On the efficiency of legume supernodulating mutants

Abstract

The supernodulating mutants of legumes lack the internal regulation of the number of symbiotic root nodules that harbour N2‐fixing nodule bacteria. On one hand, these mutants represent an efficient tool for dramatic increase in the degree of rhizobial symbiosis development. The trait of released nodulation is often associated with the desirable resistance of nodule initiation and functioning to the inhibition by ambient nitrate. On the other hand, the more intense and stable atmospheric nitrogen fixation of supernodulated plants is devalued by plant growth depression that results from the disproportion between the photosynthetic capacity of the shoot and the catabolic demands of symbiotic nodules. The deleterious effects of excessive nodulation can be neutralised or alleviated by a breeding strategy aimed at creating an ideotype of N2‐fixing legume. The growth depression can be diminished by the reduction in the nodule number typical for supernodulators, that is, 6–10‐fold of the wild type, to the level found permissive for the particular crop. This shift should be accompanied with breeding aimed at the increased photosynthetic capacity of the shoot. Forage varieties of legumes represent a reserve of high photosynthetic and shoot growth capacity, thanks to a long‐term breeding history for green biomass accumulation. Moreover, the deleterious effects of supernodulation are less perceived after introgression into the background of forage varieties in view of different criteria in their evaluation, such as nitrogen accumulation and biomass production per crop area unit. The growth of supernodulators can be further corrected by breeding for auxiliary traits such as long‐vine shoot architecture, a longer vegetation period and late flowering. The same strategy is applicable to the compensation for inherent pleiotropic changes in plant development, which are often associated with primarily symbiotic mutations. Supporting evidence for the efficiency of the described approach has already been reported.

Pea nodule gradients explain N nutrition and limited symbiotic fixation in hypernodulating mutants

“…Nodule number was positively correlated only to variables reflecting either (i) nodule biomass or its biomass proportion within the host plant or (ii) the N nutrition level (plant N concentration or NNI). Novak (2010) has recently suggested that the large increase in nodule number in hypernodulating mutants causes an imbalance between the demand of the nodulated root for carbon and the photosynthetic capacity of the host plant to sustain nodule growth and activity. Likewise, we have previously hypothesised that low nodule SNU of hypernodulating mutants could be accounted for by C competition with excessive nodule growth ).…”

“…The hypernodulating phenotype was never associated to a substantial increase in N uptake by the mutant plants as compared to their wild type. Instead, hypernodulation most often limited plant growth and productivity (Sagan et al 1993;Bhatia et al 2001;Novak 2010;Voisin et al 2013). Hence, most studies on hypernodulating mutants have reported negative pleiotropic traits related to carbon nutrition such as depressed shoot growth, shortened internodes or fasciation of the stem, and reduction of roots development and growth (e.g., pea : Postma et al 1988;Duc and Messager 1989;Sagan and Duc 1996;Krusell et al 2002;Bourion et al 2007; soybean: Matsunami et al 2004;Voisin et al 2013).…”

“…First, an internal regulation leads to an "auto-regulation of nodule number" (AON) by the host plant, through a systemic feedback repression of nodulation by pre-existing nodules (Reid et al 2011;Mortier et al 2012 for reviews). Deregulated mutants with a hypernodulating phenotype were isolated in several legume species (Novak 2010;Reid et al 2011). In the case of pea (Pisum sativum L.), hypernodulating mutants have been selected after mutagenesis and three genes have been shown to be involved in the regulation of nodulation, namely NOD3 (Postma et al 1988), SYM29, and SYM28 (Sagan and Duc 1996).…”

Improving Nitrogen Use Efficiency in Crops for Sustainable Agriculture

“…Legumes are defined by their unique flower structure, their pod, and the ability of 88% of the species examined so far to form atmospheric N 2 fixing nodules [79,80]. Legumes are only of second importance after grasses to humans, by contributing significantly to grain, pasture and forage, and forestry production [33,81].…”

Root system variability in common legumes in Central Europe

“…Legumes are known for their symbiotic relationship with Rhizobium s.l. bacteria, enabling them to utilize aerial N 2 , which is inaccessible to many other species (Garg & Geetanjali 2007;Novák 2010). Symbiotic nitrogen fixation makes legumes highly competitive especially in environments limited by mineral (NO − 3 and NH + 4 ) nitrogen and with an adequate phosphorus and potassium supply (Honsová et al 2007;Jackson et al 2008).…”