- Organ Development
The totipotency of plant cells was demonstrated in a classical paper by Skoog and Miller [85] showing
that a balance between cytokinin and auxin controlled bud and root formation in tobacco pith explant. At
high concentrations of both hormones, cells often grow amorphously without differentiation. But a high
cytokinin/auxin ratio causes induction of shoots, whereas a high auxin/cytokinin ratio enhances root for-
mation [35].
Lateral development of axillary buds is inhibited by the presence of apical bud. This was shown by
excision of the apical bud to remove correlative inhibition. But in the presence of apical bud, soaking the
entire shoot in cytokinin enhanced lateral bud growth to a large extent [86]. Treatment with hadacidin, an
inhibitor of purine synthesis, inhibited axillary bud growth following decapitation [87]. Because adenine
treatment could not reverse this inhibition, the inhibitor may not be specific. In the absence of cytokinin-
deficient mutants in higher plants, there is only indirect evidence that an endogenous cytokinin level reg-
ulated the development of axillary buds. Medford et al. [81], using genetically engineered cytokinin-over-
producing tobacco and Arabidopsisplants, observed that the most significant morphological change of
high cytokinin levels was that it caused extensive growth of the axillary buds (Figure 3). Thus, there is
strong evidence that cytokinin and auxin balance is important in organ differentiation and its further de-
velopment.
- Delayed Senescence
In plants and crops, the process of senescence is encountered at all stages of their life cycle. When a func-
tional mature leaf is excised from the main body of a plant, it switches on to its death program. Progres-
sive degradation of RNA, proteins, lipids, and chloroplast leading to the loss of chlorophylls starts when
the leaf dies. Once started, the degradation of cell constituents continues even if the cut end is dipped in
mineral salts solution. This process of senescence (i.e., breakdown of cell constituents and yellowing of
leaf), leading to ultimate death, is accelerated further if the leaf is kept in darkness. In many plant species,
adventitious roots are formed at the cut end of the petiole, which decelerates the degradative process of
the metabolites in the leaf blades. Because the supply of mineral salts did not influence the degradative
process, roots being the major source of cytokinin supply [79], this hormone may have been responsible
for delaying the process. But different species show a diversity of response to cytokinins, auxins, or gib-
berellins, in terms of loss of chlorophyll and protein, in experimental systems using detached leaves or
leaf disks [88]. However, two lines of evidence suggest that cytokinins may play an important role in de-
514 NAQVIFigure 3 Axillary bud growth (twelfth node from apex) in (A) wild-type and (B) transgenic cytokinin-over-
producing tobacco plants. (From Ref. 81; with permission from copyright owner.).