Insight Into the Multiple Branches Traits of a Mutant in Larix olgensis by Morphological, Cytological, and Transcriptional Analyses

Larix olgensis is a tall deciduous tree species that has many applications in the wood fiber industry. Bud mutations are somatic mutations in plants and are considered an ideal material to identify and describe the molecular mechanism of plant mutation. However, the molecular regulatory mechanisms of bud mutations in L. olgensis remain unknown. In this study, dwarfed (or stunted), short-leaved, and multi-branched mutants of L. olgensis were found and utilized to identify crucial genes and regulatory networks controlling the multiple branch structure of L. olgensis. The physiological data showed that the branch number, bud number, fresh and dry weight, tracheid length, tracheid length-width ratio, inner tracheid diameter, and epidermal cell area of mutant plants were higher than that of wild-type plants. Hormone concentration measurements found that auxin, gibberellin, and abscisic acid in the mutant leaves were higher than that in wild-type plants. Moreover, the transcriptome sequencing of all samples using the Illumina Hiseq sequencing platform. Transcriptome analysis identified, respectively, 632, 157, and 199 differentially expressed genes (DEGs) in buds, leaves, and stems between mutant plants and wild type. DEGs were found to be involved in cell division and differentiation, shoot apical meristem activity, plant hormone biosynthesis, and sugar metabolism. Furthermore, bZIP, WRKY, and AP2/ERF family transcription factors play a role in bud formation. This study provides new insights into the molecular mechanisms of L. olgensis bud and branch formation and establishes a fundamental understanding of the breeding of new varieties in L. olgensis.

Glutathione, carbohydrate and other metabolites of Larix olgensis A. Henry reponse to polyethylene glycol-simulated drought stress

Drought stress in trees limits their growth, survival, and productivity and it negatively affects the afforestation survival rate. Our study focused on the molecular responses to drought stress in a coniferous species Larix olgensis A. Henry. Drought stress was simulated in one-year-old seedlings using 25% polyethylene glycol 6000. The drought stress response in these seedlings was assessed by analyzing select biochemical parameters, along with gene expression and metabolite profiles. The soluble protein content, peroxidase activity, and malondialdehyde content of L. olgensis were significantly changed during drought stress. Quantitative gene expression analysis identified a total of 8172 differentially expressed genes in seedlings processed after 24 h, 48 h, and 96 h of drought stress treatment. Compared with the gene expression profile of the untreated control, the number of up-regulated genes was higher than that of down-regulated genes, indicating that L. olgensis mainly responded to drought stress through positive regulation. Metabolite analysis of the control and stress-treated samples showed that under drought stress, the increased abundance of linoleic acid was the highest among up-regulated metabolites, which also included some saccharides. A combined analysis of the transcriptome and metabolome revealed that genes dominating the differential expression profile were involved in glutathione metabolism, galactose metabolism, and starch and sucrose metabolism. Moreover, the relative abundance of specific metabolites of these pathways was also altered. Thus, our results indicated that L. olgensis prevented free radical-induced damage through glutathione metabolism and responded to drought through sugar accumulation.

Genetic transformation and growth index determination of the Larix olgensis LoHDZ2 transcription factor gene in tobacco

Homeodomain-leucine zippers (HD-Zip) are plant-specific transcription factors that participate in different plant development processes and differentially regulate metabolic processes. LoHDZ2 is an HD-ZipII subfamily transcription factor gene that we identified from a transcriptomic analysis of Larix olgensis. To understand its function, we built a LoHDZ2 expression vector and then inserted it into tobacco by genetic transformation. Transgenic plants were identified at the DNA and RNA levels. Phenotypic index analysis of transgenic tobacco showed dwarfed growth with larger leaves and earlier flowering than the wild type. LoHDZ2 was expressed differently after hormone treatment with IAA, MeJA and 2,4-D. The results suggested that LoHDZ2 may respond to hormones and be involved in regulating growth and metabolism. These results helped us better understand the function of LoHDZ2 and provided a candidate gene for Larix olgensis molecular breeding.

