Genetically Controlled Chemical Factors Involved in Absorption and

5 Genetically Controlled Chemical Factors Involved in Absorption and Transport of

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 9, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch005

Iron by Plants JOHN C. BROWN U.S. Department of Agriculture, Agricultural Research Service, Plant Stress Laboratory, Beltsville, Md. 20705 Plant use of iron depends on the plant's ability to respond chemically to iron stress. This response causes the roots to release H and "reductants," to reduce Fe , and to accu mulate citrate, making iron available to the plant. Reduc tion sites are principally in the young lateral roots. Azide, arsenate, zinc, copper, and chelating agents may interfere with use of iron. Chemical reactions induced by iron stress affect nitrate reductase activity, use of iron from Fe phosphate and Fe chelate, and tolerance of plants to heavy metals. The iron stress-response mechanism is adap tive and genetically controlled, making it possible to tailor plants to grow under conditions of iron stress. +

3+

3+

3+

3+

The amount of Fe iron available in the aqueous solutions of most cal A

Fe

careous soils is insufficient for p l a n t g r o w t h ( I ) . A b o v e p H 4.0, t h e 3 +

a c t i v i t y i n s o l u t i o n decreases a t h o u s a n d f o l d for e a c h u n i t increase

i n p H ( 2 ) . O e r t l i a n d J a c o b s o n ( I ) i n d i c a t e t h a t at p H 9, the s a t u r a t i o n c o n c e n t r a t i o n of b o t h c a t i o n forms of i r o n d r o p s b e l o w 1 0 "

20

mol/1. i n a

solution i n e q u i l i b r i u m w i t h atmospheric oxygen. Y e t a n iron-deficient (chlorotic)

a n d iron-sufficient ( g r e e n )

p l a n t of the same species c a n

g r o w side b y side ( F i g u r e 1) i n a n a l k a l i n e e n v i r o n m e n t ( 3 ) .

I n such

instances, the t w o c u l t i v a r s u s u a l l y differ o n l y i n t h e i r g e n e t i c a b i l i t y t o r e s p o n d to i r o n stress.

T h e c h e m i c a l reactions i n d u c e d b y i r o n stress

m a k e i r o n a v a i l a b l e to the p l a n t , a n d p l a n t s are classified iron-efficient i f t h e y r e s p o n d to i r o n stress a n d iron-inefficient i f t h e y d o not.

Plants

r e q u i r e a c o n t i n u i n g s u p p l y o f i r o n to m a i n t a i n p r o p e r g r o w t h . S i n c e i r o n stress occurs i n m a n y a l k a l i n e soils, iron-inefficient p l a n t s often b e c o m e 93

Raymond; Bioinorganic Chemistry—II Advances in Chemistry; American Chemical Society: Washington, DC, 1977.

94

BIOINORGANIC

CHEMISTRY

II

c h l o r o t i c a n d d i e . T h e c h e m i c a l factors i n v o l v e d i n i r o n a b s o r p t i o n a n d t r a n s p o r t i n p l a n t s are d e s c r i b e d here b y c o n t r a s t i n g iron-inefficient a n d iron-efficient p l a n t s . Iron

Supply I r o n is most c o m m o n l y s u p p l i e d to p l a n t s b y t h e seed, b y t h e g r o w t h


m e d i u m , or as a spray. T h e g e r m i n a t i n g seed u s u a l l y contains sufficient i r o n to m e e t t h e plant's r e q u i r e m e n t s i n t h e s e e d l i n g stage ( 4 ) . et a l . ( 5 )

Hyde

suggested t h a t p h y t o f e r r i t i n w a s a f o r m of i r o n stored for use

Figure 1. Iron-efficient Hawkeye (left) and iron-ineffi cient Τ203 soybean (right) grown together on an alka line soil (pH 7.5). Only Τ203 soybean developed iron deficiency. b y the y o u n g s e e d l i n g i n c o t y l e d o n s of p e a . showed

Ambler and Brown

(6)

t h a t c h e m i c a l factors t h a t interfere w i t h u p t a k e of i r o n f r o m

t h e g r o w t h m e d i u m d o n o t interfere w i t h p l a n t use of i r o n f r o m

the

cotyledons.

the

I r o n a d d e d as a s p r a y is n o t t r a n s p o r t e d a w a y f r o m

a p p l i c a t i o n area.

