468 ROBERT C. HARE 



GROWTH SUBSTANCES AND AUXIN CATABOLISM 



Boyer (1966, 1967) has shown that indoleacetic acid (IAA) induces 

 galls in infected white pines similar to those caused by blister rust. 

 Since both IAA and blister rust induce hypertrophy and hyperplasia and 

 formation of periderm, an increase in auxin may be a cause of galling. 

 Benzimidazole and gibberellic acid (GA3) , together with IAA, further 

 promoted formation of wound periderm similar to that in resistant seed- 

 lings. These results suggest that auxins, gibberellins , and cytokinins 

 are involved in gall development and resistance. 



Susceptibility to obligate parasites such as rusts appears to be 

 positively correlated with vigor of the host, whereas facultative para- 

 sites tend to attack the weaker individuals (Heimburger, 1962) . Illy 

 (1966), for example, found a positive correlation between susceptibility 

 of Pinus pinaster Ait. to the pine twist rust (caused by Melampsora 

 pinitorqua Rostr.) and 16 measures of vigor on six parent trees and 

 their progeny. Workers at Gulfport have noted that fusiform rust-infected 

 nursery seedlings are frequently taller and more vigorous than their 

 healthy counterparts. If true, this observation suggests that rust 

 infection promotes height growth or that taller seedlings are more 

 susceptible. 



To determine whether fusiform rust promotes height growth, increases 

 auxin and gibberellin production, or attenuates auxin catabolism, several 

 greenhouse experiments were carried out with slash and shortleaf pine 

 seedlings (Hare, 1970) . A basidiospore suspension was injected into the 

 epicotyls of seedlings with a fine hypodermic needle; water was injected 

 into controls. Heights were measured monthly when samples of stems, 

 including the injection point, were analyzed for auxins, gibberellins, 

 and activity of indoleacetic acid oxidase (IAAO) , IAAO inhibitor, and 

 polyphenol oxidase (PPO) . 



Of 360 slash pine seedlings inoculated, 92% developed needle symptoms 

 and galls. None of the shortleaf developed galls but many showed needle 

 spots and stem swellings that lasted up to 3 months. 



One would expect susceptible seedlings to need auxin for gall growth 

 and gibberellin for enhanced height growth (if any) . Resistance, then, 

 might conceivably take the form of failure to respond to infection by 

 producing these growth substances, perhaps through gene repression, so 

 that galls would not develop. Although rust infection did not promote 

 height growth during the 6-month study, it consistently (five experiments) 

 increased the auxin and gibberellin contents of both susceptible slash 

 and resistant shortleaf seedlings, especially during the first 2 months 

 of infection. Three zones of auxin activity and three or four of 

 gibberellin were located on chromatograms . One auxin was identified 

 colorimetrically and chromatographically as IAA; one gibberellin peak 

 coincided with that of GA3 . A strong inhibitor of both auxin and gib- 

 berellin activity, probably inhibitor 6 or abscisic acid (Milborrow, 1967) , 

 increased with time after inoculation. Auxin in galled slash seedlings 

 remained elevated until the end of the study, but in shortleaf it returned 

 to normal with the disappearance of symptoms. Gibberellins remained 

 elevated in both species for several months after inoculation. One auxin 

 chromatographing near the front and a gibberellin at Rf 0.6 to 0.8 seemed 

 to be associated with gall development, appearing in older galled slash 

 but rarely in healthy slash or in shortleaf. 



