Abstract
The mechanical strength of inflorescence stems is an important trait in cut flowers. Calcium ions (Ca2+) play a pivotal role in maintaining stem strength, but little is known about the underlying molecular mechanisms. In this study, we treated herbaceous peony (Paeonia lactiflora Pall.) with ethyl glycol tetraacetic acid (EGTA), an effective Ca2+ chelator, and used morphology indicators, spectroscopic analysis, histochemical staining, electron microscopy, and proteomic techniques to investigate the role of Ca2+ in inflorescence stem mechanical strength. The EGTA treatment reduced the mechanical strength of inflorescence stems, triggered the loss of Ca2+ from cell walls, and reduced lignin in thickened secondary walls in xylem cells as determined by spectroscopic analysis and histochemical staining. Electron microscopy showed that the EGTA treatment also resulted in significantly fewer xylem cell layers with thickened secondary walls as well as in reducing the thickness of these secondary walls. The proteomic analysis showed 1065 differentially expressed proteins (DEPs) at the full-flowering stage (S4). By overlapping the Kyoto encyclopedia of genes and genomes (KEGG) and gene ontology (GO) analysis results, we identified 43 DEPs involved in signal transduction, transport, energy metabolism, carbohydrate metabolism, and secondary metabolite biosynthesis. Using quantitative real-time polymerase chain reaction (qRT-PCR) analysis, we showed that EGTA treatment inhibited Ca2+ sensors and secondary wall biosynthesis-related genes. Our findings revealed that EGTA treatment reduced the inflorescence stem mechanical strength by reducing lignin deposition in xylem cells through altering the expression of genes involved in Ca2+ binding and secondary wall biosynthesis.
Highlights
Calcium ions (Ca2+) play a key role in regulating plant growth and development, including cell wall formation[1], osmotic regulation[2], cell division[3], and resistance to biotic and abiotic stresses[4,5,6]
The upper part of the P. lactiflora inflorescence stems was less straight after ethyl glycol tetraacetic acid (EGTA) treatment compared to the control at all four flower-development stages (flower-bud stage (S1), pigmented stage (S2), unfold-petal stage (S3), and fullflowering stage (S4); Fig. 1a)
To further investigate P. lactiflora morphological changes induced by EGTA treatment, five morphological indices, including the upper inflorescence stem mechanical strength, diameter and weight, and the flower diameter and weight at these four flower developmental stages were measured
Summary
Calcium ions (Ca2+) play a key role in regulating plant growth and development, including cell wall formation[1], osmotic regulation[2], cell division[3], and resistance to biotic and abiotic stresses[4,5,6]. Tang et al Horticulture Research (2019)6:36 reported the effects of Ca2+ deprivation on P. lactiflora inflorescence stem mechanical strength. The mechanical strength in P. lactiflora inflorescence stem is related to wall thickness[12]. Thickened cell walls are called secondary walls, and they are distributed around the vascular tissues and under the epidermal layer in stems, providing a major mechanical strength to plants[13,14]. Deposition of lignin in xylem elements and sclerenchyma cell walls is important for mechanical strength[9]. Tamara) flowers, inflorescence stem bending was associated with the absence of lignin deposition in sclerenchyma cells. This has been verified in rice (Oryza sativa L.) brittle culm 3 mutants[15]. Cellulose synthesis is essential for proper secondary wall construction[16]
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