The Physiology of Flowering Plants - KHAM PHA MOI

206 CELL GROWTH AND DIFFERENTIATIONforming the above-ground structures (Section 9.2) and the root apicalmeristem (RAM) forming the below-ground structures (Section 9.6).The development of secondary meristems leads to branchingwhilst the cambial meristems, found outside of the apical regions,allow secondary growth leading to thickening, and hence strengthening,of structures. Not all of the meristems are active at any onetime and, indeed, the entire plant may enter a dormant state,whether as an embryo (Section 11.14) or as a mature plant, with there-initiation of cell division requiring specific environmental andinternal signals. Similarly, in a growing plant, growth hormonessuch as auxin, produced in the apical meristem, may generate signalsthat inhibit the development of the secondary meristems in theaxillary buds, leading to apical dominance (Section 7.2.2).Although growth normally occurs from the division of cellswithin meristems, many (but not all) differentiated plant cells retainthe ability to de-differentiate and regenerate an entire plant. Thisability is termed totipotency. Although this may be possible inanimals under exceptionally artificial circumstances (such as thoseused to clone Dolly the sheep from a differentiated udder epithelialcell), this ability is commonplace in plants. It is widely exploited inagriculture and horticulture to regenerate entire plants from cuttingsor single cells (Section 7.2.5). This is a fundamental requirement forthe genetic modification of many plants. Figure 8.1 shows an outlineof the different processes essential to plant development. All of thesemay be occurring at any time during a plant’s life cycle, even at anearly stage such as a developing seedling.8.2.1 PolyploidyNot all cells remain totipotent. Cultured plant cells require auxins,cytokinins, sucrose and inorganic nutrients for growth. Alterationsin the relative concentrations of auxin and cytokinin can cause theundifferentiated callus cells to generate shoots or roots but thisability is often lost after prolonged culture. The reasons underlyingthis are uncertain. In some cases a correlation between a loss ofcapacity to generate new organs and the development of polyploidyhas been observed. This may result from the abnormal growth conditionsof cultured cells but it is also true that separate callus culturesinitiated from the same plant may differ markedly in their ability toFig: 8:1 Cell division, expansionand differentiation in plants.ExpansionTerminaldifferentiationDormancyCell divisionDifferentiationTotipotencySenescenceand death