Cucumber hypocotyl

Mitochondria from cucumber roots, pea (Plsum sativum L.) roots, and maize coleoptiles reacted in a manner similar to mitochondria from cucumber hypocotyls. Hence, it appears that various allelochemlcals can produce different effects on ATP production. 

In bean hypocotyl, the increase of resistance in the course of ripening was found to be accompanied292 by a decrease in the meth-oxyl content from 0.5 to 0.2%, while the content of calcium increased from 0.38 to 1.92%. An increased content of calcium was observed in a resistant variety of cucumber hypocotyl.293 These results suggest that one of the protective mechanisms of the plant against infection is the conversion of pectic acid into calcium pectate, which is resistant to the action of pectic enzymes. 

Katsumi, M. 1985. Interaction of a brassinosteroid with IAA and Ga3 in the elongation of cucumber hypocotyl sections. Plant Cell Physiol. 26 615-625. 

Treatment of the roots of tomato and radish plants with BR led to elongation of the petioles and hypocotyls, and treatment of the bases of mung bean hypocotyl cuttings led to the elongation of the epicotyls (46,47,48). In cucumber hypocotyl segments, washing with water reduced the effect of BR treatment (49), and the uptake of BR has been studied in detail in maize roots, where it accumulated independently of energy supply and 30% was irreversibly bound (50). 

Physiological Modes of Brassinolide Action in Cucumber Hypocotyl Growth... 

Adventitious Root Formation. IAA stimulates adventitious root formation in cucumber hypocotyl cuttings (28). On the other hand, BR has either no effect or is slightly inhibitory, while it stimulates hypocotyl elongation. Gibberellin is usually inhibitory. However, it has been reported that in mung bean cuttings, the formation of root primordia was not inhibited (29). 

N,N -Dicyclohexylcarbodiimide (DCCD), an inhibitor of membrane bound ATPase, has been shown to strongly inhibit IAA-induced elongation of cucumber hypocotyl sections, while it has no effect on GA-induced elongation (36). DCCD markedly inhibits BR-induced elongation (8), suggesting that BR acts differently from GA, but similarly to IAA in this particular case. 

Simultaneous Interaction. In the literature, the interaction of BR with auxin has been reported to be additive or synergistic depending on experimental systems and conditions (5). In cucumber hypocotyl sections, the interaction is definitely synergistic (5). The synergism is especially significant when the concentration of one of the two is suboptimal. The interaction at their optimal concentrations is synergistic only during the early period of incubation and becomes additive finally. A synthetic auxin, 2,4-D also interacts with BR similarly (8). 

Auxin is known to stimulate proton secretion from the cytosol to the cell wall matrix. BR also does the same in cucumber hypocotyl sections as has been reported for other plant materials (16-20). When sections with the epidermis peeled off are incubated in a weakly alkaline buffer, the pH of the buffer drops considerably, indicating that protons are secreted from the tissue to the medium. BR at 10 nM and IAA at 10 iM are almost equally effective (Figure 5). The interaction of BR and IAA is rather inhibitory at the early period of incubation. However, proton secretion continues longer in the presence of both BR and IAA, and finally exceeds those by BR or IAA alone. BR behaves similarly to IAA in this effect. 

A synergism between IAA and BR was also described by Katsumi (72) in green cucumber hypocotyl segments, and analysis by the PEST program showed the difference between data sets for IAA alone and IAA with fixed concentrations of BR can be accounted for by a change in the parameter RAMP, suggesting that the response capacity of the tissue to IAA is enhanced by BR (8). Possible explanations for this effect could be increased numbers of receptors for IAA, amplification of the LAA-induced signal, or its transmission, increased transcription or translation rates for LAA-induced protein synthesis, decreased turnover of mRNA or proteins, increased rates of delivery of cell wall components, etc. Much more research is needed to examine these possibilities. 

There is further evidence to support the claim that brassinosteroids are hormonal in their action (6). These are the effects of brassinolide on gravitropism (7), effects in conjunction with light quality (8), effects on photosynthate partitioning (9), probable effects on phytochrome (10), substitution for indole-3-acetic acid in soybean epicotyls (11), enhancement of xylem differentiation (11), stimulation of membrane permeability in cucumber hypocotyls (12), and stimulation of ATPase activity (12). Taken objectively, many of these specific physiological and biochemical functions which are attributed to brassinolide, and by inference to the brassinosteroids in general, have been attributed to the other plant hormones, especially indole-3-acetic acid and the gibberellins (6, 13). 

Conventional hormone-binding studies have led to the discovery of GA-binding proteins in the cytosol of cucumber hypocotyls and pea epicotyls. These proteins have been partially purified and display some of the characteristics expected of a receptor [11, 12, 18, 17, 14]. In vivo and in vitro GA-binding studies have been performed with aleurones of barley and wheat [ 10,8] but, at present, no GA-binding proteins have been characterised, nor has a candidate GA-receptor emerged. 

Katsumi M (1991) Physiological modes of brassinolide action in cucumber hypocotyl growth. In Cutler HG, Yokota T, Adam G (eds) Brassinosteroids chemistry, bioactivity, and applications, vol 474, ACS Symposium Series. American Chemical Society, Washington, DC, pp 246-254... 

Cleland R, Thompson M, Rayle DL, Purves WK (1968) Difference in effects of gibberellins and auxins on wall extensibility of cucumber hypocotyls. Nature 219 510-511 Clowes FAL (1954) The promeristem and the minimal construction centre in grass root apices. New Phytol 53 108-116... 

Katsumi M, Kazama H (1974) Interrelationship between auxin and gibberellin in the elongation of cucumber hypocotyl sections. In Plant growth substances 1973. Hiro-kawa, Tokyo, pp 845-852... 

Katsumi M, Purves WK, Phinney BO, Kato J (1965) The role of the cotyledons in gibberellin- and auxin-induced elongation of the cucumber hypocotyl. Physiol Plant 18 550-556... 

Table 5.2. The effect of D. ajacisDitcrpenoid Alkaloids on Normal and Gibberellic Acids (GAs)—Induced Cucumber Hypocotyl Elongation [Courtesy of Lawrence and Waller (1973a,b, 1975)]...

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