Guanosine Tetraphosphate (ppGpp) Content: A Potential Marker of Tomato (Solanum lycopersicum L.) Seed Quality

Plant homologs of bacterial RelA/SpoT proteins (RSH) metabolise guanosine tetraphosphate and guanosine pentaphosphate (ppGpp and pppGpp, respectively), which are hyperphosphorylated nucleotides (referred to as [p]ppGpp or 'alarmones of the stringent response'). These nucleotides regulate chloroplast transcription, photosynthesis, plant growth and stress responses. However, it is not yet clear at which particular stages of plant development they are produced, nor which of them are affected or regulated by these nucleotides (nor in what way), especially in plants outside the Brassicaceae family. We characterised the quality of tomato seeds collected in 1981, 1988 and 1998 and stored for 39, 32 and 22 years, respectively. This characterisation was based on the application of a germination test, RNA integrity analysis (using capillary electrophoresis of total RNA, followed by calculation of the RNA integrity number) and (for the 1981 and 1988 batches only) a tetrazolium test. Using the data obtained from the electrophoresis, we analysed the levels of different RNA fractions in the tested seed batches (and in seedlings). Next, we measured ppGpp content in the seeds by applying, for the first time for this purpose in plants, ultra-high-performance liquid chromatography-hydrophilic interaction liquid chromatography (UHPLC-HILIC). Interestingly, we found that the seeds we determined to be of low quality contained significantly fewer of these nucleotides than seeds of high quality. This suggests that high ppGpp content in tomato seeds potentially indicates longevity, warranting further studies with a broad range of seed material of various ages and qualities.