Abstract
The shoot apical meristem (SAM) is a crucial tissue located at the tops of plants which can continually grow and differentiate to develop into all aboveground parts. SAM development is controlled by a series of complicated molecular regulation networks, among which microRNAs (miRNAs) and their target genes play key roles. However, little is known about these miRNAs in woody plants. In this study, we used small RNA (sRNA) sequencing to build four libraries derived from shoot tips and mature leaf tissues of Populus tomentosa, and identified 99 known miRNA families. In addition, 193 known miRNAs, including phytohormone-, developmental-, and cellular process-related miRNAs, showed significant differential expression. Interestingly, quantitative real-time reverse transcription polymerase chain reaction (PCR) analysis of miR172, miR164, and miR393 expression showed marked changes in expression patterns during the development of shoot tips. The target genes of these miRNAs were involved in the regulation of hormone responses and stem cell function. In particular, the miR172 target APETALA2 (AP2), involved in the maintenance of stem cells in the shoot apex, was expressed specifically during the initial active stage of development. These findings provide new insights into the regulatory mechanisms of miRNAs involved in SAM development and differentiation in tree species.
Highlights
Meristems are stem cell niches in plants that are capable of unlimited self-renewal and generate differentiated descendants [1]
To cover the major stages of the reactivating–activation cycle, three developmental stages were chosen as follows: (1) Reactivating stage (Re): The samples were collected in March when no leaves were attached to the shoot tips; (2) initial active stage (IA): The samples were collected in April, with several young light-green leaves near the shoot tips; (3) stable active stage (SA): The samples were collected in June, with some mature dark-green leaves on the stem
The results showed that 105 known miRNAs were differentially expressed between the shoot apical meristem (SAM) and leaves (Figure 4a; Table S5)
Summary
Meristems are stem cell niches in plants that are capable of unlimited self-renewal and generate differentiated descendants [1]. The shoot apical meristem (SAM), located in the shoot apex, gives rise to all aboveground organs, such as the stems, leaves, and flowers, forming the aerial structure of higher plants. Previous studies on the regulatory mechanisms of the shoot apex growth cycle have mainly focused on hormonal regulation [2,3]. Auxin is a vital regulator in the development of the SAM and the formation of secondary structures in woody plants due to its endogenous spatial distribution and polar auxin transport. Inhibition of polar auxin transport prevents leaf formation at the SAM and leads to reduced cell division in the stem cell niches [4]. A newly discovered plant hormone, strigolactone (SL), regulates growth redistribution of shoots by modulating auxin transport in the apical meristem [6].
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