Discussion

Generally the factors which inhibit growth in acid soil may be summarized as follows:

1. The effects of toxic ions such as Al or Mn;

2. The effects of shortage of macro elements such as P, Ca, Mg, K etc.;

3. The effects of low pH on the soil microorganisms which are important to plant growth.

Basic Acidic Basic Acidic

» m r

(b)

-

- - —S» -

■ T

rfr--'^

» T w. ' ** *

• . —- . »

• *Jf *

Fig 27.1. Identification of sulfuric acid responsive protein. (a) Sulfuric acid treatment (pH2.0), (b) Control.

Fig. 27.2. Amino acid sequence comparison of sulfuric acid responsive protein 28kDa and those of some legumes. (a) Sulfuric acid responsive protein 28kDa, (b) Pisum sativum,2 (c) Phaseolus vulgaris.3

Phaseolus v.

AMFEl-p

AMFE2-P

Phaseolus V.

AMFEl-p

Phaseolus V.

AMFEl-p

AMFE2-P

1 : MALAPSKVSPFSGFSLSDGVGAVRNPTCSVSLSFLNKKVGSRNLGVSASTVPLTG^

1: ------------------------------------------------------

1: -------------------------------------------------------,1

IFEP IFEP IFEP

FEEVKI FEEVKt FEEVKI

lELXVPTAGQVSLARQYYADECESAINEQINVEYNASYVYHSLFAYFDRDNVAL 120 ¡VLAIPMSPQVSMARQSYADECEAAINEQINVEYNVSYVYHSLFAYFDRDNIAL 65 JMSLVPDVPQVSLARRNFSDECEAAINEQINVEYNVSYVYHAMFAYFDRDNVAL 6 5

121: KGFARXFRESSEEEREHAEKLMKYQNTRGGRWLHPIKNVPSEFEHVEKGDALYAMELAL 180 66: KGFAKFFKESSEEEREHAEKLMKYQNIRGGRWLHPIKAPPSEYEHPEKGDALYAMELAL 125 66: KGLAKFFRDSSEEERDHAEKLMEYQNKRGGRVTLQSIVMPLSEFDHDEKGDALYAMELAL 125

Phaseolus V. AMFEl-p AMFE2-p

181: SLEKLVNEKLRSVHSVADRNKDPQLADFIESEFLSEQVEAIKKISEYVAQLRMVGKGHGV 240 126: ALEKLTNEKLLHVHSVAARNNDPQMTDFIESEFLAEQVEAIKKIAEYVAQLRRVGKGHGV 185 126: SLEKLTNEKLLNLHSVASKNNDAQLADFIENEFLGEQVESIKRISEYVAQLRRVGKGHGV 185

Phaseolus V. AMFE1 -p AMFE2 -p

WHFDQSLLHDGHAA

( a ) 1 : MALTS SKFSSVFLSSIAGDSLKKPISTSTNLSV AL SATSPSLSLAPKGSGGRNLRVFAAGTEN PTÄLTGVIFEPFEEVKKDVLAIPMSPQ 90

(b) 1: MALSSSK-FSSF-SGF---SL—SPVSG-NGVQKPC—FCDLR-VGEKWGSRKFRV-SATT-AP—LTGVIFEPFEEVKKDYLAVPSVPL 75

(C) 1: MALAPSK-VSPF-SGF---SL—SDGVG-AVRHPTC—SVSLSFLMKKVGSRNLGV-SAST-VP—LTGVIFEPFEEVKKEELXVPTAGQ 7 6

(a) 91: VSMARQSYADECEAAir

(b) 7 6: VSLARQNFADECESVIÍ ( C) 77: VSLARQYYADECESAII

JINVEYWVSYVYHSLFAYFDRDNIALKGFAKFFKESS] II IERIII 5INVEYNASYVYHSLFAYFDRDNVALKGFAKFFKESS1 1 -R.: II JINVEYMASYVYHSLFAYFDRDNVALKGFARXFKESS] II IERI II

