Blue light does not inhibit nodulation in Sesbania rostrata

Abstract

Earlier, we reported that root nodulation was inhibited by blue light irradiation of Lotus japonicus. Because some legumes do not establish nodules exclusively on underground roots, we investigated whether nodule formation in Sesbania rostrata, which forms both root and "stem" nodules following inoculation with Azorhizobium caulinodans, is inhibited by blue light as are L. japonicus nodules. We found that neither S. rostrata nodulation nor nitrogen fixation was inhibited by blue light exposure. Moreover, although A. caulinodans proliferation was not affected by blue light irradiation, bacterial survival was decreased. Therefore, blue light appears to impose different responses depending on the legume-rhizobial symbiosis. ). 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. 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. KEYWORDSSesbania rostrata, a tropical legume, develops nodules on both root and stems in response to Azorhizobium caulinodans ORS571. 8,9 The crack entry infection process via adventitious root primordia on S. rostrata aerial stems, which leads to the initial formation of nodules, has been observed in the flooded roots of this species.10,11 Although root nodulation of L. japonicus is inhibited by white light, stem nodulation of S. rostrata is not inhibited under high intensity sunlight, which includes the blue component of the spectrum. Thus, we hypothesized that stem nodulation would not be inhibited by blue light, but that the subterranean roots might be affected. Because inhibition of root nodulation in L. japonicus by white light is actually caused by blue light perception, we investigated the effect of blue light irradiation on nodulation on S. rostrata underground roots in response to inoculation with A. caulinodans ORS571.To study the effect of light on root nodulation in S. rostrata, we irradiated roots with blue light supplied from above for 14 d and analyzed the nodulation process thereafter. Seeds were germinated on 0.8% (wt/vol) water agar for 3 d in the dark. ) in a vertical orientation with/without root shading in a 28 C growth chamber. For the unshaded plants, both the shoot and root were exposed to light, whereas when the root was shaded, only the shoot was exposed. Under these conditions, unshaded and shaded roots received approximately 60 mmol m ¡2 s ¡1 and approximately 5 ...

Differential light-dependent regulation of soybean nodulation by papilionoid-specific HY5 homologs

“…Root exposure to BL in inoculated plants inhibits nodulation in P. vulgaris 38 and L. japonicus, 40 but it does not affect nodulation in S. rostrata. 47 In this study, we chose to study the effects of shoot BL pretreatments on the nodulation of germinating seedlings because it is a growth response that involves changes in both nodulation and photomorphogenesis. Our results suggest that short-term BL pretreatment activates BL signaling in both shoots and roots, thereby inhibiting nodulation and hypocotyl elongation, while long-term BL pretreatment promotes photomorphogenesis and photosynthesis, thus stimulating nodulation and photomorphogenesis (Figures 1B and S1A).…”