Nodule endophytes and effect of exogenous application of 6-benzylamino- purine (6-BAP) and Indole-3-acetic acid (IAA) on nodulation

Abstract

Rhizobium legume symbiosis is a highly complex signaling process involving exchange of signals from both the symbiotic partners. Despite of this highly specialized mechanism of root hair entry of Rhizobium, presence of non-rhizobial bacteria within the root nodules have been reported. These endophytic bacteria residing the plant tissues have often been shown to promote plant growth. In order to explore the presence and diversity of endophytes within the root nodules and their role in plant growth, the test plants viz. Vigna radiata (Moongbean) and Arachis hypogaea (Groundnut) were selected. The result indicated presence of non-rhizobial endophytes within the root nodules with their unequal distribution in terms of Gram nature. Moongbean root nodules bore more Gram positive endophytes while groundnut nodules bore more Gram negative endophytes. The reason for such varying distribution of endophytes within root nodules remains to be studied. A total of 46 endophytes were isolated which were biochemically characterized and screened in vitro for their plant growth promoting traits like production of Indole acetic acid (IAA), siderophore, phosphate solublization and antifungal activity. Among the Moongbean root nodule (MBRN), 82% of endophytes produced IAA, 70% produced siderophore, 43% solublized phosphate, 33% displayed antifungal activity and 7% produced chitinase. Among the Groundnut root nodule (GNRN), all endophytes produced siderophore, 92% produced IAA, 15% solublized phosphate, 23% displayed antifungal activity and 39% produced chitinase. To determine the effect of these endophytes on seed germination of Vigna radiata, seedling vigor assay was performed. On the basis of multiple plant growth promoting activities, selected endophytes were evaluated for their effect on growth of Vigna radiata. These endophytes individually as well as in combination with native Rhizobium, significantly increased the seed germination, root length and dry weight of the host plant Vigna radiata, but have no significant effect on non-host plant Arachis hypogaea. Root nodules are the site where nitrogen fixation occurs by the Rhizobium. The number of nodules is directly related to the plant growth and development and these number of nodules is correlated with the number of lateral roots, length of longest lateral root and length of primary root. The effect of plant growth regulators namely 6-BAP and IAA on growth and development of various root parameters was studied. Results indicated that at low concentration of 6-BAP (0.5 μM) all parameters significantly increased and 2 same positive results were obtained at 10 ppm concentration of IAA while decrease in growth in all parameters was observed higher than 15 μM concentration of 6-BAP except in number of lateral roots which were more sensitive to hormone treatment and reduced higher than 5 μM concentration of 6-BAP and IAA also showed reduction in the number of lateral roots at 10 ppm concentration. Infection thread(IT) moves from root hairs straight to the cortex. Effect of 20uM 6-BAP on movement of infection thread was studied. At higher concentration of 6-BAP (20 μM) the morphology and movement of I.T was abnormal. At 10 ppm of IAA no abnormalities in I.T morphology and movement were observed