Tree Genetics and Molecular Breeding 2024, Vol.14, No.4, 166-176 http://genbreedpublisher.com/index.php/tgmb 170 4.2 Role of stem cells in tissue regeneration and repair Stem cells play a pivotal role in tissue regeneration and repair by replacing damaged or lost cells. In trees, stem cells located in the meristems are activated in response to injury or environmental stress, leading to the regeneration of tissues and organs. The dynamic regulation of stem cell fate by niche-derived cues ensures that stem cells can adapt to changing conditions and meet the needs of the tissue (Ema and Suda, 2012; Hoggatt and Scadden, 2012; Chacón-Martínez et al., 2018; Mannino et al., 2021). For instance, the redox and metabolic states of the niche can influence stem cell behavior, promoting differentiation and migration in response to oxidative stress, which is crucial for efficient healing and revascularization (Ushio-Fukai and Rehman, 2014). 4.3 Integration of stem cells in organ development The integration of stem cells into organ development is a complex process that involves coordination between multiple stem cell-niche units distributed across the tissue. In trees, this process is regulated by both local and systemic signals that ensure the proper spatial and temporal control of stem cell activity (Moore and Lemischka, 2006; Ema and Suda, 2012; O’Brien and Bilder, 2013). The specialized niches in the shoot, root, and vascular meristems provide the necessary signals to maintain stem cell pluripotency and guide their differentiation into specific cell types required for organ formation (Aichinger et al., 2012). Additionally, the interplay between intrinsic and extrinsic factors within the niche ensures that stem cells can respond to developmental cues and environmental changes, facilitating the continuous growth and adaptation of the tree (Lutolf and Blau, 2009; Singh, 2012). By understanding the multi-scale regulation mechanisms of tree stem cells, from the molecular level to ecosystems, we can gain insights into the fundamental processes that drive plant growth, regeneration, and adaptation. This knowledge can inform strategies for improving tree health and resilience in the face of environmental challenges. 5 Whole Plant Level Regulation 5.1 Coordination of stem cell activity with overall plant growth The coordination of stem cell activity with overall plant growth is a complex process that involves the integration of various signaling pathways and environmental cues. Stem cells in the shoot apical meristem (SAM) and root apical meristem (RAM) are regulated by a network of transcription factors and phytohormones. For instance, the homeodomain transcription factor WUSCHEL (WUS) and the bHLH transcription factor HECATE1 (HEC1) play crucial roles in maintaining stem cell proliferation in the SAM by controlling genes involved in metabolism and hormone signaling (Schuster et al., 2014). Additionally, the vascular cambium, a stem cell-like tissue responsible for secondary growth, is regulated by long-distance signaling molecules such as auxin and strigolactones, which coordinate cambium activity with other growth processes (Agustí et al., 2011). This intricate regulatory network ensures that stem cell activity is synchronized with the overall growth and development of the plant. 5.2 Response of stem cells to environmental cues Stem cells in plants are highly responsive to environmental cues, which allows them to adapt their growth and development to changing conditions. Environmental factors such as light, temperature, and nutrient availability influence stem cell activity through various signaling pathways. For example, cell elongation in the Arabidopsis hypocotyl is regulated by a central circuit of interacting transcription factors, including ARF6, PIF4, and BZR1, which integrate hormonal and environmental signals to modulate growth (Oh et al., 2014). Additionally, the activity of meristems is influenced by environmental conditions such as nitrate availability and drought, which affect the balance between self-renewal and differentiation of stem cells (Shimotohno and Scheres, 2019). This responsiveness to environmental cues enables plants to optimize their growth and development in diverse environments. 5.3 Long-distance signaling and its effects on stem cell regulation Long-distance signaling plays a critical role in the regulation of stem cell activity in plants. The vascular system, comprising xylem and phloem, serves as the main conduit for the transmission of long-distance signals, including RNAs, proteins, and phytohormones (Figure 3) (Kondhare et al., 2021). These mobile signals regulate various physiological processes such as flowering, leaf and root development, and stress responses. For instance, auxin
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