EFFECTS OF ACUTE HEAT STRESS DURING ANTHER AND POLLEN DEVELOPMENT IN LYCOPERSICON ESCULENTUM CV MICRO-TOM

The exposure of plants to extreme temperatures affects their geographic distribution and their yield quality and quantity (1). One of the most sensitive process to high temperature is plant reproduction, particularly the male apparatus (2). Thanks to its small size, rapid growth and easy transformation, Lycopersicon esculentum cv Micro- Tom has been proposed as one of the preferred variety of tomato plants to carry out molecular research (3). Although it is widely used as model organism, its use is quite recent and little is known about its response to heat stress. Plants at three flower bud stages: meiotic stage (2 mm long), microspore stage (4 mm long) and mature stage (6 mm long) were stressed with high temperature (42 °C for 3 hours) to study the effects on mature anthers. Investigations by light microscopy were carried out with three different stains: Alcian Blue, Toluidine Blue O and Periodic Acid Schiff. The only remarkable difference in morphological features was the absence of starch in cells of anthers treated at microspore stage. Pressman (4) reported that in tomato high temperature inhibited starch deposition in developing pollen, either through a decrease in the availability of assimilates or through the impairment of the activities of enzymes involved in starch biosynthesis or in sucrose hydrolysis, namely invertase and sucrose synthase (SuS). The immunolocalization evidenced the absence of the SuS enzyme in mature anthers treated at microspore stage, confirming that probably the HS interfered with the presence of SuS and starch. Scanning electron microscope observations revealed several alterations in shape and size of pollen grains. Pollen grains from anthers treated at the meiotic stage were reduced to collapsed exine coat; the majority of pollen grains from anthers treated at the microspore stage was crushed, whereas most of pollen grains treated at the mature stage was round in shape. Pollen viability and germinability tests showed that only pollen grains of mature treated anthers had a viability percentage similar to the control; the germination percentage of pollen grains of anthers treated at all developmental stages was extremely low. Molecular analysis on two heat shock factors (HSFA2 and HSFB3a) highlighted an attempt of HS response by treated anthers, especially for HSFA2, which showed a significant increase of expression after HS exposure. The severe physiological alterations caused by HS irreversibly affected the male apparatus of Micro-Tom plants, thus impairing its reproductive performance.