Comparative physiological and transcriptional analysis of two grapevine rootstocks in response to magnesium deficiency

Magnesium (Mg) is an essential mineral macro-nutrient for plants and its deficiency affects productivity and quality in many agricultural crops. Surprisingly, only in the last decades researchers have focused their attention to the plant physiological and biochemical response to this nutritional disorder. In addition, very limited studies are available on the molecular mechanisms involved in tolerance to Mg shortage. In grapevine Mg deficiency often occurs in acidic soil and/or in the presence of high amount of potassium (K). The shortage seems to be related to the increase in late season bunch stem necrosis (LBSN) with substantial crop loss. Although grapevine is one of the most economically important fruit crop in the world, no literature is available on the physiological and molecular aspects involved into Mg deficiency stress in this plant species. Interestingly, field observations revealed that some grapevine rootstock genotypes could exhibit tolerance or susceptibility to Mg shortage. In order to better understand the molecular mechanisms involved in response to Mg deficiency, the present work investigated the transcriptional and metabolic responses to short- and long term Mg starvation in hydroponically grown microcuttings of two grapevine rootstocks showing different tolerance to Mg shortage. In particular, 1103 Paulsen (Vitis berlandieri x Vitis rupestris) was classified as tolerant whilst SO4 was defined susceptible (Vitis berlandieri x Vitis riparia). Analysis of growth and physiological parameters, performed after 4 and 14 days of starvation, confirmed the differentially tolerance in the two genotypes, as observed in field. The susceptible SO4 after 14 days of Mg deprivation showed typical symptoms induced by Mg deficiency, such as chlorosis and soluble sugars accumulation in leaves. In addition, a significant alteration in root fresh weight and shoot/root ratio (S/R) was observed in this susceptible rootstock. The comparison of root transcriptional profiles allowed the identification of differentially expressed transcripts putatively involved in the tolerance to Mg deficiency. Transcripts involved into response to stress signaling mediated by receptor-like kinases and lignin biosynthesis were positively regulated in SO4 and negatively regulated in 1103P, both after short- and long-term Mg starvation. In contrast, transcripts related to oxidative stress responses and cell wall remodeling exhibited opposite expression profile being positively and negatively regulated in tolerant 1103P and in susceptible SO4 genotypes, respectively. Analysis of metabolites performed with gas chromatography mass spectrometry (GC-MS) showed an increase and a decrease in content of metabolites involved in oxidative stress response in root tissues of 1103P and SO4, respectively, after 14 days of treatment. These results agree with the transcriptional behavior of two rootstocks.