Gibberellins and brassinosteroids

In a wide spectrum of plant species and organs, gibberellin-induced cell elongation has been correlated with a predominance of transverse CMTs [15,83,111,113-118]. Conversely, when endogenous levels of gibberellins became depleted by means of specific inhibitors of their biosynthesis (e.g., ancymidol, 25,35 paclobutrazol), the CMTs then 

There are situations where, during the course of normal cellular development, holes appear in the CMT array. One such example is during the formation of bordered pits in secondary xylem vessel elements [61]. Pit fields also develop in the maturing cell walls of the maize root cortex and may be pre-figured by the occasional MT holes that are normally seen in these cells. However, it is too early to say whether intracellular changes in gibberellin levels are related to these particular intracellular morphogenetic events which, in the case of secondary xylem vessels, would be rather localized to only a fraction of the cells within the secondary xylem tissue as a whole. 

Many of the above-described effects of gibberellins on orientation and stability of CMTs are suspected to be regulated by unidentified protein kinases because their inhibitors (such as 6-dimethylaminopurine and okadaic acid) abolish the gibberellin-mediated growth responses [48,130]. Inhibition of protein kinase activity also prevented developmentally related changes in an isoform of a-tubuUn in azuki bean epicotyls [48], 

There are no reports on effects of gibberellins on the plant actomyosin complex, although GA3 was shown, by means of the laser trap CODA, to induce a slight increase in the tension properties of transvacuolar cytoplasmic strands [106]. 

Orientation of CMTs in ditferent regions of the root cap, as well as the quiescent centre (QC), of in vitro cultured tomato roots which are either wild-type or gib-1 mutant. The root cap probably controls division planes elsewhere in the root these are abnormal in the gib-1 apices [136]. MT orientations were scored as transverse (T), longitudinal (L), oblique (O) or random (R) with respect to the major axis of cell growth. Percentage values are given as pairs (ala), the first value being from wild-type, the second from gib-1 roots, n is the number of ceils scored. Unpublished data of P.W. Barlow and J.S. Parker 

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Two compounds common in plant metabolism are believed to be precursors of isoprenoid cytokinins in plants adenosine-5 -monophosphate (AMP) and A -isopentenylpyrophos-phate (iPP). As a final product of the mevalonate pathway, the latter substance serves also as a precursor for a wide spectrum of metabolites including some other plant hormones, as abscisic acid, gibberellins and brassinosteroids. The hypothetical scheme of reactions resulting in the formation of iPA, Z and DHZ is given in Fig. 2. The enzyme of entry into isoprenoid cytokinin formation is A -isopentenylpyrophosphate 5 -AMP-A -iso-pentenyltransferase (EC 2.5.1.8, trivially named cytokinin synthetase ). This enzyme activity was first detected in a cell-free preparation from the slime mould Dictyostelium discoideum [7,8]. Later the enzyme from higher plants (cytokinin-independent tobacco callus [9,10] and immature Zea mays kernels [11]) was described and the data were recently summarised in [12], The enzyme is very specific as far as the substrate is concerned [13,14] only the nucleotide AMP can be converted and only iPP (with a double bond in A position) may function as a side chain donor. 

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