Leaf senescence is a form of PCD that involves the degradation of chlorophyll. It is a crucial developmental process that transfers nutrients from leaves to other organs, promoting plant growth and crop productivity. P. ternata is a herbaceous plant whose leaves are the main organs that produce photosynthetic products. The leaf senescence of P. ternata is a complex regulatory process that involves the coordinated action of multiple pathways. Therefore, it is essential to comprehend how senescence signals are perceived and processed in P. ternata leaves and to understand the mechanisms of senescence regulation. This work will aid in the development of refined cultivation and management strategies. In this study, two natural leaf senescence model populations of P. ternata were constructed, including early leaf senescent populations (ES) and stay-green populations (LS). The integrated analysis of transcriptome and metabolome analysis revealed that sugar and hormone metabolisms were the underlying reason for leaf senescent differences between ES and LS groups. Comparatively to LS group, germplasms in ES group displayed more overt signs of leaf senescence, which was reflected by the changes in physiological activities, chloroplast damages, the content of sugar and hormone and the expression levels of SAGs.
Sugars are a vital messenger in the regulation of plant development. There were few studies on sugar metabolism in leaf senescence of P. ternata. To understand the roles of sugar in ES and LS groups, we analyzed gene expression levels and contents of sugar related metabolites in these groups. It showed that the expression levels of Galactinol synthase 1 (GOLS1), Galactinol synthase 2 (GOLS2) and raffinose synthase (RFS) in LS showed higher expression levels compared to those in ES group. This trend was similar to the changes in leaf senescence in grapes (Ma et al., 2023). Interestingly, Raffinose family oligosaccharides (RFOs) are worthwhile for plant growth and development that function as osmoprotectants. GOLS and RFS are critical enzymes involved in RFO biosynthesis (Jing et al., 2023). The expression levels of UDP-glucose 4-epimerase-related genes UGE1, UGE5 and RFS6 in leaves of ES group, displayed higher expression levels compared to those in LS group. In this study, the relative content of galactinol in ES group was significantly lower than that in LS group. Galactinol and raffinose effectively protected salicylate from hydroxyl radical attack in vitro. These findings suggest the possibility that galactinol and raffinose scavenge hydroxyl radicals as a novel function to protect plant cells from oxidative damage during leaf senescence in P. ternata (Nishizawa et al., 2008).
Previous research indicated that D-Arabitol is involved in the metabolism of pentose phosphate degradation and suggested that the photosynthesis of senescent leaves was inhibited. Glycerophosphoinositol (PIs) is an extra-plastidial lipid that plays a crucial role as a signaling lipid, transporting materials and maintaining cell plant structure. The increased accumulation of PI lipid classes in ES during leaf senescence may indicate the plant's efforts to cope with the stress caused by this process (Ciubotaru et al., 2023). Carbohydrates, including Xylitol (Maaloul et al., 2021)d Maltotetraose (Liang et al., 2021), primarily regulate osmotic pressure in plant cells and protect biological membranes. The present findings suggest that carbohydrates could play a significant role in investigating the leaf senescence of P. ternata.
Phytohormones play a role in leaf senescence in plants. However, the intricate regulatory mechanisms involved in leaf senescence in P. ternata remain unclear. To comprehend the hormone roles between ES and LS groups, we analyzed hormone content and the expression levels of related genes.