T h u s , i r o n n u t r i t i o n p r o b l e m s a p p e a r to b e

centered

i n t h e roots a n d o n c h e m i c a l factors t h a t affect u p t a k e a n d t r a n s l o c a t i o n of i r o n f r o m t h e g r o w t h m e d i u m .


5.

95

Absorption and Transport of Iron by Plants

BROWN

Role of the Rootstock in Iron Use R e c i p r o c a l a p p r o a c h grafts o f iron-inefficient o n iron-efficient rootstocks s h o w e d t h a t i r o n t r a n s p o r t is c o n t r o l l e d b y the rootstock of t o m a t o ( T a b l e I ) ( 7 ) a n d o f soybean

( 8 ) . A l t h o u g h the i r o n c o n c e n t r a t i o n o f

iron-inefficient T 3 2 3 8 f e r tomato roots w a s s i m i l a r t o that of iron-efficient T 3 2 3 8 F E R t o m a t o roots T3238fer

(Table

I ) , less i r o n w a s t r a n s p o r t e d t o t h e

tops because T3238fer roots d i d n o t r e s p o n d to i r o n stress.


L i k e w i s e i n soybeans, a u t o r a d i o g r a p h s s h o w e d that p o r t e d t o the n e w leaf that d e v e l o p e d

5 5

F e w a s not t r a n s -

after the iron-efficient t o p w a s

g r a f t e d t o the iron-inefficient rootstock (8). T h e n e w leaf w a s c h l o r o t i c because t h e iron-inefficient rootstock c o u l d not s u p p l y

5 5

F e to i t .

Table I. Iron Concentration i n Tops and Roots of Iron-inefficient T3238fer (t-fer) and Iron-efficient T 3 2 3 8 F E R ( T - F E R ) Tomatoes as Affected by Tomato Rootstock (7) Iron ffig/g) Experiment (a) Approach t - f e r top on t - f e r top on T - F E R top T - F E R top

Grafts

t - f e r root T - F E R root on t - f e r root on T - F E R root

Experiment (b)

top

root

top

root

63 124 43 121

715 340 229 692

12 192 17 130

1160 618 338 740

Physiologia Plantarum

"Plant

Response to Iron Stress S e v e r a l p r o d u c t s or b i o c h e m i c a l reactions o c c u r o n l y i n iron-efficient p l a n t s i n response t o i r o n stress: ( 1 ) H y d r o g e n ions are r e l e a s e d f r o m t h e roots. (2)

R e d u c i n g c o m p o u n d s are r e l e a s e d f r o m the roots.

(3)

F e r r i c i r o n is r e d u c e d at the roots.

(4)

O r g a n i c acids ( p a r t i c u l a r l y c i t r a t e ) are i n c r e a s e d i n roots.

Response (Zea

to i r o n stress is a d a p t i v e a n d g e n e t i c a l l y c o n t r o l l e d i n c o r n

mays L . )

( 9 ) , soybeans

t o m a t o (Lycopersicon

(Glycine max

esculentum M i l l )

Hydrogen Ion Release from Roots.

( L . ) Merr.)

(10),

and

(II). I n n u t r i e n t solutions w i t h n o

a d d e d i r o n a n d n i t r o g e n a v a i l a b l e o n l y as N 0 - N , t h e first i n d i c a t i o n o f 3

i r o n stress is t h a t the t e r m i n a l leaves o f iron-efficient T 3 2 3 8 F E R t o m a t o d e v e l o p i n c i p i e n t chlorosis. A l m o s t s i m u l t a n e o u s l y , h y d r o g e n ions w e r e r e l e a s e d f r o m t h e i r roots.

T h i s r e d u c e d the p H o f 8 1. o f n u t r i e n t f r o m

a p p r o x i m a t e l y 6.4 t o 4.4 i n 2 4 h r ( i r o n stress) a n d i n c r e a s e d p H f r o m 4.4 t o 6.6 w i t h i n the next 24 h r ( F i g u r e 2 ) , b e c a u s e i r o n r e p r e s s e d the


96

BIOINORGANIC C H E M I S T R Y

II

Figure 2. Plant-induced pH changes of nutrient solutions caused by differential iron-stress response of 25-day old T3238fer ( ; and T328FER ( ; tomato plants placed in nutrient solutions lacking iron and contain ing only NO -N. The pH dropped at day 3 when the T3238FER plants developed iron stress.


