Reduced mycorrhizal colonization (rmc) tomato mutant lacks expression of SymRK signaling pathway genes

Abstract

Arbscular mycorrhizal (AM) symbiosis is widely distributed in the plant kingdom, unlike rhizobial symbiosis that is observed only in four orders of the eurosid dicots.1 AM symbiosis involves provision of mineral nutrients to the plant by the fungus, which in turn derives carbon compounds from the plant.2 Other benefits of the symbiosis to plants include improved water relations and tolerance to some plant diseases.3 The fungal hyphae penetrate the root epidermis, spread inter-cellularly in the cortex and form arbuscules in the root cells of the inner cortex. Arbuscules are highly branched hyphal structures that are separated from the plant cytoplasm by a perifungal membrane. 4 They are the main sites where nutrient exchange between the symbiotic partners occurs and their formation signifies the establishment of functional symbiosis.Molecular events occurring during AM symbiosis have been studied in legumes, which also show an elaborate signaling pathway for establishing nodulation symbiosis.5 Two genes, NFR1 and NFR5 encoding receptor-like serine/threonine kinases with LysM domains, are involved in nodulation (Nod) factor perception in Lotus japonicus.6 Several downstream components of the Nod factor signaling cascade include the leucine-rich-repeat receptor kinase SYMRK, which is known to be involved in AM symbiosis besides nodulation symbiosis and is thought to act near the junction of fungal and rhizobial signaling cascades. 7Activation of SYMRK causes a transient increase in intracellular calcium levels. Downstream components of this signaling pathway include a calcium/calmodulin dependent protein kinase (CCAMK ) and a protein CYCLOPS, whose function is notComparison of the expression of 13 genes involved in arbuscular mycorrhizal (am) symbiosis was performed in a wild type tomato (Solanum lycopersicum cv. 76r) and its reduced mycorrhizal colonization mutant rmc in response to colonization with Glomus fasiculatum. Four defense-related genes were induced to a similar extent in the mutant and wild type am colonized plants, indicating a systemic response to am colonization. Genes related to nutrient exchange between the symbiont partners showed higher expression in the am roots of wild type plants than the mutant plants, which correlated with their arbuscular frequency. a symbiosis receptor kinase that is involved in both nodulation and am symbiosis was not expressed in the rmc mutant. the fact that some colonization was observed in rmc was suggestive of the existence of an alternate colonization signaling pathway for am symbiosis in this mutant.

Important innate differences in determining symbiotic responsiveness in host and non-hosts of arbuscular mycorrhiza

“…Group 1 consisted of genes showing high evolutionary constraints, which may influence the ability of plant species to become an AM-host. This group comprised of only two known candidate genes; SYMRK 54 , 55 and CCaMK 56 , 57 . These genes showed significant differences in amino acid composition of the analysed protein sequences in a host- and non-hosts specific manner in all the in silico analyses including MEME, CODEML and DIVERGE.…”

Systemic jasmonic acid modulation in mycorrhizal tomato plants and its role in induced resistance against Alternaria alternata

“…Colonisation measurements were done using roots from three independent plants 52 days post sowing, by clearing root pieces in 10% hot KOH solution followed by staining with 0.5% trypan blue, as described in Nair & Bhargava (). Extent of colonisation was measured in 30 root segments of 2‐cm length per plant using the computer program ‘Mycocalc’ (Trouvelot et al .…”