Tissue-specific expression of tomato Ribonuclease LX during phosphate starvation-induced root growth

Ribonuclease LX (RNaseLX) from tomato (Solanum lycopersicum L.) belongs to the RNase T2/S-RNase superfamily of plant endoribonucleases and this is a report on the characterization of the RNaseLX gene and its encoded protein as a member of the phosphate starvation response in tomato. RNaseLX gene sequences were cloned by a PCR-assisted approach. RNaseLX promoter sequences contained the conserved binding motif of the transcription factor PHR1 known to mediate phosphate starvation-dependent gene expression. The increase of RNaseLX transcript levels in roots during phosphate starvation correlated with high promoter activity in transgenic plants carrying a PromLX::uidA gene construct and pointed to transcriptional control of RNaseLX expression. Histochemical staining for beta-glucuronidase activity and immunodetection of RNaseLX protein revealed striking RNaseLX expression in main and lateral root tips of phosphate-starved transgenic plants, specifically in epidermal cells, as well as in lateral and adventitious root primordia. Induced RNaseLX expression in roots correlated with stimulated growth and elongation of primary and lateral roots during phosphate deprivation. Phosphate-starvation-induced RNaseLX transcript levels in roots were not modulated by auxin or ethylene. These data indicate that the role of intracellular RNaseLX in the phosphate starvation response is connected with specific RNA turnover processes at the root tip.