s

i r o n stress-response m e c h a n i s m w h e n i t w a s m a d e a v a i l a b l e . W i t h i n this same t i m e , the t e r m i n a l leaves c h a n g e d f r o m green to s l i g h t l y c h l o r o t i c a n d t h e n b e c a m e green a g a i n . T h e t e r m i n a l leaves of the iron-inefficient T 3 2 3 8 f e r t o m a t o also d e v e l o p e d

i r o n chlorosis b u t r e m a i n e d

chlorotic

t h r o u g h o u t this p e r i o d b e c a u s e i t d i d n o t r e s p o n d to i r o n stress. Reducing Compound Release from Roots. I n n u t r i e n t solutions w i t h n o i r o n a n d n i t r o g e n as N H - N a n d N 0 - N , iron-efficient s o y b e a n 4

a n d tomato i r o n stress. roots

(7)

(12)

3

released " r e d u c t a n t s " f r o m t h e i r roots i n response

T h e t e r m " r e d u c t a n t s " designates

that r e d u c e

Fe

3 +

to

Fe

2 +

(Figure 3).

compounds released "Reductant"

is

to by

released

i n t o s o l u t i o n i n greatest q u a n t i t y w h e n the p H of the n u t r i e n t is b e l o w 4.5. T h e a m o u n t of r e d u c t a n t released w a s d e t e r m i n e d s p e c t r o c h e m i c a l l y i n vitro using 2,4,6-tripyridyl-5-triazine ( T P T Z )

( 1 3 , 14, 15).

i r o n c o m b i n e s w i t h T P T Z to f o r m the c o l o r c o m p l e x F e i n w a t e r c o n f o r m s to B e e r s l a w u p to a b o u t 60 μτηοΐ F e agents ( i n v i t r o ) i n t e r f e r e d w i t h r e d u c t i o n of F e

3 +

2 +

2 +

The F e

(TPTZ)

(15).

2

which

Chelating

b y the " r e d u c t a n t s , "

b u t t h i s i n t e r f e r e n c e w a s e l i m i n a t e d b y i n c r e a s i n g the c o n c e n t r a t i o n "reductant" i n solution

2 +

of

(14).

I r o n u p t a k e b y iron-inefficient soybeans w a s n o t i n c r e a s e d t h e y w e r e p l a c e d i n n u t r i e n t solutions that c o n t a i n e d " r e d u c t a n t "

when (14).

T h i s may mean that "reductants" i n the external solution indicate a leaky r o o t r e s u l t i n g f r o m the release of h y d r o g e n i n t o the n u t r i e n t s o l u t i o n . M o r e i m p o r t a n t m a y b e the a d a p t i v e p r o d u c t i o n of " r e d u c t a n t s " i n s i d e the root or at the r o o t surface t h a t keeps i r o n i n the m o r e a v a i l a b l e F e form

(13).

We

have concluded

2 +

that i r o n a b s o r p t i o n a n d t r a n s p o r t is

c o n t r o l l e d i n s i d e t h e root, a n d i r o n u p t a k e is greatest w h i l e t h e i r o n stress-response m e c h a n i s m is f u n c t i o n i n g . T h e " r e d u c t a n t " m a i n t a i n e d its r e d u c i n g c a p a c i t y e v e n after b e i n g p a p e r c h r o m a t o g r a p h e d , d r i e d , a n d p l a c e d i n a t h i n film of f e r r i c y a n i d e f e r r i c h l o r i d e s o l u t i o n (10 μΐηοΐ) (14).

T h e f o r m a t i o n of three P r u s s i a n

b l u e spots o n the p a p e r i n d i c a t e d t h a t the roots r e l e a s e d at least three different r e d u c i n g

compounds.

O v e r the past 20 y r , c o m p o u n d s h a v e b e e n i d e n t i f i e d a n d m e c h a n i s m s e s t a b l i s h e d for m i c r o b i a l transport of i r o n .

F o r e x a m p l e , Ito a n d



5.