Evaluation of the mixed effects model and quantile regression approaches for predicting tree height in larch (Larix olgensis) plantations in northeastern China

Tree height (<i>H</i>) is one of the most important tree variables and is widely used in growth and yield models, and its measurement is often time-consuming and costly. Hence, height-diameter (H-D) models have become a great alternative, providing easy-to-use and accurate tools for <i>H</i> prediction. In this study, H-D models were developed for <i>Larix olgensis</i> in Northeast China. The Chapman-Richards function with three predictors (diameter at breast height, dominant tree height, and relative size of individual trees) performed best. Nonlinear mixed effects (NLME) models and nonlinear quantile regressions (NQR9, 9 quantiles; NQR5, 5 quantiles; and NQR3, 3 quantiles) were further used and improved the generalized H-D model, successfully providing accurate <i>H</i> predictions. In addition, the <i>H</i> predictions were calibrated using several measurements from subsamples, which were obtained from different sampling designs and sizes. The results indicated that the predictive accuracy was higher when calibrated by using any number of height measurements for the NLME model and more than 3 height measurements for the NQR3, NQR5 and NQR9 models. The best sampling strategy for the NLME and NQR models involved sampling the medium-sized trees. Overall, the newly developed H-D models can provide highly accurate height predictions for <i>L. olgensis</i>.

Establishment of callus induction system, histological evaluation and taxifolin production of Larch

Taxifolin in larch is an important natural active ingredient, which prevents and treats liver diseases and cardiovascular diseases, and has many health benefits and application values ​​such as anti-tumor, anti-virus, and anti-aging. However, the traditional extraction method is not conducive to the protection of the ecological environment, and also limits the industrial production of taxifolin from larch. Therefore, the purpose of this research is to establish a larch callus culture system and obtain taxifolin from callus. The shoots of the stem-tips of Larix olgensis were used as explants to be cultured. The light conditions, optimal medium and hormone ratio for callus induction were screened to evaluate the callus induction characteristics of larch and its influence on the accumulation of taxifolin. The results showed that: callus formation started on 28–51 days, and the best conditions for inducing callus were explored: 12 h/d light culture MS + 2,4-D 2 mg/L + 6-BA 1 mg/ L + KT 0.1 mg/L; the callus induction rate is 50%-80%; the highest condition for producing taxifolin 6-BA 1 mg/L + NAA 0.1 mg/L, reaching 0.414%. In conclusion, this study represents a suitable induction condition for larch callus culture and taxifolin accumulation.

Nitrogen uptake strategies of mature conifers in Northeastern China, illustrated by the 15N natural abundance method

Background Conifers partition different N forms from soil, including ammonium, nitrate, and dissolved organic N (DON), to sustain plant growth. Previous studies focused on inorganic N sources and specific amino acid forms using 15N labelling, but knowledge of the contribution of DON to mature conifers’ N uptake is still scarce. Here, we quantified the contribution of different N forms (DON vs. NH4+ vs. NO3−) to total N uptake, based on 15N natural abundance of plant and soil available N, in four mature conifers (Pinus koraiensis, Pinus sylvestris, Picea koraiensis, and Larix olgensis). Results DON contributed 31%, 29%, 28%, and 24% to total N uptake by Larix olgensis, Picea koraiensis, Pinus koraiensis, and Pinus sylvestris, respectively, whereas nitrate contributed 42 to 52% and ammonium contributed 19 to 29% of total N uptake for these four coniferous species. Conclusions Our results suggested that all four conifers could take up a relatively large proportion of nitrate, while DON was also an important N source for the four conifers. Given that DON was the dominant N form in study soil, such uptake pattern of conifers could be an adaptive strategy for plants to compete for the limited available N sources from soil so as to promote conifer growth and maintain species coexistence.