| Pinus massoniana Lamb.is a kind of fast growing conifer species which likes light but cannot tolerate shade.It has strong adaptability,quick growth,high yield,high comprehensive utilization degree of whole tree and excellent wood,and is widely distributed and reserved in the south of China.At present,researches on the seasonal activities of P.massoniana are mainly conducted at the macro and ecological level,such as the seasonal dynamics of plant diversity in the forest gap,the seasonal dynamics of root respiration efficiency and the seasonal dynamics of soil nitrogen mineralization.However,there are few reports on secondary vascular tissues of P.massoniana from cell biology and molecular regulation mechanism issues.The seasonal secondary vascular bundle development,cellular structure,inclusion composition change and response to the seasonal change of molecular regulatory mechanism remains to be studied.This study will help to reveal the seasonal activity regularity of the tissue development,differentiation and molecular regulation mechanism of the vascular bundle of P.massoniana,and establish a more perfect research system,which is convenient for the molecular biological research of the wood formation of P.massoniana and the subsequent research on the biological improvement and directional cultivation of the wood properties of plantation.The research results of this study are as follows:(1)The secondary phloem of P.massoniana alternated between old and new periderm during seasonal development,and the monitoring results showed that the alternations of periderm occurred in July.According to the characteristics of tissue development and differentiation of the secondary phloem of P.massoniana,the development process of the secondary phloem was divided into seven stages: functional phloem stage,ethmoid lignification stage,phloem ray bending stage,parenchyma cell dedifferentiation and division,formation of dedifferentiated cell groups,periderm transition stage and cortex shedding stage.According to cell morphology and inclusions distribution,ethmoid cells were deformed during lignification.Parenchyma and resin canal increased with development.The crystal content of the crystalline cells and the content of sugars and lipids in the phloem parenchyma cells increased with the development process,indicating that the longitudinal transport function was first lost,and then the secretion function,substance storage function and mechanical function were increased.Finally,the parenchyma dedifferentiated and differentiated to form a new cortex and percutaneous tissue,and the tissues outside the new percutaneous tissue became lignified to form a cortex,which completely developed into the protective tissue of the stem.The starch content in the secondary vascular tissue of P.massoniana increased first and then decreased with the seasons.(2)During the development of the secondary vascular tissue of P.massoniana,the structure and composition of the secondary vascular tissue produced a series of regular changes.When the cambium was differentiated into new phloem and xylem,the cell wall was enriched with cellulose and the content of cellulose in phloem was decreased.During the development of xylem tracheids from early wood to late wood,the diameter and number of grain pores decreased,the lignification degree of secondary wall increased,the area of cell cavity decreased,the proportion of cellulose content decreased,and the chord wall and radial wall thickened.The intracellular and intercellular transport capacity of xylem cells decreased and the mechanical strength increased.(3)The secondary vascular bundle of P.massoniana recovered in early March at the earliest,entered late wood formation stage from late June to early July,and entered dormancy in mid-November.The number of layers of vascular formation was lowest in dormancy period,increased from late February to early June,and decreased from early June to late November.The cell layers and cell width of phloem with transport function of P.massoniana showed bipeak distribution on April 26 and October 26.The growth rate of secondary xylem of P.massoniana increased first and then decreased.With a 365-day cycle in 2022,xylem cells were produced on the 70 th day,with an initial cell accumulation rate of 0.108 cells per day and a peak of 0.326 cells per day on the 172 nd day.The initial radial growth rate of xylem was 4.649 μm/d and reached a peak of 16.283 μm/d on day 180.Xylem activity stopped on day 321.The average accumulation rate of xylem cells was 4.649 μm/d,and the average radial growth rate was 9.951 μm/d.The daily maximum temperature,daily minimum temperature and daily moderate rainfall were positively correlated with the number of cambium cell layers,xylem cell accumulation rate and radial growth rate,while the development of phloem with transport function was not significantly correlated with temperature and rainfall.(4)The transcriptomes were interactively analyzed and WGCNA was used to construct a weighted co-expression network for the seasonal development of the secondary vascular bundle of Pinus masson.The enrichment pathways and highly correlated modules regulating the seasonal development of the secondary vascular bundle of Pinus Masson were analyzed,and the key genes involved in the seasonal development were screened.In-depth analysis showed that starting from the metabolic pathway of starch and sucrose,passing through the pathway of carbon metabolism,amino acid biosynthesis and tryptophan metabolism,and finally to the phenylpropane biosynthesis pathway,flavonoid biosynthesis pathway and plant hormone transduction pathway,there were similar expression patterns of some genes in different metabolic pathways.Co-regulate the metabolism of non-structural carbohydrates to structural carbohydrates,hormones and other metabolites.The results showed that the cell morphology,structure,differentiation rate,inclusions and gene expression in the secondary vascular bundle of Pinus massoniana changed regularly with the seasons. |
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