97


BROWN

N e i l a n d s (16) suggested that e x c r e t i o n of m e t a l - b i n d i n g p h e n o l i c acids d u r i n g i r o n d e p r i v a t i o n of Bacillus substilis m i g h t c o m b a t i r o n deficiency. W a l s h a n d W a r r e n (17) i n d i c a t e d that the p h e n o l i c acids a c c u m u l a t e d b y i r o n - d e f i c i e n t cultures of B. subtilis do n o t seem to be i n v o l v e d i n i r o n u p t a k e b u t serve to s o l u b i l i z e the i r o n i n the g r o w t h m e d i u m . B y e r s a n d L a n k f o r d (18) f o u n d t h a t the a d d i t i o n of i r o n to g r o w t h c u l t u r e s i n h i b i t e d p h e n o l i c a c i d e x c r e t i o n b y B. subtilis. O u r findings w i t h i r o n efficient s o y b e a n a n d tomato agree w i t h the a b o v e observations, except that w e h a v e not i d e n t i f i e d the " r e d u c t a n t s / ' Ferric Iron Reduction at the Root. Sites of F e r e d u c t i o n i n i r o n efficient s o y b e a n (13, 19) a n d t o m a t o (20) are p r i n c i p a l l y i n t h e y o u n g l a t e r a l roots ( F i g u r e 4 ) . Iron-inefficient T 3 2 3 8 f e r tomato s h o w e d p r a c t i c a l l y n o r e d u c t i o n i n these roots. R e d u c t i o n sites w e r e d e t e r m i n e d b y t r a n s f e r r i n g iron-stressed T 3 2 3 8 f e r a n d T 3 2 3 8 F E R tomato to n u t r i e n t solutions c o n t a i n i n g F e H E D T A ( i r o n - h y d r o x y e t h y l e n e d i a m i n e t r i a c e t i c a c i d ) a n d K F e ( C N ) (20). A b l u e p r e c i p i t a t e , P r u s s i a n b l u e , a p p e a r e d i n t h e e p i d e r m a l areas of the root w h e r e F e w a s r e d u c e d b y the root. 3 +

3

6

3 +

M o s t of the F e w a s r e d u c e d outside the root i n areas accessible to B P D S ( b a t h o p h e n a n t h r o l i n e d i s u l f o n a t e ) (20). This was established 3 +

Ό

1

~

2 ~^

3 ' DAYS

4

'

5

'

6

Figure 3. Plant-induced pH changes of nutrient solutions (top) and release of "reductant" Fe to Fe (bottom) caused by differential iron stress response of 25-day old iron-stressed T3238fer and T3238FER tomato phnts placed in nutrient solutions lacking iron and containing NH -N and N0 -N 3+

2+

A

2



98

BIOINORGANIC

CHEMISTRY

II

Figure 4. Roots of 25-day-old iron-stressed T3238FER (left) and Τ3238fer (right) tomatoes after being phced in a nutrient solution containing 5 mg Fe/l. as FeHEDTA and 33 mg/l. K Fe(CN) . The dark areas on T3238FER roots (left) are Prussian blue precipitates formed when the roots reduced Fe * to Fe . The blue precipitate indicates the reduction sites. Τ3238fer roots (right) showed no reduction of Fe . 3

6

3

2+

3+

b y a d d i n g B P D S to the n u t r i e n t solutions, 1 0 % i n excess of F e . A s t h e 3 +

Fe

w a s r e d u c e d , m o s t of i t w a s t r a p p e d i n s o l u t i o n as F e

3 +

w a s n o t t r a n s p o r t e d to the p l a n t top

2 +

BPDS

3

and

(21).

If the iron-efficient roots w e r e g i v e n i r o n as F e H E D T A for 20 h r , t h e n t a k e n out of t h e n u t r i e n t solutions, r i n s e d free of F e H E D T A , a n d p l a c e d i n n u t r i e n t solutions c o n t a i n i n g K F e ( C N ) , P r u s s i a n b l u e f o r m e d 3

6

t h r o u g h o u t the p r o t o x y l e m of the y o u n g l a t e r a l roots u p to the m e t a x y l e m (13). in

the

root

F e r r o u s i r o n w a s c o n t i n u o u s i n these areas of the roots a n d

regions

of

root

elongation

and

m a t u r a t i o n of

the

primary

Increase in Roots.

Organic

(13,19). Organic A c i d (Particularly Citrate)

acids k e e p i r o n m o b i l e i n e x t e r n a l s o l u t i o n , a n d Rogers a n d S h i v e suggested t h a t t h e y m i g h t f u n c t i o n s i m i l a r l y i n s i d e the p l a n t .

p l a n t s u s u a l l y c o n t a i n m o r e c i t r i c a n d m a l i c acids t h a n n o r m a l plants

(23,24,25,26).


