Validation of ACFVA for different plant cells
We first demonstrated that ACFVA could accurately measure many different biologically important features of suspensions cells using several plant cell types, including live Arabidopsis protoplasts and Dunaliella cells because these cells are particularly challenging to identify by automated image analysis (Lobet et al., 2013; Sekulska-Nalewajko et al., 2016), and they enable rapid chemical substance detection such as pollutants in the environment or generation of large quantities of active ingredients such as Plant-derived therapeutic proteins using living cell after genetic modification (Feng et al., 2022; Liu and Stewart Jr, 2015). Using the basic cell-culture methods described previously (Hosseini Tafreshi and Shariati, 2009; Yoo et al., 2007), we prepared Arabidopsis protoplasts and Dunaliella cells for experiments shown in Figs. 2 and 3. Direct comparison of image analysis method is difficult because results from image analysis can be heavily skewed by how the software or method is tuned and commercial software packages are numerous and expensive. Furthermore, the algorithms in commercial software are proprietary and so cannot be directly compared apart from the entire software package, including image preprocessing methods. The best practical comparison, therefore, is for image analysis method to release the crucial results of these method on the same image. For plant live-cell imaging analysis, in which living cells are imaged over a period of time using phase contrast and/or fluorescence microscopy, an important and necessary procedure is image segmentation of single cells which give crucial results of microscopy images such as cell counts, cell localization and cell size which was also important in animal cells microscopy images analysis(Chupeau et al., 2013; Stöter et al., 2013).
Cell count which is a straightforward phenotype is used to probe cell proliferation/apoptosis/death in cytological research. As Arabidopsis protoplasts is a novel and convenient chassis cell in synthetic biology (Zhu et al., 2021), we choosed three concentration (low/medium/high) Arabidopsis protoplasts as plant living cells for photoing using Confocal microscopes. Their microscopy images under brightfield were used for microscopy images analysis comparison because it is the most convenient way to understand the state of cells. For methods used to analysis cell microscopy images, we choose manual statistics methods using ImajeJ software by two testers and automatic statistics method using CellProfile software, a free, open-source system designed for flexible, high-throughput cell image analysis (Stöter et al., 2013) comparison with our new automatic statistics method. For small amount of cells (low concentration), there is no obvious difference among various methods in cell count (30, 29, 45, 29 cells per image). However, the cell numbers counted by CellProfile were much higher (> 2 fold of medium and > 20 fold of high cell concentration) when the amount of cells is not small. In both medium and high concentration cells, our new automatic method obtain similar results as manual methods (Fig. 2A). Object identification is the most challenging step in automatic microscopy image analysis and its accuracy determines the accuracy of the resulting cell measurements that has, nonetheless, proved challenging for many existing software due to their poor ability of separating target cell and contaminants when objects in picture are numerous and complex such as Arabidopsis protoplasts microscopy image of medium and high concentration used in this study. ACFVA was originally developed for the detection of a single cell type whose threshold was set precisely for automatic distinction single cells from numerous contaminants and accurately filtered out impurities. It made our new method can be as accurate as manual cell identification, but less time consuming than manual identification.
Beside cell count, cell size is also an important phenotype for determining cell healthy and the ability of measurement of cell size is a direct indicator of the accuracy of a method to identify individual cells. In all cell concentrations, there was no difference in cell size of Arabidopsis protoplasts (20-40nm) determined by our automatic method and manual method (Fig. 2B), which were both consistent with previous reports(Yoo et al., 2007). Because only in low concentration, the identification of plants cells by CellProfile was accurate. We only used this method measure the cell size of low concentration according to its software introduction. Unlike our method which give the information including cell localization and cell diameter of single cell, CellProfile just gave us three results:10th pctile diameter (the average diameter of top 10% small cells) was 13.6 µm, medium diameter (average diameter of all the cells) was 20.1 µm and 90th pctile diameter (the average diameter of top 10% large cells) was 31.3 µm. To make comparison between these two automatic methods, we obtain the same three results 22.92 µm, 28.66 µm and 34.48 µm using our new method. Although there was no significant difference (< 2 folds) among diameters measuring by two methods, the average diameter of top 10% small cells of CellProfile was smaller than the standard diameter of Arabidopsis protoplasts. It may cause by some small concomitants were counted as cells by CellProfile method. Moreover, the correlation coefficient R2 also proved the diameters obtained by our new method were more stable and accurate than CellProfile (Fig. 2C).
In order to test whether ACFVA can automatically identify a wide range of plant single cells, a microscopy image of Dunaliella cells and a low resolution microscopy image of plant single cells randomly downloaded from Internet (Chupeau et al., 2013) were also analyzed using different methods. In terms of cell counts as same as the result above, the cell numbers counted automatically by our new method in two types of cells were almost identical to manual methods (< 1.1 folds). Because the cell numbers of two pictures were not small enough (> 50), the cell numbers of CellProfile were much higher (> 100 folds of Dunaliella cells and > 20 folds of other plant cells) than other methods (Figure D and E). The large errors of Dunaliella cell numbers counted by CellProfile may be due to the irregular cell shape of Dunaliella. The cell diameter of these two types of cell ranged from 10.26–11.48 µm and 11.13 to 23.69 µm, which were both in line with the previous reports (Hosseini Tafreshi and Shariati, 2009). Consistent with the results above, the correlation coefficient R2 of cell diameter automatically measured by CellProfile of these two types of cells were lower than our new method. Taken together, our new method can automatically (no complex parameter setting), rapidly (the analysis was run on a desktop computer at a rate of > 1 image/minute) and precisely (including cell localization, cell size, cell shape etc) identify various single plant cell.
