CUHK Passions and Pursuits

12 T he various life-sustaining functions of a plant cell are performed by its organelles which act like the human organs. Classic textbooks in plant biology differentiate organelles of the endomembrane system into endoplasmic reticulum (ER), Golgi apparatus, multivesicular bodies and vacuoles. The last is by far the largest and most important organelle. Vacuoles are essential in regulating a plant’s growth and development. They are the master regulator responsible for the lytic function (degradation and waste storage), the storage of proteins and sugar, maintenance of turgor pressure, the balance of cell volume and defence responses. Despite their significant roles very little is known about their genesis or evolution. In the past 40 years two models have been proposed to explain their biogenesis. One school claims that vacuoles are formed by the fusion of endosomes. The other school hypothesizes that the vacuole is derived from the ER as a single interconnected organelle and that there is only one vacuole in every cell. With the advent of 3D transmission electron microscope technology with nanometre resolution (one nanometre (nm) is one billionth of a metre), Prof. Jiang Liwen took on the challenge to engage in the debate on vacuole biogenesis. His team used a state-of-the-art 3D electron tomography (ET) equipment to study vacuole biogenesis at different developmental stages in the Arabidopsis root cells. At the nano-level, a new picture of the morphology and distribution of the organelles was revealed. In the early stages of the development of a cell, many small vacuoles (SVs) with sizes ranging from 400 nm to 1,000 nm in diameter are observed. In later stages, however, the number of such SVs decreases and in their place larger vacuoles (1,000–2,000 nm, then over 2,000 nm in diameter) can be found. This suggests that vacuoles are formed by the fusion of smaller vacuoles, a direct refutation of the ‘one single interconnected vacuole’ model mentioned above. Professor c c Professor Jiang and postdoctoral fellow Dr. Cui Yong (left)