Effects of acute heat stress on the female reproductive apparatus of Lycopersicon esculentum cv micro-tom

Temperatures above the normal optimum are sensed as heat stress (HS) by all living organisms, including plants. HS affects fertility, yield and production quality of crops with important consequences on food security and the well-being of human population. These negative effects can depend on the fact that all the phases of sexual reproduction in angiosperms, namely gametophyte development (from meiosis to pollination), progamic phase (from pollination to zygote formation) and embryo development (from zygote to seed), are more vulnerable to temperature stress than vegetative processes (1). Presently there are limited data about the effects of HS on the female part (pistil) and its interaction with pollen grains. The present research gives more insight into the effects of acute HS on the pistil's morphology and the progamic phase of tomato (Lycopersicon esculentum cv Micro-Tom). A morphological analysis of Micro-Tom pistils treated at 42°C for 3 h at different developmental stages has been performed by light microscopy using Toluidine Blue O, Periodic Acid Schiff, Alcian Blue and Calcofluor White stains. Moreover, the pattern distribution of α-tubulin has been investigated in the heat-treated pistils. In addition, the carbohydrate epitope of arabinogalactan proteins (AGPs), involved in pollen tube guidance to the ovary, has been localized by JIM8 antibody. The consequences of acute HS have been also evaluated on stigmatic receptivity, pollen performance and in vivo pollen tube growth using epifluorescence microscopy. Results herein reported show that the general organization of the pistil tissues is substantially unaltered under HS. A significant observation is that HS seems to alter the microtubule distribution in the ovules: in control ovules, microtubules resulted well preserved in the inner integument tissue, whereas in treated ovules they were diffused in the cytoplasm. The epitope immunorecognized by JIM8 antibody seemed to disappear under HS in the transmitting tissue of the styles, thus denoting a possible alteration in the pollen tube growth pathway; on the contrary its localization was similar to the control in heat-treated ovules. Both stigma receptivity and pollen performance resulted sensitive to HS. The present work confirmed the sensitivity of sexual plant reproduction to HS: like in other species, such as peach (2) and sweet cherry (3), the receptivity of Micro-Tom stigma was reduced under HS and thus, the possibility of fertilization. Molecules involved in pollen tube guidance to the ovary (AGPs) and structural proteins (microtubules) showed an altered pattern with possible functional consequences on pistil function.