During hydroponic screening of tomato strains (L. esculentum) for increased capacity to absorb P under low-P stress (97 umol P), one strain, PI 121665 (USDA Plant Introduction accession from Canada), was identified as extremely efficient in P uptake. This strain, referred to as cottony root, was distinguished by exceptionally high numbers of root hairs. No other strain from among 200 strains observed in nutrient culture displayed a similar phenotype. For example, PI 102716 was similar to cottony root in P absorption efficiency, expressed as mg P absorbed per g of root dry weight, but did not display the cottony root phenotype. Expression of cottony root is inhibited by increasing the P concentration to 400 umol. Cottony root bears no resemblance to any of the tomato root mutants reported by Zobel (2.3,4).
Staining attempts to locate mycorrhizal fungi on the roots of cottony root failed to detect their presence. The entire root system of cottony root displayed an extremely dense proliferation of root hairs while only sporadic root hairs were present on roots of non-cottony. Mature root hairs of cottony root ranged from 1.8 to 3.1 mm in length and from 9 to 10 um in diameter. There were approximately 180 hairs per mm of root measured in a segment of maximum root hair proliferation occurring approximately 1 cm from lateral root tips and extending basipetally for approximately 3 cm.
The F 1 between cottony root and non-cottony produced the same root phenotype as non-cottony indicating a recessive mode of inheritance for cottony root (Table 1). A lack of cytoplasmic inheritance was evident in the data from reciprocal F1`s (data not shown). Segregation ratios for root type were approximately 1:1 and 3:1 non-cottony to cottony root in test cross and F populations, respectively, indicating a single recessive gene conditioning the cottony root phenotype. Using Yates correction factor, significant deviations from expected ratios were not observed (1). Similar results were obtained in a repeat experiment (data not shown). Supporting evidence of the proposed pattern of inheritance is shown in the segregation of several F3 families derived from non-cottony F2 individuals (Table 2). In accordance witb rules of tomato gene nomenclature, the symbol crt is being proposed for cottony root.
Table 1. Inheritance of cottony root phenotype in the cross PI 102716 X PI 121665.
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No. plants observed
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Expected Chi
Generation Non-cottony Cottony ratio^y^ square
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PI 102716(P1) 10 0 1:0 __
PI 121665(P2) 0 11 0:1 __
F1 (P1 x P2) 7 0 1:0 __
BC1 (P1 x P2)xP1 17 0 1:0 --
BC1 (P1 xP2 )xP2 16 20 1:1 .26
F2 (P1 xP2 ) 45 14 3:1 .01
^Y^Assuming segregation of single locus.
Table 2. Segregation of cottony root phenotype in several F3
lines descended from non-cottony F2 individuals.
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No. plants observed
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Expected Chi
F3 family Non-cottony Cottony ratio^y^ square
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No. 31 27 6 3:1 .49
No. 34 33 1 1:0 .01
No. 35 26 8 3:1 (.01
No. 41 33 0 1:0 --
No. 66 27 4 3:1 1.81
No. 88 30 3 3:1 3.64
^Y^Assuming segregation of single locus.
References:1. Steel, R. G. D. and J. H. Torrie. 1980. Principles and procedures of statistics. McGraw-Hill, New York.
2. Zobel, R. W. 1968. Linkage and phenotype studies with lz-3. Tomato Genetics Coop. Rpt. 18:46-47.
3. Zobel, R. W. 1971. Root mutants of the tomato. Tomato Genetics Coop. Rept. 21:42.
4. Zobel, R. W. 1975. The genetics of root development. pp. 261-275. In: J. Torrey and D. Clarkson (eds.). The Development and Function of Roots. Academic Press, London.