(22)

Chlorotic green

5.

99


BROWN

S c h m i d a n d G e r l o f f (27)

reported a naturally occurring iron-com-

p l e x i n g agent i n t o b a c c o x y l e m e x u d a t e that p r e v e n t e d i r o n p r e c i p i t a t i o n a n d w a s a v e h i c l e for t r a n s p o r t i n g i r o n i n the p l a n t . T i f f i n a n d B r o w n s h o w e d that i r o n i n x y l e m exudate is m o s t l y i n n e g a t i v e l y c h a r g e d

(28) forms.

I r o n stress, as i t controls i r o n s u p p l y to t h e p l a n t , m a y b e

c o n t r o l l i n g factor affecting the a p p e a r a n c e of citrate i n x y l e m (21, 29).


exudate

the

exudate

T h e p a r a l l e l b e t w e e n i o n t r a n s p o r t e d a n d citrate i n the x y l e m was

striking.

When

i r o n i n c r e a s e d , the c i t r a t e i n c r e a s e d ;

a

decrease i n i r o n w a s p a r a l l e l e d b y a decrease i n citrate. T h i s r e l a t i o n s h i p h e l d true w h e t h e r i r o n stress w a s p r o d u c e d i n the p l a n t b y l i m i t i n g the i r o n s u p p l y or b y u s i n g z i n c , a z i d e , or arsenate to i n d u c e i r o n stress Trapping F e

2 +

at the root as F e

2 +

BPDS

p l a n t a n d s i m u l t a n e o u s l y decreased

3

(29).

decreased the i r o n s u p p l y to the

the citrate i n stem exudate

(21).

T i f f i n (30, 31,32) i d e n t i f i e d f e r r i c citrate i n the x y l e m exudate of s e v e r a l p l a n t species b y u s i n g electrophoresis

to f o l l o w

the m i g r a t i o n of

the

c h e l a t e d i r o n . Sufficient citrate w a s a l w a y s present to chelate the m e t a l , and

any

excess m i g r a t e d

as a n iron-free

fraction

behind

the

iron-

citrate b a n d . C l a r k et a l . (33) w e r e ineffective

f o u n d that m a l i c , acetic, a n d frans-aœnitic acids

in moving

5 9

F e electrophoretically

isocitrate, frans-aconitate, a n d m a l a t e buffers. anodically whenever

i n acetate,

citrate,

Citric acid moved

present o n the e l e c t r o p h e r o g r a m

and

iron

successfully

c o m p e t e d w i t h the other acids for i r o n . C l a r k et a l . ( 33 ) f u r t h e r s h o w e d t h a t iron-efficient c o r n a b s o r b e d a n d t r a n s p o r t e d m o r e i r o n i n t h e x y l e m exudate t h a n iron-inefficient c o r n . B u t w h e n x y l e m exudate

5 9

F e w a s a d d e d i n v i t r o to

f r o m the iron-inefficient c o r n , the

5 9

F e moved

as

5 9

citrate, i n d i c a t i n g that there w a s sufficient c i t r i c a c i d i n t h e exudate chelate the a d d e d

5 9

Fe.

T h e iron-inefficient c o r n roots d o not

Fe to

respond

t o i r o n stress a n d l a c k the m e c h a n i s m to s u p p l y i r o n for m o v e m e n t

into

t h e root. T h e t r a n s l o c a t i o n of i r o n i n the p l a n t i n v o l v e s m o r e t h a n citrate c h e l a t i o n of i r o n i n the root. Mechanism

of Iron Absorption

and

Transport

Iron-efficient a n d iron-inefficient p l a n t s c a n h a v e several h u n d r e d / x g F e / g of root, b u t the iron-inefficient p l a n t m a y d i e f r o m l a c k of i r o n i n its tops.

I n contrast, iron-efficient p l a n t s r e s p o n d to i r o n stress, a n d

the root makes i r o n a v a i l a b l e for t r a n s p o r t a n d use i n tops. w a y , i r o n m a y r e m a i n i n the n u t r i e n t s o l u t i o n as F e

3 +

In a similar

chelate or F e

3 +

p h o s p h a t e a n d not be t r a n s p o r t e d to the p l a n t top u n t i l i t is m a d e a v a i l a b l e for transport t h r o u g h c h e m i c a l reactions i n d u c e d b y i r o n stress. T h e s e observations stress the i m p o r t a n c e of a p l a n t b e i n g a b l e to r e s p o n d to i r o n stress. I r o n is u s u a l l y u s e d i n p l a n t tops once i t is m a d e a v a i l a b l e for t r a n s p o r t b y the roots.


