Desiccation tolerance in the streptophyte green alga Klebsormidium: The role of phytohormones

The origin of phytohormones has been a puzzling question for decades; however, with the availability of large transcriptomic and genomic datasets of the early branching streptophytic green alga Klebsormidium this question can be addressed from a fresh perspective. Klebsormidium has recently been examined extensively for physiological and structural reactions to desiccation or cold temperatures, natural factors in soil crust living algae. Ecological influences have been made responsible for fine scaled structuring of genotypes and differentiation of cryptic species. But how do these organisms sense their changing environments? In this addendum article, we explore our own data set from a transcriptomic study of severe desiccation stress in Klebsormidium crenluatum. The cells were desiccated for 2.5 h under monitored conditions over silicagel at »10% relative humidity. The relative water content of the desiccated cells was 6.54 § 1.89%. For the molecular analysis we established a high-coverage reference transcriptome database which contained 24,183 contigs with a mean sequence length of 1,327b (N50 D 1,462). This database was used to evaluate which phytohormone pathways are present in Klebsormidium and might be involved for cellular response to desiccation stress. Desiccation is well studied in embryophytes, and the cytokinin, ethylene and abscisic acid (ABA) signaling pathways have been implicated in stress response. Given these facts we wondered whether ABA and/or ethylene and/or cytokinin signaling are involved in desiccation tolerance in K. crenulatum, and searched for the most similar transcripts to the K. flaccidum proteins reported to be putative orthologues of these 3 (ABA, cytokinin, ethylene) plant phytohormones signaling components. Here, we propose that at least 3 major signaling pathways for land plant hormone response are functional in Klebsormidiophyceae. Based on our transcriptomic data of severe desiccation stress auxin mediated signal transduction seem to be missing and in the case of jasmonic acid (JA) only the receptor JAR1 was found, but the further steps were absent in the analyzed K. crenulatum transcriptome.