A Legume TOR Protein Kinase Regulates Rhizobium Symbiosis and Is Essential for Infection and Nodule Development

Abstract

The target of rapamycin (TOR) protein kinase regulates metabolism, growth, and life span in yeast, animals, and plants in coordination with nutrient status and environmental conditions. The nutrient-dependent nature of TOR functionality makes this kinase a putative regulator of symbiotic associations involving nutrient acquisition. However, TOR's role in these processes remains to be understood. Here, we uncovered the role of TOR during the bean (Phaseolus vulgaris)-Rhizobium tropici (Rhizobium) symbiotic interaction. TOR was expressed in all tested bean tissues, with higher transcript levels in the root meristems and senesced nodules. We showed TOR promoter expression along the progressing infection thread and in the infected cells of mature nodules. Posttranscriptional gene silencing of TOR using RNA interference (RNAi) showed that this gene is involved in lateral root elongation and root cell organization and also alters the density, size, and number of root hairs. The suppression of TOR transcripts also affected infection thread progression and associated cortical cell divisions, resulting in a drastic reduction of nodule numbers. TOR-RNAi resulted in reduced reactive oxygen species accumulation and altered CyclinD1 and CyclinD3 expression, which are crucial factors for infection thread progression and nodule organogenesis. Enhanced expression of TORregulated ATG genes in TOR-RNAi roots suggested that TOR plays a role in the recognition of Rhizobium as a symbiont. Together, these data suggest that TOR plays a vital role in the establishment of root nodule symbiosis in the common bean.In plants, growth occurs as a result of cell growth coupled with cell proliferation and cell expansion caused by nutrient availability. Unlike animals, in plants, postembryonic growth is sustained by specialized zones (shoot meristems and root apices) and depends upon the availability of nutrients that are present externally or generated by various cellular processes. These local and systemic nutrient and energy sources are integrated, and information is communicated to metabolic regulators through signaling networks to control cellular processes.The evolutionarily conserved target of rapamycin (TOR) is a Ser/Thr kinase that acts as a master regulator of metabolism and growth. In mammals, TOR exerts translational control of cell proliferation (Dowling et al., 2010), tumorigenesis, metastasis (Hsieh et al., 2012), and insulin signaling (Hsu et al., 2011). Loss-of-function TOR mutations in Saccharomyces cerevisiae and Schizosaccharomyces pombe (Barbet et al., 1996;Weisman and Choder, 2001), Drosophila melanogaster (Zhang et al., 2000), Caenorhabditis elegans (Long et al., 2002), and mice (Murakami et al., 2004) resulted in embryo lethality and other growth-associated problems.Unlike other eukaryotes, knowledge about TOR in plants is limited due to a lack of molecular and biochemical assays for endogenous TOR kinase activity.

Phylogenetic analysis of target of rapamycin (TOR) kinase gene of some selected plants species

“…Our results are also supported by another investigation carried out by Dobrenel et al (2011) in which protein sequences of TOR kinase from plants, animals, yeasts, algae and moss were aligned and monocots and dicotsgrouped in separate clusters. The plants species Nanjareddy et al (2016) during the phylogenetic analysis of bean TOR gene which confi rmed that this gene belongs to the legume group and is closely related to the G. max and M. truncatula TOR genes which showed the highly conserved nature of TOR gene at family level. In our studies all the plants species including in present investigation are clustered together according to their family showed the similar nature of gene as mentioned by Nanjareddy et al (2016).…”

“…The plants species Nanjareddy et al (2016) during the phylogenetic analysis of bean TOR gene which confi rmed that this gene belongs to the legume group and is closely related to the G. max and M. truncatula TOR genes which showed the highly conserved nature of TOR gene at family level. In our studies all the plants species including in present investigation are clustered together according to their family showed the similar nature of gene as mentioned by Nanjareddy et al (2016).…”

“…This result also revealed that the relatedness of these two families were higher than the other family included in the present studies. While, subgroup I-b comprised 7 by availability of nutrients and energy source that are present outwardly or generated by various cellular processes (Nanjareddy et al 2016). However, so far, very few secrets have been revealed on mechanism that how this information is perceived and transduce into coherent growth and developmental decisions.One of the most important pathways that are found in all eukaryotes is the one related to the target of rapamycin (TOR) protein kinase.…”

Celebrating 20 Years of Genetic Discoveries in Legume Nodulation and Symbiotic Nitrogen Fixation

“…Knockdown of PvPI3K suppressed multiple autophagy genes, including PvBECLIN. Independent knockdown of PvBECLIN and another autophagy-related kinase gene, PvTOR (TARGET OF RAPAMYCIN), phenocopied the PvPI3K-silenced lines and led to a drastic reduction in nodule number (Estrada-Navarrete et al, 2016;Nanjareddy et al, 2016).…”

INCREASING NODULE SIZE1 Expression Is Required for Normal Rhizobial Symbiosis and Nodule Development

“…Nodule development from the initiation of nodule primordia through maturation and the formation of bacteroids is essential for N 2 fixation (Oke and Long, 1999). Although several genes, small peptides, and noncoding RNAs involved in nodule development in different legume species have been characterized (Boualem et al, 2008;Imin et al, 2013;Djordjevic et al, 2015;Mohd-Radzman et al, 2016;Nanjareddy et al, 2016;Cai et al, 2017;Di Giacomo et al, 2017;Hobecker et al, 2017), the regulatory elements of nodule development still need to be explored further.…”