Summary Xylem development is a process of xylem cell terminal differentiation that includes initial cell division, cell expansion, secondary cell wall formation and programmed cell death (PCD). PCD in plants and apoptosis in animals share many common characteristics. Caspase‐3, which displays Asp‐Glu‐Val‐Asp (DEVD) specificity, is a crucial executioner during animal cells apoptosis. Although a gene orthologous to caspase‐3 is absent in plants, caspase‐3‐like activity is involved in many cases of PCD and developmental processes. However, there is no direct evidence that caspase‐3‐like activity exists in xylem cell death. In this study, we showed that caspase‐3‐like activity is present and is associated with secondary xylem development in Populus tomentosa. The protease responsible for the caspase‐3‐like activity was purified from poplar secondary xylem using hydrophobic interaction chromatography (HIC), Q anion exchange chromatography and gel filtration chromatography. After identification by liquid chromatography‐tandem mass spectrometry (LC‐MS/MS), it was revealed that the 20S proteasome (20SP) was responsible for the caspase‐3‐like activity in secondary xylem development. In poplar 20SP, there are seven α subunits encoded by 12 genes and seven β subunits encoded by 12 genes. Pharmacological assays showed that Ac‐DEVD‐CHO, a caspase‐3 inhibitor, suppressed xylem differentiation in the veins of Arabidopsis cotyledons. Furthermore, clasto‐lactacystin β‐lactone, a proteasome inhibitor, inhibited PCD of tracheary element in a VND6‐induced Arabidopsis xylogenic culture. In conclusion, the 20S proteasome is responsible for caspase‐3‐like activity and is involved in xylem development.

Summary Regeneration is a common strategy for plants to repair damage to their tissue after attacks from other organisms or physical assaults. However, how differentiating cells acquire regenerative competence and rebuild the pattern of new tissues remains largely unknown. Using anatomical observation and microarray analysis, we investigated the morphological process and molecular features of secondary vascular tissue regeneration after bark girdling in trees. After bark girdling, new phloem and cambium regenerate from differentiating xylem cells and rebuild secondary vascular tissue pattern within 1 month. Differentiating xylem cells acquire regenerative competence through epigenetic regulation and cell cycle re‐entry. The xylem developmental program was blocked, whereas the phloem or cambium program was activated, resulting in the secondary vascular tissue pattern re‐establishment. Phytohormones play important roles in vascular tissue regeneration. We propose a model describing the molecular features of secondary vascular tissue regeneration after bark girdling in trees. It provides information for understanding mechanisms of tissue regeneration and pattern formation of the secondary vascular tissues in plants.

Eucommia ulmoides Oliv. (Eucommiaceae), a traditional Chinese medicinal plant, was used to study phloem cell differentiation during bark regeneration after girdling on a large scale. Here it is shown that new sieve elements (SEs) appeared in the regenerated tissues before the formation of wound cambium during bark regeneration after girdling, and they could originate from the transdifferentiation of immature/differentiating axial xylem cells left on the trunk. Assays of water-cultured twigs revealed that girdling blocked sucrose transport until the formation of new SEs, and the regeneration of the functional SEs was not dependent on the substance provided by the axis system outside the girdled areas, while exogenous indole acetic acid (IAA) applied on the wound surface accelerated SE differentiation. The experiments suggest that the immature xylem cells can transdifferentiate into phloem cells under certain conditions, which means xylem and phloem cells might share some identical features at the beginning of their differentiation pathway. This study also showed that the bark regeneration system could provide a novel method for studying xylem and phloem cell differentiation.

These authors contributed equally to this work. SummaryPPF1 is a gibberellin-induced, vegetative growth-specific gene, first isolated from short-day (SD)-grown G2 pea plants. In the current work, we found that transgenic Arabidopsis plants overexpressing the PPF1 gene (PPF1 (þ)) flowered much later and had a significantly longer lifespan compared to control plants, whereas suppression of this gene (PPF1 (-)) resulted in a very rapid reproductive cycle. Western blotting analyses of PPF1 (þ) and (-) plant lines revealed a positive correlation between the amount of antibody-reactive protein and the time of flowering. Green flourescent protein (GFP) co-expression assays showed that the PPF1 protein is likely localized in chloroplast membranes. Transgenic expression of PPF1 affected the calcium storage capacities since chloroplasts isolated from PPF1 (þ) plants contained high Ca 2þ levels while chloroplasts of PPF1 (-) plants contained very low amounts of calcium ion. Using Novikoff human hepatoma cells, we demonstrated that expression of PPF1 leads to a significant inward calcium ion current that was absent in untransformed cells. We conclude that, as a putative calcium ion carrier, PPF1 affects the flowering time of higher plants by modulating Ca 2þ storage capacity within chloroplasts.

Abstract. Gas chromatography – selected ion monitoring – mass spectrometry was used to measure the level of indole‐3‐acetic acid (IAA) in the cambial region at the top and bottom of the branchless portion of the main stem of three large Scots pine trees, at weekly intervals from 28 April to 13 July. During this period, the cambium reactivated from the dormant state and entered its ‘grand’ period of xylem and phloem production, which was monitored by microscopy. The total amount of IAA (ng cm−2) increased steadily from 28 April until late June, and thereafter remained constant. In contrast, the concentration of IAA (ng g−1 fresh weight) was high at the start of cambial reactivation, declined when the number of differentiating tracheids began to increase, and then rose as the number of cells decreased. The timing and magnitude of the changes in xylem and phloem production and in IAA level were similar at the two sampling positions. It is concluded that the seasonal changes in cambial activity in the conifer stem cannot be ascribed simply to a fluctuation in the level of endogenous IAA in the cambial region.