Auxins and cytokinins

Auxins and cytokinins are the most widely studied plant hormones on account of their importance for cell growth and division and, from a practical point of view, for their profound impact on organogenesis when plant cells and tissues are cultured in vitro. Not surprisingly, therefore, both these hormones were included in pioneering studies of hormonal effects on the orientation of MTs in relation to cell growth (e.g. references 

Summary of alterations to cell or organ elongation rate in relation to hormone-induced (auxin and cytokinin) alterations of CMT orientation. Rates were either increased or decreased with respect to some reference rate, and the CMTs became reoriented (— ) from random (R) or longitudinal/oblique (L/O) to transverse (T) with respect to the cell axis, or vice versa. The filled circle ( ) indicates the report of a particular correlation, the open circle (O) that such a correlation has not been found the numbers indicate the reference in which these findings are mentioned. The data are from seedling shoot tissues of various species (both monocot and dicot) treated with auxin (references [16,18,19,21-26]) or, in one case, cytokinin (reference [16]) 

More detailed matrix of proposed positive correlations (indicated by ) between (a) the effect of changes in CMT numbers along periclinal cell walls in shoot tissue in relation to growth rate, and (b) changes in CMT orientation in relation to cell or organ growth polarity 

Exogenously applied auxin (0.1-1 p,M lAA) was found to inhibit root growth and to reorient specifically the CMTs of root cells from the transverse to the longitudinal arrangement [37,39]. This change consistently occurred in the cortex, whereas epidermis and stele parenchyma cells were less sensitive. Treatment of maize roots with benzoic acid (10 p,M), however, did not reorient the CMTs, suggesting that the auxin-mediated effects on MT arrangements were not due to an increased acidification but do indeed represent 

One of the ways in which auxin could affect MTs is through a control of their dynamic instability and turnover rates [53], the latter being characteristically rapid in plant cells [54]. We suggest that such effects could allow a swift alteration of auxin level to act as a potent morphogenetic factor since an auxin-related intervention into the pattern of MT assembly (Fig. 1), if confined only to certain cells, or even certain walls of a cell (or cells), might be sufficient to invoke a new structural framework for subsequent growth and 

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The auxin and cytokinin, whose production is normally controlled, are now formed in large amounts and cause uncontrolled tumor growth. The opines are used by Agrobacterium as a unique source of energy and of metabolites for biosynthesis. The host plant cells, however, cannot catabolize the opines. 

The cambium layer of plant stems (Fig. 1-16) differentiates continuously to form phloem on the outside of the cambium and xylem on the inside. At the same time, cambium cells are retained. Thus, at each cell division one daughter cell becomes a differentiated cell, while another remains the less differentiated cambium. This pattern of continuous differentiation from a line of stem cells with constant properties is found in animals as well as in plants. In the differentiation of cambium it appears that chemical signals obtained from the surrounding cells on either the inside or the outside of the cambium layer determine whether the differentiated cell becomes phloem or xylem. Sucrose, auxin, and cytokinins are all involved. 

Shinshi, H., Mohnen, D. Meins, F. (1987). Regulation of a plant pathogenesis-related enzyme Inhibition of chitinase and chitinase mRNA accumulation in cultured tobacco tissues by auxin and cytokinin. Proceedings of the National Academy of Sciences (USA) 84, 89-93. 

Barker, S.J., Tagu, D. The role of auxins and cytokinins in mycorrhizal symbiosis. J. Plant Growth Regul 2000 19 144-154. 

It is generally known that the equilibrium between auxin and cytokinin concentrations is determinative for the type of organ regenerated from callus tissue in vitro. High cytokinin to auxin ratio leads to the shoot Ibrmation, whereas the change in the ratio on behalf of auxin eventuates mostly in root proliferation [253]. Concerning apical dominance and... 

The leaf segments ca. 5x5 mm) of axenic shoots were cultured on MS solid medium supplemented with auxin or combinations of auxin and cytokinin at 25 C in the dark. Calli were formed at the cut end of leaf segments within 2 to 3 weeks on MS solid medium containing 2,4-dichlorophenoxyacetic acid (2,4-D, 1-3 mg/1) and Kin (0-0.1 mg/1), while adventitious roots appeared after intermediary callus on MS solid medium containing either lAA (1-3 mg/1), NAA (0.5-3 mg/1), or NAA (1 mg/1) plus Kin (0.1 mg/1). The calli and adventitious roots grew well and could be maintained in the same cultural conditions as was used for their induction. Fig. (28). 

Specificity. Using the first member of the family, brassinolide (BR), an extensive survey of its effects in 17 bioassays, which varied in their responses to gibberellins, auxins and cytokinins, showed that BR did not behave exclusively as any one of those hormones. In some supposedly specific bioassays BR was as effective, or more so, as the hormone the assay was supposed to detect (9,10). This also applies to the rice lamina inclination assay (11), which is now frequently used. 

The effect of homoBR on flowering tissues was to produce bisexual and pistillate flowers on a staminate inflorescence. Also, sepals were deformed (24), and one would suspect some of these effects were due to induced ethylene biosynthesis, as the dosage of the brassinosteroid used was very high. Excess hormone levels are known to induce the biosynthesis of ethylene BR can also do this, and it interacts with auxin and cytokinin in the induction (43). BR can also affect endogenous auxin and abscisic acid levels in treated tissue (21,44,45). Thus BR does have multiple and modulatory effects. 

It has been reported that brassinolide (BR) has some actions similar to auxin and cytokinin (7). In order to compare the physiological effects of BR in these respects and to determine if this is so, we studied three aspects as follows ... 

The three experiments with brassinolide showed that the physiological effect of brassinolide seems to be that of both an auxin and cytokinin. The twining growth effect, however, is the characteristic response evoked by brassi-nosteroids, and it was thought that the twining movement of the tendril and the twiner (climber) reported for auxin, or abscisic acid (ABA), or ethylene earlier is doubtful. In other words, brassinolide as a steroid may act on membrane systems. 

FIGURE 9-27 Metabolites produced in Agrobacterium-mierAeA plant cells. Auxins and cytokinins are plant growth hormones. The most common auxin, indoleacetate, is derived from tryptophan. Cytokinins... 

Since plant cells can be maintained for long periods in the apparent absence of all known plant hormones, it seems safe to conclude that no hormone is essential just to maintain the viability of plant cells. Some plant hormones seem to be needed for essential developmental processes, however, with the result that no plant can develop in their absence. The hormones auxin and cytokinin appear to fit this description. Both are present in all plants at all times, and in all the major organs [39], No mutant which totally lacks either of these hormones has ever been found [40]. Plants completely deficient in auxin or cytokinin may sometime be discovered, but the failure to find such plants so far suggests that these two hormones play roles that cannot be dispensed with by plants. 

Each of these early studies revealed the ability of hormones to rearrange the cortical MTs (CMTs), lying in the cell cortex, just beneath the plasma membrane, in ways that seemed relevant to the respective growth response. Despite the paucity of subsequent studies on MT responses to cytokinin, the conclusions from the mid-1970s survive up to the present day. As in the case of the other classes of plant hormones, data are lacking on auxin and cytokinin interactions with the actin MFs. 

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