Phospholipids as plant growth regulators A. Keith Cowan Nutra-Park Inc., Suite 140, 3225 Deming Way, Middleton, WI 53562, USA; (e-mail: kcowan@nutrapark.com) Received 17 August 2005; accepted in revised form 27 November 2005 Key words: Lysophospholipids, Phospholipids, Plant growth regulators Abstract In this paper the potential to use phospholipids and lysophospholipids as plant growth regulators is discussed. Recent evidence shows that phospholipids and phospholipases play an important signalling role in the normal course of plant development and in the response of plants to abiotic and biotic stress. It is apparent that phospholipase A (PLA), C (PLC) and D (PLD), lysophospholipids, and phosphatidic acid (PA) are key components of plant lipid signalling pathways. By comparison, there is very little information available on the effect of exogenously applied phospholipids on plant growth and development. This paper serves to introduce phospholipids as a novel class of plant growth regulator for use in commercial plant production. The biochemistry and physiology of phospholipids is discussed in relation to studies in which phospholipids and lysophospholipids have been applied to plants and plant parts. Implicit in the obser- vations is that phospholipids impact the hypersensitive response and systemic acquired resistance in plants to improve crop performance and product quality. Based on published data, a scheme outlining a possible mode of action of exogenously applied phospholipids is proposed. Introduction Phospholipids are a major and vital component of all biological membranes and play a key role in processes such as signal transduction, cytoskeletal rearrangement, and in membrane trafficking. For example, phospholipids act as co-factors for membrane-localized enzymes involved in signalling cascades and facilitate protein–lipid and protein– protein interaction in which protein activation is achieved by either a lipid-induced conformational change or spatial rearrangement of the proteins within the lipid bilayer. Phospholipids and related catabolites (e.g. lysophospholipids) can change the physical properties of membranes to increase or decrease ion flux and membrane transport, vesicle formation, and endo- and exocytosis. Furthermore, genetic studies using Arabidopsis thaliana confirm that changes in phospholipid homeostasis pro- foundly affect plant growth and development. Over-accumulation of phosphatidylinositol-4,5- bisphosphate (PI4,5P 2 ) and intositol-1,4,5-phos- phate (IP 3 ) is a characteristic of the sac9 mutants which show a constitutive stress response (Williams et al. 2005), a reduction in levels of phosphatidyl- glycerol (PG) in the pho1 mutant causes ultra- structural alterations in thylakoid organization (Ha¨ rtel et al. 1998), and impairment of root hair tip growth in the srh1 (cow1) mutant is due to loss of an essential PI transfer protein (Bo¨hme et al. 2004). In recent years, the role of phospholipids in plant growth and development and the response of plants to biotic and abiotic stress has concentrated largely on the signalling mechanisms involved Plant Growth Regulation (2006) 48:97–109 Ó Springer 2006 DOI 10.1007/s10725-005-5481-7