Analysis of the Purslane (Portulaca oleracea L.) Transcriptome under Drought Stress

Purslane (Portulaca spp.) is a xerophyte that can survive severe drought conditions and is found in many arid regions around the world. In addition to its extreme drought tolerance, some purslane accessions have displayed tolerance to high salinity, heavy metals, and other toxins in soil. Purslane shoots and leaves also contain an abundance of phytochemicals with well known human health benefits, including phytosterols, omega-3 fatty acids, and antioxidants, thus purslane consumption is gaining popularity. Limited genomic information is available for purslane, although information behind its novel adaptations to stress and biosynthesis of phytochemicals would prove extremely valuable. Therefore, this study was conducted as a first step in characterizing the purslane transcriptome in response to drought. Our previous studies characterized drought tolerance of 9 different purslane accessions during germination and seedling development. The accession Tokombiya showed exceptional drought tolerance and recovery and was thus selected for transcriptome analysis using 454-sequencing. Water was withheld from 20-day-old seedlings, while untreated control seedlings were regularly watered.  After approximately 30 days without water, seedlings began to wilt and were subsequently collected for analysis every 2 days for two weeks. cDNAs derived from RNA extracted from entire seedlings were pooled into early (week 1) and late (week 2) responses and untreated control (week 1 & 2) and were analyzed using 454-sequencing. Approximately 8,000 contigs were assembled per library and compared to sequences in public databases; around 60% of contigs showed significant similarity to known proteins. Genes involved in stress responses constituted around 4% of the assigned genes in well-watered control seedlings and 5% in both early and late responses to drought. Interestingly, 70% of the stress related genes were related to heat or drought stress in all libraries. Several genes involved in pathways and processes of interest, including melatonin biosynthesis and arsenic transport, were identified and analyzed using qRT-PCR.