Studies On The Mechanism Of The Decrease Of Phosphatidylglycerol Content In Leaves Of Higher Plant Under Phosphate Deficiency

Phosphatidylglycerol(PG) is the sole phospholipid existing in thylakoid membrane and thylakoid PG has specific structure, with palmitic acid (16:0) or Δ3-trans-hexadecenoic(16:1t) esterified at sn-2 position of glycerol. It has been demonstrated by many researchers that PG plays an important role in structure and function of thylakoid membranes. Several articles have reported that under phosphate deprivation conditions, the lipid composition of the thylakoid membrane obviously changed by decreasing the relative proportions of PG. Researcher generally considered that because the phosphor was diverted from PG biosynthesis to more critical metabolic processes such as nucleic acid biosynthesis, the biosynthesis of PG was restricted and therefore the relative content of PG decreased. However,there is no experimental evidence to sustain the point, and whether other factors may contribute to the PG loss still remain unclear so far. In the present paper, we compared the changes of the relative content of PG and the activity of PG hydrolases induced by phosphate deficiency in wheat and tobacco leaves at different developmental stage, meanwhile, the main hydrolysis products of exogenous PG degraded by the crude extracts of phosphate-deficient leaves, and the effects of n-butanol and neomycin sulfate on the in vitro enzyme reaction were also analyzed to investigate the reasons for the decrease of PG in leaves of wheat and tobacco plants grown in phosphate-deficient conditions. When the first and third leaves just fully developed at days 6 and 18, respectively, phosphate deprivation induced a decrease of PG content in third leaves but in first leaves not, while PG hydrolases activities in these two sets of phosphate-deficient leaves were similarly low. These results suggested that before the leaves fully developed, PG biosynthesis inhibition was the major factor resulting in PG loss, and the inhibition would get more severe in the later-formed leaves. By comparing the change of PG content and the PG hydrolase activity of wheat leaves at different developmental stage, we found that when the first leaves just fully developed, the relative amount of PG in phosphate-deficient leaves was similar to that of control, and that the difference between their PG hydrolase activity was also slight. However, with the advancing age, great losses of PG accompanied by a rapidly increased PG hydrolase activity were detected in phosphate-deficient leaves. These results suggested that the activity of PG hydrolases was dramatically enhanced in phosphate-deficient leaves, acceleration of PG deterioration was one crucial reason for the decrease of PG content in leaves of wheat plants grown in phosphate-deficient conditions. Phospholipases are key enzymes catalyzing lipid hydrolysis in plants. In higher plants, they are grouped in families, designated phospholipase A, C or D (PLA, PLC or PLD), depending upon their site of cleavage. An array of products, both polar and non-polar, can be produced by the individual or coordinate action of these phospholipases on phospholipids. In the present study, the main breakdown products of PG degraded by the crude enzyme extract of phosphate-deficient leaves of wheat plants were phosphatidic acid (PA), diacylglycerol (DAG) and free fatty acid (FFA). Inclusion of the reported inhibitors of PA generation via PLD pathway, n-butanol, to the in vitro enzyme reaction mixture caused reductions of PA and DAG levels in the enzyme reaction mixture, indicating a possible role for PLD activity in PG degradation. On the other hand, neomycin sulfate, a non-specific PLC activity inhibitor, also inhibited DAG and FFA generation. These results suggested that PLD and PLC are both responsible for PG degradation in the leaves of phosphate-deficient wheat. In addition, the effects of phosphate deficiency on the relative content of PG, the activity of PG hydrolase and the transcriptional levels of PLC,PLDα,PLDβ and PAT-1 gene in the young and old leaves of tobacco plants grown in phosphate-deficient and phosphate-sufficient conditions were analyzed as well. The results indicated that the decrease of PG content in tobacco leaves resulted from reduced PG biosynthesis and enhanced PG degradation together; PLC and PLD activities were involved in PG degradation in older leaves of tobacco plants grown inphosphate-deficient conditions. Remarkably, PLC,PLDα,PLDβ transcript accumulation paralleled the PG hydrolase activity profile, suggesting that the enzymatic activity measured in phosphate-deficient leaves maybe resulted from expression of these genes. This also means that the activity of PG hydrolase is probably mainly regulated at the transcriptional level. On the contrary, phosphate deficiency had no effect on the transcriptional level of PAT-1 gene.