The secondary growth seen in monocots like Draceana need not be considered “abnormal secondary growth” (as considered by many anatomists) and should be considered as “true secondary growth’, as proposed by Fisher (1973) and DeMason (1994,) because similar to the origin of interfascicular cambium from interfascicular ground tissue inner to pericycle in normal secondary growth in Dicotyledons, in monocots it is the pericycle itself giving rise to secondary cambium producing both secondary vascular tissues and parenchyma in a unidirectional manner.
Evidences from Molecular studies (Mathew et al., 2017).
Transcript scaffolds are a representation of expressed genes and Mathew and co-workers compared gene expression in the monocot cambium samples (of Cordyline australis and Yucca gloriosa) to genes expressed in the cambium and wood-forming tissues for two forest tree species, Populus and Eucalyptus. Gene Ontology (GO) terms when compared revealed a surprising degree of overlapping processes, with 5, 345 of a total of 6, 527 GO terms shared between two monocots and 2 dicot species. Only 127 GO terms were uniquely associated with the two monocot species and not shared by Populus or Eucalyptus. Candidate regulatory genes that vary between the monocot and vascular cambia were also identified, and included members of the KANADI and CLE families involved in polarity and cell-cell signaling, respectively.
This suggests that in certain monocots, where there is a secondary thickening, the regulatory principles of cambium regulation may have been reactivated and thus the monocot cambium evolved.
Earlier studies
According to Fisher, the peculiar monocot cambium produces secondary growth that can be considered as a true secondary growth (Fisher, 1973; Fisher et al., 1974; DeMason, 1994) because it is the product of divisional activity of a secondary meristem (Tomlinson & Zimmermann, 1969 and earlier workers). Diggle and DeMason (1983b), opines that the transition of primary meristem into secondary meristem in the monocotyledonous species is analogous to the transition of procambium to vascular cambium in the woody dicotyledonous stem. On the basis of a detailed study of Yucca whipplei, covering the histology (Diggle & DeMason, 1983a, b) and audiography (DeMason & Diggle, 1984) observations, it was concluded that the primary thickening meristem (PTM) and the monocot cambium (referred to by them as STM) are ontogenetically related to each other and “function as a single entity during the growth and development of the vegetative stem”. This idea found support from the subsequent observations on Cordyline terminalis (DeMason & Wilson, 1985). Earlier, Fahn (1967) also pointed out that if these two meristematic tissues are present in one plant, they could be two developmental phases of the same meristem. Diggle and DeMason (1983a, b) held that the PTM and the STM are histologically similar and are recognizable as a region of radially flattened cells arranged in anticlinal files. A distinction between these meristems was possible usually because of the cell arrangement in derivative tissues, especially those within the vascular bundles.
Role of peptides in vascular differentiation (Fukuda and Hardtke (2020))
The formation of the vascular tissue is a well-organized plant developmental process, whose
central step is the regulation of vascular stem cell fates. It is governed by cell-to-cell communication, and symplastic movement of signaling molecules contributes to vascular development. Various secreted peptides as well as plant hormones play crucial roles in cell-to-cell communication for regulating vascular development (Fukuda and Ohashi-Ito, 2019). Among the peptides, several members of the CLAVATA3 (CLV3)/EMBRYO SURROUNDING REGION, or CLE, family act at different points of key processes in vascular development (Hazak and Hardtke, 2016; Fukuda and Ohashi-Ito, 2019). The CLE family genes are conserved among land plants. In the Arabidopsis (Arabidopsis thaliana) genome, there are 32 genes encoding 27 distinct CLE peptides (Yamaguchi et al., 2016). CLE precursor proteins contain an N-terminal signal peptide and at least one conserved C-terminal 12–14 amino acid CLE domain, from which a mature CLE peptide is produced through proteolytic cleavage. The activity of some processed peptides is increased by modifications such as hydroxylation on Pro residues, and in some cases by glycosylation with three arabinose residues (Ito et al., 2006; Ohyama et al., 2009; Matsubayashi, 2011; Shinohara and Matsubayashi, 2013). During vascular development, CLE41/CLE44, CLE45, and CLE9/CLE10 peptides function in distinct processes. In addition, other peptides such as phytosulfokine and EPIDERMAL PATTERNING FACTOR-LIKE (EPFL) peptides are thought to contribute to the regulation of vascular development(Ikematsu et al., 2017; Holzwart et al., 2018). Crosstalk among peptides and between peptides and plant hormones is an important current topic in vascular development.
References
Fukuda, h and C.S. Hardtke (2020) Peptide Signaling Pathways in Vascular Differentiation Plant Physiology, 182 (4), April 2020, Pages 1636–1644, https://doi.org/10.1104/pp.19.01259
Matthew Z., Suzanne G. and G. Andrew (2017) Transcript profiling of a novel plant meristem, the monocot cambium, JIPB 59 (6)436–449.
Mammen Daniel
