Selective autolysis of protoplasmic components during development of secondary-phloem sieve-tube elements in

Abstract

Observing changes in plasma components during the development of sieve-tube elements (SE) in secondary-phloem is very important in the study of their physiological function. We investigated the development of SE in the secondary-phloem of Populus deltoides with the aid of an electron microscope. The developmental process of SE in secondary-phloem can be divided into three stages: immature, mature and degenerated, based on the changes of plasma components and cell structures. The immature stage is the development period before the vacuole membrane was ruptured. The radial extension of cells, cell wall incrassation and generation of plastid sieve elements and P-protein occurred during this period. The mature stage started when the vacuole membrane was ruptured. Selective autolysis of protoplasmic components formed mature SEs, with the characteristics that the organelles, such as dissociative ribosome, Golgi body, endoplasmic reticulum and nuclei were disassembled quickly. Two distinctive ways of nuclear degeneration occur. One is an early degenerated nuclear membrane, with dispersed karyoplasms, joined by P-protein. The other is early agglomerated chromatin and subsequently degenerate karyotin with two clear layer structures in the nuclear membrane. One sign in the degenerated stage is the disassembled plasma membrane. During this stage, plastid membranes become disorganized, the starch grains are dispersed in the chamber of SE and the mitochondria dissembled. The callose appears synchronously in sieve plates and P-protein disaggregates. Opened sieve plates are then formed because of callose autolysis, after the protoplasm disappears completely. Finally, the physiological function of SE is lost.

References

1. Esau K, Gill R H (1971). Aggregationof endoplasmic reticulum and its relation to the nucleus in a differentiatingsieve element. J Ultrastruct Res, 34(2): 144–158. doi:10.1016/S0022-5320(71)90010-4
2. Evert R F, Deshpande B P (1969). Electronmicroscope investigation of sieve element ontogeny and structure in Ulmus americana. Protoplasma, 68: 403–432. doi:10.1007/BF01293611
3. Fahn A (1990). Plant Anatomy (in Chinese, trans. Wu S M, Liu D Y).Tianjin: Nankai University Press
4. Hoefert L L (1979). Ultrastructure of developing sieve elements in Thlaspi arvense L. I. The immature state.Amer J Bot, 66(8): 925–932. doi:10.2307/2442233
5. Hoefert L L (1980). Ultrastructure of developing sieve elements in Thlaspi arvense L. (II) Maturation.Amer J Bot, 67(2): 194–201. doi:10.2307/2442642
6. Kluck R M, Bossy W E, Green D R, Newmeyer D D (1997). The release of cytochrome c from mitochondria: a primarysite for bcl-2 regulation of apoptosis. Science, 275(5303): 1132–1136. doi:10.1126/science.275.5303.1132
7. Li Z L, Zhang X Y (1983). PlantAnatomy. Beijing: Higher Education Press (in Chinese)
8. Ouyang X Z, Xie S P, Li B J (1998). Differentiation and ultrastructureof the protophloem sieve elements of minor veins in the maize leaves:Changes in the protoplast. Acta Bot Sin, 40(1): 14–21 (in Chinese)
9. Singh A P, Srivastava L M (1972). The finestructure of corn phloem. Can J Bot, 50(5): 839–846. doi:10.1139/b72-100
10. Walsh M.A, Evert R F (1975). Ultrastructureof metaphloem sieve elements in Zea mays. Protoplasma, 83: 365–388. doi:10.1007/BF01418595
11. Wang Y, Hu S Y (1993). Cytoplasmicultrastructural changes during microsporogenesis of Gossypinum hirsutum: with emphasis on “cytoplasmreorganization”. Acta Bot Sin, 35(4): 225–260 (in Chinese)
12. Wu H, Zheng X F (2003). Ultrastructuralon the sieve element in root protophleom of Arabidopsis thaliana. ActaBot Sin, 45(3): 322–330
13. Yang J, Liu X, Bhalla K, Kim C N, Ibrado A M, Cai J Y, Peng T-I, Jones D P, Wang X D (1997). Prevention of apoptosis by bcl-2: release of cytochrome c from mitochondriablocked. Science, 275(5303): 1129–1132. doi:10.1126/science.275.5303.1129
14. Zee S Y, O'Brien T P (1971). The vasculartissue of the lodicules of wheat. AustralJ Biol Sci, 4: 805–809