100

BIOINORGANIC C H E M I S T R Y

II

T h e i n d i v i d u a l reactions affected b y i r o n stress c a n b e c o n s i d e r e d as regulated biochemical pathways, although regulation b y understood.

i r o n is

not

T h e m e c h a n i s m of i r o n a b s o r p t i o n a n d t r a n s p o r t i n v o l v e s

the release of h y d r o g e n ions b y the root, w h i c h lowers the p H of the root zone.

T h i s favors F e

3 +

s o l u b i l i t y a n d r e d u c t i o n of F e ' to

Fe .

3

2 +

" R e d u c t a n t s " are released b y roots or a c c u m u l a t e i n roots of p l a n t s t h a t are u n d e r i r o n stress. T h e s e " r e d u c t a n t s , " a l o n g w i t h F e


the root, r e d u c e F e

3 +

to F e , a n d F e 2 +

2 +

3 +

reduction by

c a n enter the root.

Ferrous iron

has b e e n d e t e c t e d t h r o u g h o u t the p r o t o x y l e m of the y o u n g l a t e r a l roots. The Fe

2 +

is p r o b a b l y k e p t r e d u c e d b y t h e " r e d u c t a n t " i n the root, a n d

i t m a y or m a y n o t h a v e e n t e r e d the root b y a c a r r i e r m e c h a n i s m . root-absorbed F e

2 +

The

is b e l i e v e d to b e o x i d i z e d to F e , c h e l a t e d b y c i t r a t e , 3 +

a n d t r a n s p o r t e d i n the m e t a x y l e m to the tops of the p l a n t for use. assume F e

2 +

measureable F e exudate (30,

We

is o x i d i z e d as i t enters t h e m e t a x y l e m b e c a u s e t h e r e is n o 2 +

there ( 13), a n d F e

citrate is t r a n s p o r t e d i n the x y l e m

3 +

31,32).

Effect of Iron Stress—Response Mechanism Biochemical Reactions

on Other

I n d u c e d i r o n stress alters b i o c h e m i c a l reactions m o r e i n iron-efficient than i n iron-inefficient plants, w h i c h makes the former more versatile t h a n the latter. L i s t e d b e l o w are some examples w h e r e this m a y occur. Nitrate Reductase A c t i v i t y . T h e r e are s i m i l a r i t i e s b e t w e e n i n d u c e d n i t r a t e reductase a c t i v i t y a n d i n d u c e d i r o n stress response.

In both,

b i o c h e m i c a l reactions are i n d u c e d , a n d a substrate is r e d u c e d ; N 0 N0

2

b y nitrate reductase a n d F e

response to i r o n stress.

to F e

3 +

2 +

3

to

by a reductant activated i n

C h e m i c a l reactions i n d u c e d b y i r o n stress i n -

creased the use of i r o n , a n d s i m u l t a n e o u s l y i n c r e a s e d n i t r a t e r e d u c t a s e a c t i v i t y i n roots ( F i g u r e 5 )

a n d i n tops o f iron-efficient t o m a t o .

i n d u c e d n i t r a t e reductase a c t i v i t y d e c l i n e d w h e n i r o n w a s m a d e

This avail-

a b l e to t h e p l a n t s . Use of Iron from F e

3+

Phosphate.

u s e d b y iron-efficient soybeans w h e n F e Fe

2 +

w a s d e t e c t e d i n s o l u t i o n as F e

bis(4-phenylsulfonic acid)-1,2,4,

2 +

Iron phosphate precipitate was 3 +

w a s r e d u c e d to F e

ferrozine

trizine]. 3 +

T h e iron-inefficient soybeans

3 +

acid)).

3 +

to F e , a n d 2 +

phosphate.

Use of Iron from F e E D D H A (Iron-ethylenediamine phenylacetic

(19). The

[Fe 3-(2-pyridyl)-5,6-

d e v e l o p e d i r o n chlorosis b e c a u s e t h e y d i d n o t r e d u c e F e t h e y c o u l d n o t use the i r o n f r o m F e

2 +

2 +

Iron-inefficient T 3 2 3 8 f e r

tomato

di(o-hydroxydeveloped

d e f i c i e n c y because i t c o u l d n o t a b s o r b i r o n f r o m F e E D D H A

(7).

iron In

contrast, t h e iron-efficient T 3 2 3 8 F E R t o m a t o u s e d i r o n f r o m F e E D D H A because it c o u l d reduce F e

3 +

to F e . C h a n e y et a l . (34) 2 +

showed that for



5.

