No abstract

Secondary cell walls (SCWs) accumulate in specific cell types of vascular plants, notably xylem vessel cells. Previous work has shown that calcium ions (Ca 2+ ) participate in xylem vessel cell differentiation, but whether they function in SCW deposition remains unclear. In this study, we examined the role of Ca 2+ in SCW deposition during xylem vessel cell differentiation using Arabidopsis thaliana suspension-cultured cells carrying the VND7-inducible system, in which VND7 activity can be post-translationally upregulated to induce transdifferentiation into protoxylem-type vessel cells. We observed that extracellular Ca 2+ concentration was a crucial determinant of differentiation, although it did not have consistent effects on the transcription of VND7-downstream genes as a whole. Increasing the Ca 2+ concentration reduced differentiation but the cells could generate the spiral patterning of SCWs. Exposure to a calcium-channel inhibitor partly restored differentiation but resulted in abnormal branched and net-like SCW patterning. These data suggest that Ca 2+ signaling participates in xylem vessel cell differentiation via post-transcriptional regulation of VND7-downstream events, such as patterning of SCW deposition.

Plant cell walls are typically composed of polysaccharide polymers and cell wall proteins (CWPs). CWPs account for approximately 10% of the plant cell wall structure and perform a wide range of functions. Previous studies have identified approximately 1000 CWPs in the model plant Arabidopsis thaliana; however, the analyses mainly targeted primary cell walls, which are generated at cell division. In contrast, little is known about CWPs in secondary cell walls (SCWs), which are rigid and contain the phenolic polymer lignin. Here, we performed a cell wall proteome analysis to obtain novel insights into CWPs in SCWs. To this end, we tested multiple methods for cell wall extraction with cultured Arabidopsis cells carrying the VND7‐VP16‐GR system, with which cells can be transdifferentiated into xylem‐vessel‐like cells with lignified SCWs by dexamethasone treatment. We then subjected the protein samples to in‐gel trypsin digestion followed by LC‐MS/MS analysis. The different extraction methods resulted in the detection of different cell wall fraction proteins (CWFPs). In particular, centrifugation conditions had a strong impact on the extracted CWFP species, resulting in the increased number of identified CWFPs. We successfully identified 896 proteins as CWFPs in total, including proteases, expansins, purple phosphatase, well‐known lignin‐related enzymes (laccase and peroxidase), and 683 of 896 proteins were newly identified CWFPs. These results demonstrate the usefulness of our CWP analysis method. Further analyses of SCW‐related CWPs could be expected to produce information useful for understanding the roles of CWPs in plant cell functions.

Supplementation with rare earth elements (REEs) such as lanthanum and cerium has been shown to promote plant elongation and/or increase crop yields. On the other hand, there are reports that REE supplementation of plants has no such effect. The appropriate modes for REE utilization and the underlying mechanism are not fully understood. In this study, we investigated how REE supplementation of plants under low light stress affects plant growth and gene expression. Under low light stress conditions, tomato root elongation was observed to be reduced by about half. This suppression of root elongation was found to be considerably alleviated by 20 mM lanthanum ion supplementation. This effect was plant-species-dependent and nutrient-condition-dependent. Under low light stress, the expression of the genes for phytochrome-interacting factor, which induces auxin synthesis, and several auxin-synthesis-related proteins were markedly upregulated by lanthanum ion supplementation. Thus, we speculate that REE supplementation of plants results in auxin-induced cell elongation and alleviates growth suppression under stress conditions.