Blue Light Perception by Both Roots and Rhizobia Inhibits Nodule Formation in Lotus japonicus

Abstract

*The e-Xtra logo stands for "electronic extra" and indicates that twelve supplementary figures and one supplementary table are published online.

Seeing the Light: The Roles of Red- and Blue-Light Sensing in Plant Microbes

“…Light represses nodulation in many legumes, and this repression is influenced by the quantity and quality of light (106). For the symbiosis between Mesorhizobium loti and Lotus japonicus, nodule numbers are reduced on blue light-exposed roots as compared to on shaded roots, but not on red light-exposed roots (101). Reduced nodulation correlates with fewer infection threads, possibly due to blue light inhibiting bacterial growth and reducing cell numbers for infection thread initiation.…”

Blue light does not inhibit nodulation in Sesbania rostrata

“…1a and b). However, although the root nodule number of L. japonicus unshaded roots was drastically decreased compared with that of the shaded roots (shaded; 4.11 § 0.20 vs. unshaded; 1.76 § 0.21) 4 , the number of nodules developed on S. rostrata unshaded roots was slightly, but significantly increased, compared with the shaded roots (Fig. 1c).…”

“…We recently reported that root nodulation of Lotus japonicus, a model legume, was inhibited by white light irradiation, and that the inhibition of nodulation is caused by blue light perception by both the host plant roots and the rhizobia. 4 Moreover, blue light exposure to L. japonicus roots resulted in reduced nodule weight and decreased acetylene reduction activity (ARA). Higher plants are known to develop avoidance mechanisms, such as root negative phototropism 4,5 and the shade avoidance syndrome, 7 to survive under conditions of biotic or abiotic stress.…”

“…4 Moreover, blue light exposure to L. japonicus roots resulted in reduced nodule weight and decreased acetylene reduction activity (ARA). Higher plants are known to develop avoidance mechanisms, such as root negative phototropism 4,5 and the shade avoidance syndrome, 7 to survive under conditions of biotic or abiotic stress. We concluded that inhibition of nodulation by light is one of several avoidance responses that plants such as L. japonicus use to conserve energy especially under environmental stress because nitrogen fixation is an energy-intensive process.…”

“…4 To investigate whether blue light exposure influences rhizobial growth, A. caulinodans (1.0 £ 10 6 cells per ml) were cultured under blue light. We cultured the bacteria without shaking at 28 C in either blue light (90 mmol m ¡2 s ¡1 ) or in the dark because it was difficult to make certain that the cells in each tube would be exposed to sufficient light (90 mmol m ¡2 s ¡1 ) under shaking condition.…”