Mycorrhizal development in a low nodulating pea mutant

Abstract

Summary• E107 is a pleiotropic mutant of Pisum sativum (pea) characterized by a decreased ability to form nodules. Its colonization by the arbuscular mycorrhizal fungus Glomus aggregatum is reported, and the mycorrhizal phenotype compared with the nodulation phenotype.• Four cleared lateral roots from 21-d-old plants were chosen randomly, and their lengths scanned for any fungal structures. Colonization success was determined by counting the numbers of extraradical hyphae, and calculating epidermal entry and cortical entry. Graft experiments were performed to establish which organ regulates mycorrhiza-formation.• Two blocks to infection were obvious, at the root surface, and at the interface between the epidermis and the outermost cortical cell layer. Once the fungus breached this interface, it spread within the cortex and formed normal arbuscules. The mutant phenotype was controlled by the shoot.• The E107 mycorrhizal phenotype was designated as app + , low pen, low coi. The development of the fungal infection was similar to that of rhizobial infection and under the same shoot control. The phenotype of the mutant E107 strongly supports the hypothesis that mycorrhiza formation and nodulation are regulated by the same processes.

A model for the development of the rhizobial and arbuscular mycorrhizal symbioses in legumes and its use to understand the roles of ethylene in the establishment of these two symbioses

“…Bonfante et al (2000) further studied these mutants and reported the LjSym4-1 and LjSym4-2 phenotypes. The LjSym4-1 phenotype is very similar to that of the brz pea mutant (Resendes et al 2001), except in LjSym4-1, a root surface block is not evident. The LjSym4-2 mutant exhibits a much stronger block to epidermal exit in that AM do not form because hyphae never exit the epidermis (Bonfante et al 2000).…”

“…Even in wild-type pea, numerous IT abort in the base of the epidermal cell (Guinel and LaRue 1992). Additional evidence for tight regulation of both nodulation and AM fungal colonization at this point is given by the brz mutant, in which the parallel between the two plant-microbe interactions is striking (Resendes et al 2001). In each symbiosis, there is a similar reduction in the number of infections, a block at the epidermis-cortex interface, and a few successful infections and the two phenotypes are controlled by the shoot.…”

“…Recently, Genre and Bonfante (2002) proposed that LjSym4 is critical in establishing the proper rearrangement of the Csk in the epidermis; its correct expression would allow the AM fungus to cross the epidermal layer without any obstruction. Thus, we can conclude with certainty that the epidermis, especially at its interface with the cortex, is a strong checkpoint in AM colonization (Geurts et al 1997;Bonfante et al 2000;Senoo et al 2000;Resendes et al 2001). …”

“…Indirect evidence is given by brz, a low-nodulating pea mutant that has a third less rhizobial infections (Guinel and LaRue 1992) and appressoria (Resendes et al 2001) than its parent 'Sparkle'. Neither AVG nor silver (Fig.…”

“…For the IT to form and expand, the plant cell has to coordinate secretion of matrix, plasma membrane, and cell wall components (Rae et al 1992). The breaching of the outer cortex by the IT appears to be one of the most important checkpoints in the development of this symbiosis (e.g., Guinel and LaRue 1992;Resendes et al 2001) (also see below).…”