Abscisic acid (ABA) plays an essential role in leaf senescence. In the current investigation, ES exhibited a significantly higher level of ABA content than LS. The expression levels of two ABA signaling and response-related genes PYL3 and PYL9 were lower in ES leaves compared to LS. Ethylene and strigolactone (SL) promote senescence and abscission of plant leaves. In this study, ETH and SL contents in ES group showed significantly higher levels compared to LS group. SLs could modulate the capacity of leaves to capture light energy by altering the components of photosynthetic pigments (Alvi et al., 2022). Further studies on the role of ETH and SLs in controlling chloroplast degradation during leaf senescence are needed. Auxin regulates cell enlargement and plant growth, but the role of auxin in leaf senescence is complex, and the potential mechanism is still much less understood. In this study, ES showed significantly higher expression levels of IA and IAA-Val-Me contents compared to the LS. This phenomenon was similar to that observed in senescent leaves of Arabidopsis thaliana, where free IAA levels were significantly higher than that in non-senescent leaves (Quirino et al., 1999). The expression levels of auxin signaling and response-related genes IAAs, ARFs in leaves of ES, showed lower expression levels compared to LS group. In previous studies, IAAs and ARFs were considered as negative regulators of auxin signaling, with expression levels decreasing with aging (Ellis et al., 2005; van der Graaff et al., 2006). Meanwhile, the results of the expression of auxin signaling and response-related genes GH3s, SAURs in leaves of ES showed higher expression levels compared to the LS. This trend is also consistent with studies in Arabidopsis thaliana, where AtGH3.1, a member of the GH3 family involved in the early response to auxin, was up-regulated in senescent leaves (Buchanan-Wollaston et al., 2005). Similarly, AtSAUR36, a member of the SAUR family involved in the early response to auxin, positively regulates leaf senescence (Bemer et al., 2017; Hou et al., 2013). The traditional concept is that auxin is a negative regulator of leaf senescence. In recent years, more experimental evidence has shown that auxin is a positive regulator of leaf senescence (Mei et al., 2019). Interestingly, the auxin levels and expression levels of related genes in senescent leaves of P. ternata in this study also support the argument that auxin may be a positive regulator during leaf senescence. CK has an anti-senescence effect by increasing the antioxidant activity, the content of chlorophyll, and protein in the plant. The cytokinin signaling and response related genes HK5, CKI1, GLK1, RR1 were lower in ES leaves compared to LS. This effect proves that the degree of senescence of ES leaves was more severe (Peng et al., 2021).
Many plants rely on intricate crosstalk between nutrients and hormones, an effective way of coupling nutritional and developmental information and adjusting their growth and senescence (Singh and Roychoudhury, 2023). Sugar and hormones are thought to work synergistically to regulate plant growth, development and environmental responses (Hu et al., 2017). Sugars in their different forms such as sucrose, glucose, fructose and trehalose-6-P and the hormone family are major regulators of the shoot and root functioning throughout the plant life cycle (Qian et al., 2020). Their combined effects have unexpectedly received little attention, resulting in many gaps in current knowledge. In Fig. 5, we tried to establish the crosstalk of sugar metabolism and hormone signaling pathways, and summarize gene and metabolite changes. The current theory in plant research could explain the synergistic relationship between sugar and ABA signaling, that high sugar levels in plants can increase ABA synthesis and activate the ABA signaling pathway (Arenas-Huertero et al., 2000). The T6P-SnRK1 pathway and sugar-ABA interaction are thought to be involved in processes such as plant senescence (Liu et al., 2023). ABA signaling is involved in regulating leaf senescence by T6P and SnRK1 pathways. Several studies have reported that the sugar signal of T6P directly inhibited SnRK1 activity in vitro both in Arabidopsis and wheat grains. In our study, the Tre and T6P contents of LS leaves were relatively higher than those of ES, and the expression levels of genes related to its ABA signaling pathway were also significantly higher than those of ES. This trend is consistent with wheat grains during senescence and irrigation. Further study demonstrated that Tre metabolic pathway might be the central regulatory system for sucrose allocation and sugar–ABA interactions in wheat grains. These findings suggest that accelerating the biosynthesis or signal transduction of senescence-related hormones at grain filling may be one of the ways via which TaTPP-7A synchronously enhanced grain filling and maturation. Based on the above experimental results, it is suggested that future work should investigate whether the signaling regulation of leaf senescence in P. ternata is also related to the process of nutrient sucrose storage during the maturation stage.
In this study, we constructed a population model with different natural senescent processes and comprehensively analyzed them from physiology, cytology, transcriptome and metabolomics. Based on this model, we investigated the hub genes and metabolites that are closely related to the natural leaf senescence process. In the next step, we will focus on the functional validation of key genes related to sugar and hormone signaling. Additionally, we will also investigate whether exogenous application of key metabolites could maintain metabolic homeostasis in P. ternata to delay leaf senescence at a certain time.