Optical tweezers for the micromanipulation of plant cytoplasm and organelles

Laser tweezers, often known as optical tweezers or optical traps, permit the capturing and micromanipulation of microscopic particles along X, Y and Z axes using the radiation pressure generated by a focused laser beam, normally in the infrared region of the spectrum. For trapping to be successful, the object to be captured must have a higher refractive index than that of its surrounding medium and forces generated by individual traps must be in the piconewton range [1]. Single optical traps are generally used to capture particles in the micron range, although multiple traps have been developed that can be utilized to move larger objects. Optical traps are commonly used in single-molecule techniques where, for example, they can be used to measure forces exerted on polymer beads coated with motor proteins. Thus, for instance, the mechanism of kinesin walking on microtubules or myosin on actin can be probed [[2] and [3]]. At the other end of the spectrum the optical traps are often used for the mechanical stimulation of cells [4] or manipulation of whole cells such as germinating fungal conidia [5]. What is, however, very apparent is that the full potential for using optical tweezers to manipulate organelles within living cells has yet to be fully exploited, with the majority of reports working at the whole-cell or single-molecule levels. Ashkin and Dziedzic [6•] were perhaps the first to show that cytoplasmic particles can be manipulated in vivo, when they demonstrated the pulling cytoplasm strands across vacuoles of onion epidermal cells and displacement of Spirogyra chloroplasts. Redirection of the growth of fungal hyphae by manipulation of the Spitzenkörper has been elegantly demonstrated through lateral displacement of this tip organelle [7]. Recently, and often in combination with confocal microscopy, it has been demonstrated that optical traps can be a very powerful tool in unravelling cytoplasmic dynamics. Here we discuss some of the few applications that have revealed new physical aspects of cytoplasmic and organelle dynamics in plant cells.