The Brt (branched roots) and Lrt (long roots) genes control the development of roots in peas (Pisum sativum L.)

Sidorova, K.K., Shumny, V.K., Vlasova, E.Yu.,      Inst. of Cytology and Gen., Russian Acad. of Sci.

Glianenko, M.N. and Mishchenko, T.M.                                                             Novosibirsk, Russia

Until very recently only three mutants in pea (Pisum sativum L.) were known of with altered root systems [1-3]. Following exposure of seeds of cultivar ‘Rondo’ to 0.15% ethyl methanesulfonate, three mutant phenotypes, К8а, К17а, and К18а, were identified as having altered roots.  К8а and К17а have strongly branching roots, whereas К18а is noted for a very long root (Fig. 1).  Here, we provide a summary of the genetic analysis that was performed on these mutants and the results of experiments performed to determine levels of endogenous auxin and gibberellin in the roots of the mutants and the original cultivar.

      Each of the mutants was crossed to the original cultivar, and the phenotypically similar К8а and К17а were crossed for an allelism test (Table 1).

      The results of the genetic analysis suggest that these are monogenic recessive mutants. Furthermore, crossing К8а x К17а revealed allelism. The mutant gene controlling the extensively branched root phenotype seen in К8а and К17а was denoted by brt (branched roots). The mutant gene producing the long root phenotype in К18а was designated by lrt (long roots).

The mutant lines were examined for altered nodulation response using Rhizobium leguminosarum (strain 250А).  К8а and К17а were found to form few or no nodules.  К18а displayed a relatively normal nodulation phenotype, nodulating well at low or moderate nitrate levels and developing few nodules in the presence of high nitrates.

      Auxin and gibberellin levels were determined at the onset of flowering, both with and without Rhizobium leguminosarum, under greenhouse conditions. The plants were grown in pots. The substrate used was bloating clay and a standard mineral solution. The light regime was 16/8 hrs (day/night); temperature 20-21^oC at day and 12-13^oC at night. Auxin and gibberellin levels were determined using techniques described elsewhere [3].

Table 1. Segregation in F₂ hybrid progenies.

            Auxin levels were reduced in both mutants compared to those found in Rondo (Fig. 2).  This reduction occurred either with or without the presence of Rhizobium leguminosarum.  The presence of the bacterium reduced root auxin concentration in all genotypes tested.  The long-rooted К18а was found to have a high level of gibberellin relative to the Rondo control and Kita (Fig. 2).  This significant increase in the level of gibberellin was observed both with and without Rhizobium leguminosarum. 

Fig. 2. Auxin and gibberellin levels in the roots of the symbiotic mutants and the original cultivar, Rondo, at flowering.

1.      Blixt, S.  1970.  Pisum Newsletter. 2: 11-12

2.      Murfet, I.C. and Reid, J.B.  1993.  In: Casey, R. and Davies, D.R. (eds.).  Peas—Genetics, Molecular Biology and Biotechnology. CABI, Wallingford, UK, pp 165-216.

3.      Tsyganov, V.E., Pavlova, Z.B., Kravchenko, L.V., Rozov, S.M., Borisov, A.Y., Lutova, L.A. and Tikhonovich, I.A.  2000.  Ann. Bot. 86: 975-981.

4.      Kholodar, A.V., Sidorova, K.K. and Shumny, V.K.  2001.  Russian Journal of Genetics 37(11): 1517-1521.