Osmoregulation and nutritional relationships between Orobanche foetida and faba bean

Abstract

The present study aims at comparing the phloem composition of the tolerant XBJ90.03-16-1-1-1 and the susceptible Bachaar genotypes and the impact of the faba bean genotype on the levels of the major solutes and invertase activities in the parasite Orobanche foetida. In comparison to Bachaar, the XBJ90.03-161-1-1 genotype limited the growth of orobanche tubercles under in vitro conditions. The limited growth was due to low soluble invertase activity, low osmotic potential of the infected roots and the organic nitrogen deficiency of the host phloem sap. The faba bean genotype did not affect the osmoregulation process of O. foetida. Among the organic solutes, stachyose, hexoses, starch and free amino acids, mainly asparagine and aspartate were highly accumulated in orobanche. However, asparagine/aspartate, glutamine/glutamate, alanine, serine, gamma amino butyric acid, stachyose, sucrose were identified as the main organic components in the host phloem exudates. The key role of the enzymes α-galactosidase, asparagine synthetase and aspartate oxaloglutarate aminotransferase in the utilization of the host solutes is proposed in O. foetida parasitizing faba bean.

Keywords: Vicia faba; amino acid; asparagine; broomrape; invertase; osmolarity; stachyose; sucrose.

Figure 1

Osmolarity (mosmol/L) of O. foetida tubercles (stage 4) and non infected and infected roots of the two faba bean genotypes. Data are means ± SE (n = 5, p = 0.05). * Arrows represent amplitude of osmolarity difference between infected root and tubercle. Values for the same parameter sharing the same letters did not differ significantly at 0.05 (Duncan test).

Figure 2

Schematic representation of the conversion of the host-derived sugar in O. foetida (stage 4).

Figure 3

Schematic representation of the conversion of the host-derived amino acids in O. foetida (stage 4). The hypothesis of the implication of GDH activities was not taken into consideration in this model. ASP, Aspartate; ASN, Asparagine; GLU, Glutamate; GLN, Glutamine; ALA, Alanine; SER, Serine; GABA, Gamma Amino N butyrique acid; αCG, α-cétoglutarate; PYR, Pyruvate; SUCC, Succinate; AOA, Oxaloacetic acid; AS, Asapargine synthetase; GOT, Glutamate oxaloacetate aminotransferase.