Abstract
SummaryWood production is dependent on the activity of the vascular cambium, which develops from the fascicular and interfascicular cambia. However, little is known about the mechanisms controlling how the vascular cambium is developed in woody species. Here, we show that PtrHB4, belonging to the Populus HD‐ZIP III family, plays a critical role in the process of vascular cambium development. PtrHB4 was specifically expressed in shoot tip and stem vascular tissue at an early developmental stage. Repression of PtrHB4 caused defects in the development of the secondary vascular system due to failures in interfascicular cambium formation. By contrast, overexpression of PtrHB4 induced cambium activity and xylem differentiation during secondary vascular development. Transcriptional analysis of PtrHB4 repressed plants indicated that auxin response and cell proliferation were affected in the formation of the interfascicular cambium. Taken together, these results suggest that PtrHB4 is required for interfascicular cambium formation to develop the vascular cambium in woody species.
Highlights
The plant vascular system, which enabled green plants to successfully colonize terrestrial land, is a pipeline for the transport of water, nutrients, signalling molecules and other materials over long distances, and a skeleton to provide mechanical support for vertical growth
The results suggest PtrHB4 is required for interfascicular cambium formation, likely via a mechanism which influences the process of auxin response during vascular cambium development in woody species
PtrHB4 expression was examined from the top shoot apical meristem (SAM) tissue successively down to internodes (IN) undergoing secondary growth in the Populus stem
Summary
The plant vascular system, which enabled green plants to successfully colonize terrestrial land, is a pipeline for the transport of water, nutrients, signalling molecules and other materials over long distances, and a skeleton to provide mechanical support for vertical growth. The primary vascular system comprises of a group of discrete vascular bundles containing fascicular cambium, primary phloem and primary xylem. The primary vascular bundles are originated from procambium cells at the peripheral region of the rib zone of the shoot apical meristem (SAM). Fascicular cambium located at the centre of primary vascular bundles undergoes extension into the interfascicular region and generates interfascicular cambium tangentially to form a ring of vascular cambium. Molecular understanding regarding how the ring of vascular cambium is developed in woody species is limited due to challenges faced in performing forward genetic analysis in trees. A number of genes has been identified for their role in regulating Arabidopsis cambium activity (Agusti et al, 2011; Parker et al, 2003; Pineau et al, 2005; Suer et al, 2011). WOX4, a WUSCHEL-related HOMEOBOX gene, regulated by CLE41/44 (CLAVATA3/ESR-related 41/44)/TDIF (tracheary element differentiation inhibitory factor) peptide and its receptor PXY
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