Abstract
Key messageMacroscopic, ultrastructural, and molecular features—like a ball shape, the presence of starch granules, and the up-regulation of genes involved in carbohydrate metabolism and secondary metabolite biosynthesis—distinguish PT regions within a callus.The modification of the mass of pluripotent cells into de novo shoot bud regeneration is highly relevant to developmental biology and for agriculture and biotechnology. This study deals with protuberances (PT), structures that appear during the organogenic long-term culturing of callus (OC) in kiwifruit. These ball-shaped regions of callus might be considered the first morphological sign of the subsequent shoot bud development. Sections of PT show the regular arrangement of some cells, especially on the surface, in contrast to the regions of OC beyond the PT. The cells of OC possess chloroplasts; however, starch granules were observed only in PTs’ plastids. Transcriptomic data revealed unique gene expression for each kind of sample: OC, PT, and PT with visible shoot buds (PT–SH). Higher expression of the gene involved in lipid (glycerol-3-phosphate acyltransferase 5 [GPAT5]), carbohydrate (granule-bound starch synthase 1 [GBSS1]), and secondary metabolite (beta-glucosidase 45 [BGL45]) pathways were detected in PT and could be proposed as the markers of these structures. The up-regulation of the regulatory associated protein of TOR (RAPTOR1) was found in PT–SH. The highest expression of the actinidain gene in leaves from two-year-old regenerated plants suggests that the synthesis of this protein takes place in fully developed organs. The findings indicate that PT and PT–SH are specific structures within OC but have more features in common with callus tissue than with organs.
Highlights
The formation of plant callus tissue, composed in large part of uniform cells, is not considered a regression in the developmental lineage but rather an opening to increase the developmental potency (Feher 2019)
The specific shape of the structures, the regular composition of the cell layers, the presence of plastids with starch granules, and the cell wall modification—which has not been detected beyond PT—might be indications of their distinctness, even in the absence of shoot bud meristems
We found gene expression patterns in PT and PT with visible shoot buds (PT–SH) related to development and metabolic pathways
Summary
The formation of plant callus tissue, composed in large part of uniform cells, is not considered a regression in the developmental lineage but rather an opening to increase the developmental potency (Feher 2019). Investigation of the development of shoot meristems from a disorganized callus tissue is crucial for basic research and for applications in agriculture and biotechnology (Niazian et al 2019). Adventitious shoot formation via a callus is a useful method for plant micropropagation and improvement of genotypes through the techniques used in plant biotechnology programs. A wide spectrum of interest in the Actinidia genus concern different levels of research—from the taxonomic concept to whole-genome sequencing (Ferguson 2016; Rey et al 2020)
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