Broad applicability of ACFVA with plant cells
Plant cells such as Arabidopsis protoplasts are ideal as chassis cells in synthetic biology; they are eukaryotic, allowing exogenous DNA molecules to enter the chassis cell and be encoded correctly; they are cellularly totipotent, with complete energy and metabolic pathways; and they are simple and economical to prepare compared to animal cells. Therefore, we applied ACFVA to perform transfection efficiency calculation and detecting environmental change (high-salinity stress) ---two important and useful analysis in molecular and synthetic biology using Arabidopsis protoplasts after having demonstrated its ability to accurately identify plant cells and measure a large number of relevant phenotypes.
Gene transfection is a widely used technique for molecular studies, which could make a huge impact on subsequent experiments. Therefore, accurate calculating the transfection efficiency of the cells is a necessary and important pre-requisite for for most biological research. Counting the ratio of cell with positive fluorescent light among sufficient cells is a directly methods for calculating transfection efficiency. In this study, we applied ACFVA to compare the transfection efficiency of two systems both using Arabidopsis protoplasts. It was easy and convenient to identify more than 50 cells per picture of two systems using our new method, which ensure a sufficient number of cells are counted. In addition, the fluorescent light of chloroplast which were easily measured by our new method were used to evaluate the cells’ activity of two systems. Unlike animal cells, healthy plant cells including Arabidopsis protoplasts have strong and stable chloroplast’s fluorescent. This property not only helps to identify cells but also to determine their activity, making plant cells more versatile as chassis cells for synthetic biology. In this study, we compared the transfection efficiency of with-carrierDNA and without-carrierDNA systems using the same vector 35S::AtVDA3-EGFP in Arabidopsis protoplasts. AtVDA3 was reported to involved in metabolite exchange between the organelle and the cytosol which are prominently localized in the outer mitochondrial membrane, chloroplast and nucleolus.With the help of our new method, there were 54 cells in with-carrierDNA and 84 cells in without-carrierDNA systems identified both in bright and chloroplast’s fluorescent field. It confirmed that our new method can automatically and precisely found single healthy Arabidopsis protoplasts again. There were no significant differences in the fluorescent light of chloroplast measured in two systems (Fig. 3A), which suggested the cells in these two systems were both healthy and appropriate for gene transfection. Cells with fluorescent light of both EGFP and chloroplast were set as positive cells in this study. There were 49 positive cells in with-carrierDNA and 12 positive cells in without-carrierDNA systems identified both in chloroplast and EGFP fluorescent field, and this positive cell can also be found in BF and EGFP fluorescent field. There is also no significant difference in EGFP fluorescent light of positive cells between two systems (Fig. 3A), which indicated that the host cell activity and positive cell activity of these two Arabidopsis protoplast transfection systems are consistent and the transfection efficiency were their main difference. In this study, the ratio of positive and hots cells was calculated as transfection efficiency. The transfection efficiency was 90.74% and 14.29% in with-carrierDNA and without-carrierDNA systems, respectively. It was consistent with previous reports, all suggesting that carrierDNA can improve gene transfection efficiency(Uherek and Wels, 2000). All the above data suggested that using ACFVA with Arabidopsis protoplasts can analysis gene transfection efficiency quickly and accurately.
Increasingly, synthetic biology research requires biological models that can rapidly and accurately sense changes in the external environment such as chemical stress. The development of modern gene editing techniques and fluorescent tags enable the use of fluorescent signals from biological models to detect external environment changes (Bennett et al., 2008). As we all known, various external stress such as NaCl, dehydration, ABA and cold treatments can lead to synergistic activation of Responsive-to-Dehydration 29A (RD29A), which encodes a hydrophilic protein (Yamaguchi-Shinozaki and Shinozaki, 1994) of unknown function (Msanne et al., 2011). In this study, we construct a biological model using Arabidopsis protoplasts with high transfection efficiency as chassis cells, Arabidopsis RD29A as sensor and fluorescent tags EGFP as reporter to detect the exist of NaCl in external environment (250µm NaCl). The results of chloroplast fluorescent which were also detected to evaluate the healthy of chassis cells showed no significant differences between two treatments (Fig. 3B). It proved that the this biological model was strong enough to detection of external NaCl signals, and the change of reporter fluorescent was caused by NaCl not cell activity. The reporter fluorescent (EGFP) was significantly increased when NaCl was present in the external environment (Fig. 3B), which confirmed the accurate and rapid response to NaCl of our biological model combining with ACFVA. High-salinity water poses hazards for the environment as well as affecting agriculture, infrastructure and communication under seawater. Salinity is one of the most important variables for ocean monitoring, marine environment, seasonal weather forecasting, aquaculture and solar engineering. Therefore, an effective method for sensing salt solution has been much sought for application in many fields, such as agriculture (Li and Kang, 2020), public health (Lugli and Lutz, 1999), and environmental management (Zhao et al., 2003). Many techniques have been proposed to measure salt concentration such as optical techniques (Yin et al., 2018), infrared attenuated total reflection spectroscopy (Rauh and Mizaikoff, 2016), microwave sensing(Harnsoongnoen et al., 2018) and bio-chemical sensing(Kabaa et al., 2019). ACFVA allowed rapid and accurate identification of individual plant cells and the measurement of relevant phenotypic indicators such as fluorescence values which can. Thus, compared to other methods mentioned above, using our method, on the one hand, we can make full use of the easy preparation of plant cells and detect a sufficiently large volume of sample data (billions of samples) to increase accuracy; on the other hand, it allows an increasing wealth of molecular mechanisms and fluorescent labels to be used in practice.