Absorption and Transport of Iron by Phnts

BROWN

101

Fe, mg/l Figure 5. Nitrate reductase activity and nutri ent solution pH for 28-day-old T3238fer and T3238FER tomatoes grown for 8 days at vari ous levels of iron stress. In solutions lacking iron and where iron had been removed from the roots, iron stress developed in T3238FER tomato, and the pH of the nutrient solution decreased from 7.1 (day 2) to 4.35 (day 4). Nitrate reductase activity (NRA) increased in the roots from 2.8 (day 2) to 8.5 (day 4). No significant differences were noted between NRA of T3238fer and T3238FER roots when they did not respond to iron stress. *, signifi cantly different at 1% level according to Dun cans multiple range test. **, some iron was removed from roots with 21 μΜ NaEDDHA (ethyleneaiamine di( o-hydroxyphenylacetic acid)) before these plants were placed in the nutrient solution. p l a n t s t o use F e reduce F e

3 +

3 +

f r o m several F e

chelate to F e

2 +

3 +

chelates, i t w a s first necessary to

chelate. T h e l a t t e r u s u a l l y has a m u c h l o w e r

s t a b i l i t y constant t h a n t h e f o r m e r . Tolerance to H e a v y Metals.

I n m a n y p l a n t s , h e a v y metals i n d u c e

i r o n stress. T h e s e metals s e e m to interfere w i t h t h e i r o n stress-response m e c h a n i s m (13)

a n d i n this w a y cause i r o n chlorosis to d e v e l o p .

Plants

u n d e r these c o n d i t i o n s w i l l d i e unless t h e y c a n r e s p o n d to i r o n stress and make more iron available (35).

A d d i t i o n a l i r o n counteracts t h e effect

of the h e a v y m e t a l s . Zinc Stress Induction of Iron Uptake. F o r some u n e x p l a i n e d rea son, z i n c stress m a y i n d u c e s y m p t o m s s i m i l a r to, i f n o t the same, as i r o n stress (36).

I n these p l a n t s , i r o n u p t a k e m a y i n c r e a s e so m u c h t h a t i r o n


102

BIOINORGANIC

toxicity symptoms develop.

CHEMISTRY

II

A d d e d z i n c decreases the a b s o r p t i o n a n d

t r a n s p o r t of i r o n (13, 29) b y d e c r e a s i n g the efficiency of the i r o n s t r e s s response m e c h a n i s m s . Future

Needs

S o m e of the c h e m i c a l factors i n v o l v e d i n the m e c h a n i s m of i r o n absorption a n d transport i n plants have been established. These reactions Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 9, 2016 | http://pubs.acs.org Publication Date: June 1, 1977 | doi: 10.1021/ba-1977-0162.ch005

are g e n e t i c a l l y c o n t r o l l e d , w h i c h m a k e s i t p o s s i b l e to select o r d e v e l o p iron-efficient plants for use i n s o i l that causes i r o n stress. I n a d d i t i o n w e n e e d to k n o w : ( 1 ) H o w a n d w h e r e i r o n stress changes the m e t a b o l i s m o f the p l a n t to a c c e n t u a t e i r o n u p t a k e . ( 2 ) T h e source of h y d r o g e n ions r e l e a s e d b y roots. ( 3 ) T h e i d e n t i t y of "reductants" r e l e a s e d b y roots. 3 +

( 4 ) T h e reason y o u n g l a t e r a l roots are so effective i n r e d u c i n g F e . (5 ) How Fe

2 +

(6) Where F e

m o v e s i n the root. 2 +

is o x i d i z e d to F e

3 +

a n d c h e l a t e d b y citrate.

( 7 ) H o w h e a v y metals, arsenate, a n d a z i d e i n h i b i t c h e m i c a l reac tions i n d u c e d b y i r o n stress. ( 8 ) H o w the c h e m i c a l reactions i n d u c e d b y i r o n stress are r e l a t e d to n i t r a t e n u t r i t i o n a n d the a c t i v i t y of n i t r a t e reductase. ( 9 ) H o w iron from F e

3 +

is u s e d i n leaves.

