No abstract

Phospholipases D (PLD) and C (PLC) hydrolyze the phosphodiesteric linkages of the head group of membrane phospholipids. PLDs and PLCs in plants occur in different forms: the calcium-dependent phospholipid binding domain-containing PLDs (C2-PLDs), the plekstrin homology and phox homology domain-containing PLDs (PX/PH-PLDs), phosphoinositide-specific PLC (PI-PLC), and non-specific PLC (NPC). They differ in structures, substrate selectivities, cofactor requirements, and/or reaction conditions. These enzymes and their reaction products, such as phosphatidic acid (PA), diacylglycerol (DAG), and inositol polyphosphates, play important, multifaceted roles in plant response to abiotic and biotic stresses. Here, we review biochemical properties, cellular effects, and physiological functions of PLDs and PLCs, particularly in the context of their roles in stress response along with advances made on the role of PA and DAG in cell signaling in plants. The mechanism of actions, including those common and distinguishable among different PLDs and PLCs, will also be discussed.

Rapid activation of phospholipase D (PLD), which hydrolyzes membrane lipids to generate phosphatidic acid (PA), occurs under various hyperosmotic conditions, including salinity and water deficiency. The Arabidopsis thaliana PLD family has 12 members, and the function of PLD activation in hyperosmotic stress responses has remained elusive. Here, we show that knockout (KO) and overexpression (OE) of previously uncharacterized PLDa3 alter plant response to salinity and water deficit. PLDa3 uses multiple phospholipids as substrates with distinguishable preferences, and alterations of PLDa3 result in changes in PA level and membrane lipid composition. PLDa3-KO plants display increased sensitivities to salinity and water deficiency and also tend to induce abscisic acid-responsive genes more readily than wild-type plants, whereas PLDa3-OE plants have decreased sensitivities. In addition, PLDa3-KO plants flower later than wild-type plants in slightly dry conditions, whereas PLDa3-OE plants flower earlier. These data suggest that PLDa3 positively mediates plant responses to hyperosmotic stresses and that increased PLDa3 expression and associated lipid changes promote root growth, flowering, and stress avoidance.

Summary The activation of phospholipase D (PLD) produces phosphatidic acid (PA), a new lipid messenger implicated in cell growth and proliferation, but direct evidence for PLD and PA promotion of growth at an organismal level is lacking. Here we characterized a new PLD, PLDε, and show that PLDε plays a role in promoting Arabidopsis growth. PLDε is mainly associated with the plasma membrane and is the most permissive of all PLDs tested in activity requirements. Knockout (KO) of PLDε decreases, whereas overexpression (OE) of PLDε enhances root growth and biomass accumulation. The level of PA was higher in OE, but lower in KO than in wild-type plants, and suppression of PLD-mediated PA formation by alcohol alleviated the growth-promoting effect of PLDε. OE and KO of PLDε had the opposite effect on lateral root elongation in response to nitrogen (N). Increased expression of PLDε also promoted root hair elongation and primary root growth at severe N deprivation. The results suggest that PLDε and PA promote organismal growth and play a role in N response. The lipid signaling process may play a role in translating the membrane sensing of nutrient status to increasing plant growth and biomass production.

ABSTRACT

SummarySeed aging decreases the quality of seed and grain and results in agricultural and economic losses. Alterations that impair cellular structures and metabolism are implicated in seed deterioration, but the molecular and biochemical bases for seed aging are not well understood. Ablation of the gene for a membrane lipidhydrolyzing phospholipase D (PLDa1) in Arabidopsis enhanced seed germination and oil stability after storage or exposure of seeds to adverse conditions. The PLDa1-deficient seeds exhibited a smaller loss of unsaturated fatty acids and lower accumulation of lipid peroxides than did wild-type seeds. However, PLDa1-knockdown seeds were more tolerant of aging than were PLDa1-knockout seeds. The results demonstrate the PLDa1 plays an important role in seed deterioration and aging in Arabidopsis. A high level of PLDa1 is detrimental to seed quality, and attenuation of PLDa1 expression has the potential to improve oil stability, seed quality and seed longevity.

Phospholipase Dalpha1 (PLDalpha1) has been shown to mediate the abscisic acid regulation of stomatal movements. Arabidopsis plants deficient in PLDalpha1 increased, whereas PLDalpha1-overexpressing tobacco decreased, transpirational water loss. In the early stage of drought, the decrease in water loss was associated with a rapid stomatal closure caused by a high level of PLD in PLDalpha1-overexpressing plants. However, in the late stage of drought, the overexpressing plants displayed more susceptibility to drought than control plants. PLDalpha1 activity in the overexpressing plants was much higher than that of control plants in which drought also induced an increase in PLDalpha1 activity. The high level of PLDalpha1 activity was correlated to membrane degradation in late stages of drought, as demonstrated by ionic leakage and lipid peroxidation. These findings indicate that a high level of PLDalpha1 expression has different effects on plant response to water deficits. It promotes stomatal closure at earlier stages, but disrupts membranes in prolonged drought stress. These findings are discussed in relation to the understanding of PLD functions and potential applications.

Wrinkled1 (WRI1) belongs to the APETALA2 transcription factor family; it is unique to plants and is a central regulator of oil synthesis in Arabidopsis. The effects of WRI1 on comprehensive lipid metabolism and plant development were unknown, especially in crop plants. This study found that BnWRI1 in Brassica napus accelerated flowering and enhanced oil accumulation in both seeds and leaves without leading to a visible growth inhibition. BnWRI1 decreased storage carbohydrates and increased soluble sugars to facilitate the carbon flux to lipid anabolism. BnWRI1 is localized to the nucleus and directly binds to the AW-box at proximal upstream regions of genes involved in fatty acid (FA) synthesis and lipid assembly. The overexpression (OE) of BnWRI1 resulted in the up-regulation of genes involved in glycolysis, FA synthesis, lipid assembly, and flowering. Lipid profiling revealed increased galactolipids monogalactosyldiacylglycerol (MGDG), digalactosyldiacylglycerol (DGDG), and phosphatidylcholine (PC) in the leaves of OE plants, whereas it exhibited a reduced level of the galactolipids DGDG and MGDG and increased levels of PC, phosphatidylethanolamide, and oil [triacylglycerol (TAG)] in the siliques of OE plants during the early seed development stage. These results suggest that BnWRI1 is important for homeostasis among TAG, membrane lipids and sugars, and thus facilitates flowering and oil accumulation in B. napus.