mechanism coordinating and regulating Ca²⁺ and ROS signaling in plant immunity (Trapet et al., 2015). (5) Internal and external signals such as photoperiod, vernalization, gibberellins, and the circadian clock induce reproductive development. NO treatment delayed floral transition in Arabidopsis (He et al., 2004), however, NO level increase in apical meristem during vegetative/reproductive transition in wheat (Kolbert et al., 2011), and three NO donors induce floral transition in Lemna aequinoctialis (Khurana et al., 2011). (6) Fruit ripening and senescence. NO fumigation suppressed respiration and ethylene production and delay in ripening of commercial fruits (Singh et al., 2009; Manjunatha et al., 2010). NO acts antagonistically to ethylene in ripening and senescence, NO delays senescence in cotyledons, cut flowers, and leaves (Procházková and Wilhelmová, 2011).