( 10) H o w l i g h t affects i r o n use. A n u n d e r s t a n d i n g o f the b a s i c p h y s i o l o g y , b i o c h e m i s t r y , genetics, a n d nutrient element interactions involved i n iron nutrition w i l l con t r i b u t e t r e m e n d o u s l y to o u r u n d e r s t a n d i n g of p e r t i n e n t b i o l o g i c a l processes.

Literature Cited 1. Oertli, J. J., Jacobson, L., Plant Physiol. (1960) 35, 683. 2. Lindsay, W. L., Micronutrients Agri, Proc. Symp. (1972) 341-357. 3. Brown, J. C., Ambler, J. E., Chaney, R. L., Foy, D. D., Micronutrients Agri., Proc. Symp. (1972) 389-418. 4. Brown, J.C.,Adv. Agron. (1961) 13, 329. 5. Hyde, Β. B., Hodge, A. J., Kahn, Α., Birnstiel, M. L., J. Ultrastruct. Res. (1963) 9, 248. 6. Ambler, J. E., Brown, J.C.,Agron.J.(1974) 66, 476. 7. Brown, J.C.,Chaney, R. L., Ambler, J. E., Physiol. Plant. (1971) 25, 48. 8. Brown, J.C.,Holmes, R. S., Tiffin, L. O., Soil Sci. (1958) 86, 75. 9. Bell, W. D., Bogorad, L., McIlrath, W. J., Bot. Gaz. Chicago (1958) 120, 36. 10. Weiss, M.G.,Genetics (1943) 28, 253. 11. Wann, Ε. V., Hills, W. Α.,J.Hered. (1973) 64, 370. 12. Brown, J.C.,Holmes, R. S., Tiffin, L. O., Soil Sci. (1961) 91, 127. 13. Ambler, J. E., Brown, J.C.,Gaugh, H.C.,Agron. J. (1971) 63, 95. 14. Brown, J.C.,Ambler, J. E., Agron. J. (1973) 65, 311. 15. Diehl, Α., Smith, G. H., "The Iron Reagents," pp 41-56, G. Frederick Smith Chem. Co., Columbus, 1965.



5. BROWN

Absorption and Transport of Iron by Plants 103

16. Ito, T., Nielands, J. B.,J.Am. Chem. Soc. (1958) 80, 4645. 17. Walsh, B. L., Warren, R. A. J.,J.Bacteriol. (1968) 95, 360. 18. Byers, B. R., Lankford, C. E., Biochim. Biophys. Acta (1968) 165, 563. 19. Brown, J.C.,Agron. J. (1972) 64, 240. 20. Brown, J.C.,Ambler, J. E., Physiol. Plant. (1974) 31, 221. 21. Brown, J.C.,Chaney, R. L., Plant Physiol. (1971) 47, 836. 22. Rogers, C. H., Shive, J. W., Plant Physiol. (1932) 7, 227. 23. DeKock, P.C.,Morrison, R. J., Biochem. J. (1958) 70, 272. 24. Iljin, W. S., Plant Soil (1951) 3, 239. 25. Ibid. (1952) 4, 11. 26. Rhoades, W. Α., Wallace, Α., Soil Sci. (1960) 89, 248. 27. Schmid, W. E., Gerloff, G.C.,Plant Physiol. (1961) 36, 226. 28. Tiffin, L. O., Brown, J.C.,Plant Physiol. (1961) 36, 710. 29. Brown, J.C.,Tiffin, L. O., Plant Physiol. (1965) 40, 395. 30. Tiffin, L. O., Plant Physiol. (1966) 41, 510. 31. Ibid. (1966) 41, 515. 32. Ibid. (1970) 45, 280. 33. Clark, R. B., Tiffin, L. O., Brown, J.C.,Plant Physiol. (1973) 52, 147. 34. Chaney, R. L., Brown, J.C.,Tiffin, L. O., Plant Physiol. (1972) 50, 208. 35. Brown, J.C.,Jones, W. E., Commun. Soil Sci. Plant Anal. (1975) 6, 421. 36. Ambler, J. E., Brown, J.C.,Agron.J.(1969) 61, 41. RECEIVED July 26, 1976.


Genetically Controlled Chemical Factors Involved in Absorption and

Recommend Documents