Abstract
Peanut root invasion by Bradyrhizobia is through a crack entry, which is different from many other legumes applying an infection thread entry in root hair. Understanding the role of root hair in the crack entry of Bradyrhizobia invasion of peanut root and subsequent peanut nodulation would facilitate improvement of biological nitrogen fixation in cultivated peanut. The objective of this study was to investigate the involvement of root hair in Bradyrhizobial invasion of peanut. Seedling roots of a nodulating peanut cultivar were observed for root hair emergence, its life span, and nodule formation at the base of the lateral roots with and without rhizobia inoculation for 14 days after germination (DAG). Scanning electron microscopy (SEM) was utilized to observe rhizobia accumulation at lateral roots at 24 hours after inoculation (HAI) before the emergence of root hair. Root hair emerged at 7 DAG with or without rhizobia inoculation. Two variations of rosette hair (RoH) were observed, the transient-thin RoH had life span of 3 days after root hair emergence and the thick and densely distributed RoH type stayed till the time of nodule emergence (9 days after inoculation). The lateral root devoid of root hair at the top 2 cm region was found to produce nodules. The SEM observation of seedling roots at 24 HAI showed that Bradyrhizobia invaded the roots at epidermis, protoplasm of cortical cell, and cortical cells of the main root near the newly emerged lateral root in the absence of RoH. The observations validated that root hair is not required in the Bradyrhizobia invasion of peanut root in the crack entry mode. Results from this study provided important morphological information for the hypothesis of close relationship between RoH and peanut nodulation for further genetic study of crack entry mechanism and signaling pathway of symbiosis between Bradyrhizobia and peanut.
Highlights
IntroductionLegumes can form a symbiotic relationship with rhizobia to fix nitrogen in root nodules, where the rhizobia reduce atmospheric nitrogen into ammonia (a process known as biological nitrogen fixation-BNF) to supply the hosts’ nitrogen need and in return to get nutrients from the host plant
Legumes can form a symbiotic relationship with rhizobia to fix nitrogen in root nodules, where the rhizobia reduce atmospheric nitrogen into ammonia to supply the hosts’ nitrogen need and in return to get nutrients from the host plant
Our observation showed that rosette hair (RoH) started to appear at 7 days after germination (DAG) of both inoculated and un-inoculated peanut roots at the base of the lateral roots when the main roots were approximately 6 cm for both transient-thin root hair and densely populated RoH (Figure 1(b) and Figure 2(a), respectively)
Summary
Legumes can form a symbiotic relationship with rhizobia to fix nitrogen in root nodules, where the rhizobia reduce atmospheric nitrogen into ammonia (a process known as biological nitrogen fixation-BNF) to supply the hosts’ nitrogen need and in return to get nutrients from the host plant. Rhizobia must firstly interact with and enter the host root cells. The root epidermis, as a barrier, must be overridden by rhizobia for infection [1]. Different ways of root invasion have been described in the symbiotic relationship of legumes and rhizobia, which is under the genetic control of host plants [2]. The rhizobia engulfed in the ITs penetrate the root hair through ITs into the cells, which were inductively formed by actively dividing cortical cells below the site of bacterial infection in the epidermis [6]. Rhizobia are released in the root cortical nodule primordium cells to differentiate into bacteroids. The nodule primordium cells will eventually form nodules for subsequent bacteroids N2 fixation
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