.EKLMKYQNIRGGRWLHPIKAPPSEYEH 180 .EKLMKYQNTRGGRWLHPIKDVPSEFEH 165 .EKLMKYQNTRGGRWLHPIKHVPSEFEH 166

PEKGDALYAMELALAL VEKGDALYAMELALSL VEKGDALYAMELALSL

ÍLTNEKLLHVHSVAARNNDPQMTDFIESEFLAEQVEA1KKIAEYVAQLRMVGKGHGVWHFDQRLLEENGHA— 2 68 ÍLTNEKLLNVHSVAERNNDLEMTHFIEGEYLAEQVEAIKKISEYVAQLRRVGKGHGVWHFDQRLLHGVHGA— 253 ÍLVNEKLRSVHSVADRNKDPQLADFIESEFLSEQVEAIKKISEYVAQLRMVGKGHGVWHFDQSLLHDGHAA— 254

Fig. 27.4. Comparison of amino acid sequence between sulfuric acid responsive protein and ferritin precursors of some legumes. (a) AMFE1-1, (b) Pisum sativum,4 (c) Phaseolus vulgaris.5 Underline: Transit peptide part; Iron-binding residues.

From the results of the previous studies, the mechanism producing acid resistance was considered to be somewhat as follows: for example, absorption of toxic ions by organic acid secreted from roots, absorption of toxic substances by mucus secreted from roots, increase in soil pH by substances secreted from roots. Recently, the effect of organic acid on acid resistance was vigorously analyzed. Kojima reported that an Al tolerant carrot cell line secretes a similar amount of citric acid as that from roots, and it forms a chelate compound with Al ions to decrease Al toxicity.7

There have been many studies on isolating stress resistance genes. For example, the heat shook proteins, the proteins induced by drought and the specific proteins for infection have been analyzed. But, there are few studies on the acid soil resistance. As described before, the mechanism that gives acid resistance is very complicated. Factors cooperate with each other, and might produce acid resistance in the plants. In this report, it has been revealed that ferritin was induced by sulfuric acid treatment of the roots of Acacia mangium. Its full length cDNA was isolated; its size was about 1.1 kbp. The putative molecular weight is 24 kDa, but the actual molecular weight calculated from electrophoresis was 28 kDa. This difference of molecular weight might be caused by modification of the protein after translation.

The iron storage protein ferritin is widely distributed in living organisms. Plant ferritin exists mostly in chloroplasts; it forms large molecular subunits of about 440-550 kDa. Its

Fig. 27.6. Tolerance of the transformed plants against excessive iron. 1: Non-transformed tobacco, 2-10: Transformed strains of tobacco.

function is the storage of iron, and the supply of iron to heme proteins. The ferritin cDNA of Acacia mangium contains an in-frame coding sequence with the characteristics of a transit peptide for plastid targeting, as reported for other plant ferritin cDNAs. In this experiment, ferritin cDNA was isolated from roots. Ferritin might be accumulated in plastids. In previous studies in suspension cells of Phaseolus vulgaris4 and Glycin max,8 ferritin was increased by the addition of iron. It was reported that ferritin might act for the detoxification of excessive iron.9 Further, researchers working within the electric power industry have shown that plants acquired tolerance to excessive iron by inducing the ferritin gene. Our results also support these results. Considering the relationship between ferritin and treatment with sulfuric acid, it was supposed that ferritin might be induced by the increase in free iron caused by soil acidification. In this case, through the detoxification of iron, ferritin played a part in acid resistance in Acacia mangium. In fact, in a tropical soil with acid sulfate, excessive iron is included.

It has been reported that there are some isoforms of ferritin in maize, and that their expression pattern was changed by the kinds of stress.10 By RT-PCR, it was revealed that two kinds of ferritin also exist in Acacia mangium. It is guessed that the expression of ferritin isoforms is changed by the kinds of stress, as in maize. Therefore, the ferritin induced by a stress may be involved in the resistance to each stress. So, we are considering that the ferritin proteins might play an important role in the detoxification of acid stress. Further, experiments will be made in the future to reveal the relationship between ferritin and acid resistance by analyzing the expression pattern of ferritin.

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