HORMONAL REGULATION OF PLANT CELL GROWTH 195 



absence of inhibitory concentrations of Ca++, exist to only a minor 

 extent and are not of critical importance to wall expansion. 



Alberslieim and Bonner (1959) recently suggested that the pro- 

 motive effect of auxin on methyl incorporation does not involve 

 metliylation of existing polyuronides, but instead synthesis of new^ 

 pectin. If true, this would seem to eliminate entirely the hypothesis 

 that action on existing double salt cross-links is the cause of promo- 

 tion of growth by auxin. However, besides the effect on methyl 

 incorporation, further indication that some aspect of pectin metabo- 

 lism may be affected b\' auxin is given by the reports of auxin effects 

 on activit\' of pectin methylesterase (see reviews cited in the in- 

 troduction ) . 



Promotion of pectin sipithesis (Albersheim and Bonner, 1959) 

 might soften or plasticize the cell wall, since pectin is a rather hy- 

 drated and "soft" material, as has often been mentioned. With 

 growing tobacco and potato tissue it has been reported that auxin 

 causes larger promotions of pectin synthesis than of cellulose syn- 

 thesis (Wilson and Skoog, 1954; Carlier and Buff el, 1955). The in- 

 crease in pectin content in oat coleoptile sections treated with auxin 

 was very slight compared to those noted in these other tissues. But 

 it is conceivable that, of the various cell wall polysaccharide frac- 

 tions, the "softening" effect of the hot-water-soluble fraction would 

 be largest because, as mentioned above, it is the least strongly bonded 

 material. Introduction of these molecules might weaken or break 

 linkages between other components. This would make the meaning 

 of an auxin effect specifically on it more understandable than would 

 the supposition that it bears the critical bonding forces. Since it is 

 a minor constituent, this would fit in with the observation that 

 growth can occur without substantial wall synthesis. The manner 

 in which it acts could, of course, involve either a molecular mosaic 

 or an increase in passive plasticity. This reasoning could also apply 

 to the large promotion by auxin of synthesis of "cold water soluble 

 pectin" found by Albersheim and Bonner ( 1959 ) , who suggested 

 that this material might be a very weakly bonded cell wall con- 

 stituent. 



Bishop, Bayley, and Setterfield (1958) reported analyses of oat 

 coleoptiles from which they concluded that the cell walls contain 

 less than 1 per cent polyuronides or pectin. They felt that this was 

 such a minor proportion that to explain growth, attention should be 



