HORMONE RESPONSES TO SHORT DAILY UV-B IRRADIATION IN TOMATO PLANTS: WHAT HAPPENS IN THE UPPER AND BELOW-GROUND ORGANS?

UV-B radiation is known to influence many aspects of plant physiology and biochemistry through a signalling route triggered by UV-B perception by the specific photoreceptor UVR8. However, most of these studies were mainly addressed to investigate the behaviour of the above-ground organs since they are directly exposed to the solar UV-B radiation (Ulm et al., 2004; Brown & Jenkins, 2008). Little is instead known about the root responses, despite UV-B radiation affecting the morphology of this organ as well. This suggests a perceiving mechanism also in the roots and/ or a shoot-to-root signalling transmission (Tong et al., 2008; Leasure et al., 2009). The present research aimed to understand whether low doses of UV-B radiation (1.19 KJ/m 2 per day, 15 min a day, using narrow-band lamps) applied above-ground influenced the hormonal balance in both leaves and roots of Micro-Tom tomato (Solanum lycopersicum L.) plantlets. Twenty-five-day-old plantlets received daily UV-B irradiation for 11 days under controlled conditions (PPFD 228 μmol m −2 s -1 , 80% R.H., 22°C). Changes in hormone level, including ethylene, abscisic acid, indoleacetic acid and salicylic acid, were monitored at 8 (UV8) and 11 (UV11) days of UV-B treatment and 3 days after the end of irradiation (UV11+3). Gene expression of the enzymes involved in ethylene biosynthesis was investigated by qRT-PCR. Photosynthesis performance was monitored by non-destructive techniques to ensure that the UV-B dose was not stressful for the plantlets. Finally, H2O2 and O2. - content and the antioxidant activity were evaluated as markers of possible UV-B-induced oxidative stress.The irradiated leaves of Micro-Tom displayed a significant decrease in ethylene emission of 38% and 42% after 8 days and 11 days respectively, confirming a previous report on the UVR8-mediated down-regulation of ethylene biosynthesis (Hectors et al., 2007). However, this decrease was transient since ethylene emission of UV 11+3 was similar to the control. Roots of UV-B treated samples responded differently to leaves with respect to ethylene emission. Abscisic, indoleacetic and salicylic acid levels in leaves were different depending of the irradiation period (UV8 and UV11) with respect to the control and they were transient in some cases, since hormone levels in UV11+3 samples were similar to the control. Levels of these plant hormones were also evaluated in roots. All these results revealed an intricate UV-B response mechanism between above- and below-ground organs which will be discussed.