PHYTOCHROME ACTION IN ORYZA SATIVA L. III. THE SEPARATION OF PHOTOPERCEPTIVE SITE AND GROWING ZONE IN COLEOPTILES, AND AUXIN TRANSPORT AS EFFECTOR SYSTEM

• 1 In 4-day-old etiolated rice seedlings, 3 mm of the coleoptile tip did mainly perceive the photostimulus to cause the phytochrome-dependent inhibition of coleoptile elongation. At this age, cell elongation occurred most in the middle portion of coleoptiles in the dark, and was reversibly controlled by a brief exposure of the tip to red and far-red light. Thus, the photoperceptive site was evidently separated from the growing zone in intact rice coleoptiles.
• 2 The red-light-induced inhibition of coleoptile elongation was nullified by the removal of tip followed by the exogenous application of IAA. The sensitivity of thus treated coleoptiles to IAA was gradually lost during intervening darkness between the irradiation and the decapitation, and a 50% loss was obtained at ca. 6th hour at 26°C.
• 3 Polar auxin transport from coleoptile tips was remarkably prevented at the period between, at least, 2nd and 4th hour after red irradiation, and it recovered to the level of dark control by the 6th hour. Far-red light given immediately after red irradiation reversed the yield of diffusible auxin up to that of far-red control.

     

PHYTOCHROME ACTION IN ORYZA SATIVA L.: I. GROWTH RESPONSES OF ETIOLATED COLEOPTILES TO RED, FAR-RED AND BLUE LIGHT

1. Under continuous irradiation, the growth of intact rice coleoptilewas strongly inhibited by red light, and somewhat preventedby blue and far-red light. The inhibitory effect of red lighton coleoptile elongation was caused by a low-energy brief irradiation,and a single exposure of 1.5 kiloergs cm^–2 incidentenergy of red light brought about the 50% inhibition. This photoinhibitionof growth was observed only after the coleoptile had elongatedto about 10 mm or longer. The red light-induced effect was reversedby an immediately following brief exposure to far-red light,and the photoresponses to red and far-red light were repeatedlyreversible. The escape reaction of red lightinduced effect tookplace at a rate so that 50% of the initial reversibility waslost within 9 hr in darkness at 27. The inhibition by bluelight and reversal by far-red irradiation was also achievedrepeatedly with successive treatments of the coleoptiles. Theevidence for a low intensity red far-red reversible controlof coleoptile growth, indicative of control by phytochrome,seems clearly established in etiolated intact seedlings.
2. Incontrast, the elongation of apically excised rice coleoptilesegments was promoted by a brief exposure to red light in 0.02M phosphate buffer, pH 7, and the effect was almost completelynullified by an immediately subsequent exposure to far-red light.It becomes evident that the growth of intact coleoptiles wasinhibited by a exposure to red light, while that of excisedsegments in a buffer was rather promoted by red irradiation.The direction of red light induced responses, either promotiveor inhibitory, depends upon the method of bioassay using intactcoleoptiles or their excised segments.

(Received July 24, 1967; )     

The effect of ethylene on auxin-induced growth of excised rice coleoptile segments

Elongation growth induced by exogenous auxin of apical coleoptilesegments of etiolated rice seedlings was promoted by ethylene.In the absence of exogenous auxin, growth promotion was notobserved. The highest promotion by ethylene was obtained at10^–6 M of indole-3-acetic acid, a suboptimal concentrationfor auxin-induced elongation. Level of ethylene which achievedthe effect was less than 1 µl per liter of an incubationatmosphere. ¹Present address: The Ocean Research Institute, University ofTokyo, Nakano, Tokyo, Japan (Received May 27, 1970; )