Pearl millet is a cereal crop known for its high tolerance to drought, heat and salinity stresses as well as for its nutritional quality. The molecular mechanism of drought tolerance in pearl millet is unknown. Here we attempted to unravel the molecular basis of drought tolerance in two pearl millet inbred lines, ICMB 843 and ICMB 863 using RNA sequencing. Under greenhouse condition, ICMB 843 was found to be more tolerant to drought than ICMB 863. We sequenced the root transcriptome from both lines under control and drought conditions using an Illumina Hi-Seq platform, generating 139.1 million reads. Mapping of sequenced reads against the foxtail millet genome, which has been relatively well-annotated, led to the identification of several differentially expressed genes under drought stress. Total of 6799 and 1253 differentially expressed genes were found in ICMB 843 and ICMB 863, respectively. Pathway and gene function analysis by KEGG online tool revealed that the drought response in pearl millet is mainly regulated by pathways related to photosynthesis, plant hormone signal transduction and mitogen-activated protein kinase signaling. The changes in expression of drought-responsive genes determined by RNA sequencing were confirmed by reverse-transcription PCR for 7 genes. These results are a first step to understanding the molecular mechanisms of drought tolerance in pearl millet and lay a foundation for its genetic improvement.

Abscisic acid is a stress-related phytohormone that has roles in dehydration and rehydration. In Arabidopsis (Arabidopsis thaliana), two genes that inactivate abscisic acid, CYP707A1 and CYP707A3, are rapidly up-regulated upon rehydration. The factors that regulate CYP707A1/3 are not well characterized. We expressed a bZIP protein, VIP1, as a green fluorescent protein fusion protein in Arabidopsis and found that the nuclear localization of VIP1 was enhanced within 10 min after rehydration. A yeast one-hybrid assay revealed that the amino-terminal region of VIP1 has transcriptional activation potential. In a transient reporter assay using Arabidopsis protoplasts, VIP1 enhanced the promoter activities of CYP707A1/3. In gel shift and chromatin immunoprecipitation analyses, VIP1 directly bound to DNA fragments of the CYP707A1/3 promoters. Transgenic plants expressing VIP1-green fluorescent protein were found to overexpress CYP707A1/3 mRNAs. The time course of nuclear-cytoplasmic shuttling of VIP1 was consistent with the time courses of the expression of CYP707A1/3. These results suggest that VIP1 functions as a regulator of osmosensory signaling in Arabidopsis.

A high-affinity K⁺transporter PutHKT2;1 cDNA was isolated from the salt-tolerant plant Puccinellia tenuiflora. Expression of PutHKT2;1 was induced by both 300 mM NaCl and K⁺-starvation stress in roots, but only slightly regulated by those stresses in shoots. PutHKT2;1 transcript levels in 300 mM NaCl were doubled by the depletion of potassium. Yeast transformed with PutHKT2;1, like those transformed with PhaHKT2;1 from salt-tolerant reed plants (Phragmites australis), (i) were able to take up K⁺ in low K⁺ concentration medium or in the presence of NaCl, and (ii) were permeable to Na⁺. This suggests that PutHKT2;1 has a high affinity K⁺-Na⁺ symport function in yeast. Arabidopsis over-expressing PutHKT2;1 showed increased sensitivities to Na⁺, K⁺, and Li⁺, while Arabidopsis over-expressing OsHKT2;1 from rice (Oryza sativa) showed increased sensitivity only to Na⁺. In contrast to OsHKT2;1, which functions in Na⁺-uptake at low external K⁺ concentrations, PutHKT2;1 functions in Na⁺-uptake at higher external K⁺ concentrations. These results show that the modes of action of PutHKT2;1 in transgenic yeast and Arabidopsis differ from the mode of action of the closely related OsHKT2;1 transporter.