In BPS1 Downregulated Roots, the BYPASS1 Signal Disrupts the Induction of Cortical Cell Divisions in Bean-Rhizobium Symbiosis.

Manoj-Kumar Arthikala,Miguel Lara,Kalpana Nanjareddy

doi:10.3390/genes9010011

Abstract

BYPASS1 (BPS1), which is a well-conserved gene in plants, is required for normal root and shoot development. In the absence of BPS1 gene function, Arabidopsis overproduces a mobile signalling compound (the BPS1 signal) in roots, and this transmissible signal arrests shoot growth and causes abnormal root development. In addition to the shoot and root meristem activities, the legumes also possess transient meristematic activity in root cortical cells during Rhizobium symbiosis. We explored the role of Phaseolus vulgaris BPS1 during nodule primordium development using an RNA-interference (RNAi) silencing approach. Our results show that upon Rhizobium infection, the PvBPS1-RNAi transgenic roots failed to induce cortical cell divisions without affecting the rhizobia-induced root hair curling and infection thread formation. The transcript accumulation of early nodulin genes, cell cyclins, and cyclin-dependent kinase genes was affected in RNAi lines. Interestingly, the PvBPS1-RNAi root nodule phenotype was partially rescued by exogenous application of fluridone, a carotenoid biosynthesis inhibitor, which was used because the carotenoids are precursors of BPS1 signalling molecules. Furthermore, we show that the PvBPS1 promoter was active in the nodule primordia. Together, our data show that PvBPS1 plays a vital role in the induction of meristematic activity in root cortical cells and in the establishment of nodule primordia during Phaseolus-Rhizobium symbiosis.

Highlights

The most important class of plant pigments, carotenoids, are abundant isoprenoid-derived molecules that are mainly C40 tetraterpenoids with a series of double bonds [1,2]
Among the PvBPS1 genes, a slight but not significant difference was observed in the transcript levels between PvBPS1.1 and PvBPS1.2 under Rhizobium symbiotic condition. These results suggest that PvBPS1.1 and PvBPS1.2 are expressed in various organs of the common bean plant and that their expression increases under P. vulgaris-R. tropici symbiotic conditions
The promoter activity was detected in central tissues of nodules expressing PvBPS1.1 or PvBPS1.2 promoter-GUSA fusions (Figure 2E,F). These results show that both PvBPS1.1 and PvBPS1.2 promoters are expressed in nodule primordia and in mature nodules of P. vulgaris

Summary

Introduction

The most important class of plant pigments, carotenoids, are abundant isoprenoid-derived molecules that are mainly C40 tetraterpenoids with a series of double bonds [1,2]. Carotenoid biosynthesis occurs in the plastids, where carotenoids are incorporated into the light-harvesting and photosynthetic reaction centre complexes. In these complexes, carotenoids serve to both absorb light energy and dissipate excess energy (photo-protection) [3,4,5]. Auxins and cytokinins have been widely studied as classical long-distance signalling molecules in various aspects of plant development [10,11,12,13]. In this context, CCD7 and CCD8 are shown to be associated with the synthesis of the hormone strigolactone (SLs), a root-derived signal that

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