Characterization of the development dynamics within the linear growth bamboo leaf

Anatomical Mechanisms of Leaf Blade Morphogenesis in Sasaella kogasensis 'Aureostriatus'

There are limited studies on the cytology of bamboo leaf development from primordium to maturity. This study delves into the leaf morphological characteristics and growth patterns of Sasaella kogasensis 'Aureostriatus' and provides a three-dimensional anatomical analysis of cell division, expansion, and degradation. Leaves on the same branch develop bottom-up, while individual leaves develop the other way around. Like bamboo shoots and culms, the leaves follow a "slow-fast-slow" growth pattern, with longitudinal growth being predominant during their development. The growth zones of individual leaves included division, elongation, and maturation zones based on the distribution of growth space. By measuring 13,303 epidermal long cells and 3293 mesophyll cells in longitudinal sections of rapidly elongating leaves, we observed that in the rapid elongation phase (S4-S5), the division zone was located in the 1-2 cm segment at the bottom of the leaf blade and maintained a constant size, continuously providing new cells for leaf elongation, whereas in the late rapid elongation phase (S6), when the length of the leaf blade was approaching that of a mature leaf, its cells at the bottom of the blade no longer divided and were replaced by the ability to elongate. Furthermore, to gain an insight into the dynamic changes in the growth of the S. kogasensis 'Aureostriatus' leaves in the lateral and periclinal directions, the width and thickness of 1459 epidermal and 2719 mesophyll cells were counted in the mid-cross section of leaves at different developmental stages. The results showed that during the early stages of development (S1-S3), young leaves maintained vigorous division in the lateral direction, while periplasmic division gradually expanded from the bottom to the top of the leaf blade and the number of cell layers stabilized at S4. The meristematic tissues on both sides of the leaf were still able to divide at S4 but the frequency of the division gradually decreased, while cell division and expansion occurred simultaneously between the veins. At S6, the cells at the leaf margins and between the veins were completely differentiated and the width of the leaf blade no longer expanded. These findings revealed changes in cell growth anisotropically during the leaf development of S. kogasensis 'Aureostriatus' and demonstrated that leaf elongation was closely related to the longitudinal expansion of epidermal cells and proliferative growth of mesophyll cells, whereas the cell division of meristematic tissues and expansion of post-divisional cells contributed to the increases in blade width and thickness. The presented framework will facilitate a further exploration of the molecular regulatory mechanisms of leaf development in S. kogasensis 'Aureostriatus' and provide relevant information for developmental and taxonomic studies of bamboo plants.

Characterization of anatomical features, developmental roadmaps, and key genes of bamboo leaf epidermis

The exact developmental roadmaps of bamboo leaf epidermis and the regulating genes are largely unknown. In this study, we comprehensively investigated the morphological features of the leaf epidermis of bamboo, Pseudosasa japonica. We also established the developmental roadmaps of the abaxial epidermis along the linearly growing leaf. A variant of P. japonica, P. japonica var. tsutsumiana, with smaller stomata and higher stomata density, was identified. Further analysis revealed that the higher stomata density of the variant was due to the abnormal increase in stomata columns within the single stomata band. This abnormal development of stomata bands was observed as early as the guard mother cell stage in the leaf division zone (DZ). Interestingly, the developmental pattern of the single stomata was similar in P. japonica and the variant. Molecular data showed that PjDLT (Dwarf and Low Tillering) was significantly downregulated in leaves DZ of the variant. Overexpression of PjDLT in Arabidopsis and rice results in smaller plants with lower stomata density, whereas downregulation or mutation of OsDLT results in increased stomata density. Our results highlight the morphological features and developmental schedule of the leaf epidermis of bamboo and provide evidence that DLT plays an important role in regulating stomata in bamboo and rice.

Cellular differentiation, hormonal gradient, and molecular alternation between the division zone and the elongation zone of bamboo internodes

Bamboo differentiates a cell division zone (DZ) and a cell elongation zone (EZ) to promote internode elongation during rapid growth. However, the biological mechanisms underlying this sectioned growth behavior are still unknown. Using histological, physiological, and genomic data, we found that the cell wall and other subcellular organelles such as chloroplasts are more developed in the EZ. Abundant hydrogen peroxide accumulated in the pith cells of the EZ, and stomata formed completely in the EZ. In contrast, most cells in the DZ were in an undifferentiated state with wrinkled cell walls and dense cytoplasm. Hormone detection revealed that the levels of gibberellin, auxin, cytokinin, and brassinosteroid were higher in the DZ than in the EZ. However, the levels of salicylic acid and jasmonic acid were higher in the EZ than in the DZ. Transcriptome analysis with qRT-PCR quantification revealed that the transcripts for cell division and primary metabolism had higher expression in the DZ, whereas the genes for photosynthesis, cell wall growth, and secondary metabolism were dramatically upregulated in the EZ. Overexpression of a MYB transcription factor, BmMYB83, promotes cell wall lignification in transgenic plants. BmMYB83 is specifically expressed in cells that may have lignin deposits, such as protoxylem vessels and fiber cells. Our results indicate that hormone gradient and transcriptome reprogramming, as well as specific expression of key genes such as BmMYB83, may lead to differentiation of cell growth in the bamboo internode.

Cellular and molecular characterizations of the irregular internode division zone formation of a slow-growing bamboo variant

The key molecular mechanisms underlying the sectionalized growth within bamboo or other grass internodes remain largely unknown. Here, we genetically and morphologically compared the culm and rhizome internode division zones (DZs) of a slow-growing bamboo variant (sgv) having dwarf internodes, with those of the corresponding wild type (WT). Histological analysis discovers that the sgv has an irregular internode DZ. However, the shoot apical meristems in height, width, outside shape, cell number and cell width of the sgv and the WT were all similar. The DZ irregularities first appeared post apical meristem development, in 1-mm sgv rhizome internodes. Thus, the sgv is a DZ irregularity bamboo variant, which has been first reported in bamboo according to our investigation. Transcriptome sequencing analysis finds that a number of cell wall biogenesis and cell division-related genes are dramatically downregulated in the sgv DZ. Interestingly, both transcriptomic and brassinosteroid (BR) contents detection, as well as quantitative real-time PCR analyses show that these irregularities have resulted from the BR signaling pathway defects. Brassinosteroid defect might also cause the erect leaves and branches as well as the irregular epidermis of the sgv. These results suggest that BR signaling pathway plays critical roles in bamboo internode DZ and leaf development from a mutant perspective and also explain the upstream mechanisms causing the dwarf internode of the sgv bamboo.