Abstract:

Objective: Gravity plays an important role in growth and development, in terms of morphogenesis, physiology, metabolism and other biological process in plant. The change of gravity condition will induce a series of changes in plant growth and orientation. In response to the change of gravity conditions, the activities of cambium in woody plant increase unevenly, leading to the eccentric growth of stem and the formation of reaction wood with the obvious changes of xylem structure and cell wall components. Monocotyledons have no cambium, and the stem node elongated through the activity of intermediate meristem. In order to explore the response process and structural changes of monocotyledons under gravistimulation, we use maize to analyze its dynamic process and morphological changes in response to gravistimulation. By comparing with xylophyta, the differences of gravity response between monocotyledons and xylophyta were revealed. It provides a basis for further studies on the gravity response of monocotyledonous woody plants (such as bamboo, etc.) and other trees. Method: The inbred line B73 of maize was used to grow seedlings until 10 leaf stage, the seedlings were laid down horizontally, and the seedlings used as control were still kept upright. The dynamic changes and time points of maize gravity response process were recorded by delay photography. Through tissue sections and special staining, the changes of tissue structure and the differences of cell wall components in maize stem nodes response to gravity were analyzed. Result: The node which the first flag leaf came from was counted as the first one, then the stem nodes were numbered from the bottom to the top. Following the gravity induction, the response processes changed as followed: 1) Gravity response process: Maize plants sense the gravity from the top down subsequently. The plants began to respond to gravity 10 hours after induction. In the next 20 hours, the plants were bent up from the top to the fourth stem node successively, until the plant was perpendicular to ground that is the termination state. 2) Morphology and structure: The repeated experiments showed that only the fourth, fifth and sixth stem nodes had morphological changes, where different degrees of tissue expansion occurred at the side facing ground. The tissue growth and internal vascular bundles in expanded parts of these three stem nodes showed upward bending. The corresponding stem nodes of the control plants showed normal morphology. 3) Vascular bundles: Compared with the control group, the number of proximal vascular bundle cells increased and showed spindle shape, and the cell wall became significantly thinner without obvious vessel tissue, while the number of cells and the thickness of cell wall in the distal vascular bundles varied with the developmental status. In addition, the fourth internode changed most significantly. 4) Cell wall components of vascular bundles: The sections of normal and experimental stem were stained with phloroglucinol and Calcofluor white respectively. The results showed that the content of lignin in the cell walls of proximal vascular bundles was low, and the content of cellulose was higher than that of the control. There was no significant content change in the cell wall of the distal vascular bundles compared with the control. Conclusion: In response to gravity condition, maize can adjust the extension direction of stem node quickly, and cell number of the proximal vascular bundles increased dramatically. At the same time, the cell walls of the proximal vascular bundles became thinner with the content of lignin decreased but cellulose increased. Therefore, it is speculated that after responding to gravity, maize may produce tissues similar to tension wood and compressed wood by adjusting the activities of the intercalary meristem, but different from them. This study lays a foundation for the study of the regulation mechanism of vascular tissue development (wood formation) of woody plants in gravity response.

Key words: monocotyledons, maize, gravity response, cell wall, lignin, cellulose