Transcriptional profiling of maturing tomato (Solanum lycopersicum L.) microspores reveals the involvement of heat shock proteins, ROS scavengers, hormones, and sugars in the heat stress response

Abstract

Above-optimal temperatures reduce yield in tomato largely because of the high heat stress (HS) sensitivity of the developing pollen grains. The high temperature response, especially at this most HS-sensitive stage of the plant, is poorly understood. To obtain an overview of molecular mechanisms underlying the HS response (HSR) of microspores, a detailed transcriptomic analysis of heat-stressed maturing tomato microspores was carried out using a combination of Affymetrix Tomato Genome Array and cDNA-amplified fragment length polymorphism (AFLP) techniques. The results were corroborated by reverse transcription-PCR (RT-PCR) and immunoblot analyses. The data obtained reveal the involvement of specific members of the small heat shock protein (HSP) gene family, HSP70 and HSP90, in addition to the HS transcription factors A2 (HSFA2) and HSFA3, as well as factors other than the classical HS-responsive genes. The results also indicate HS regulation of reactive oxygen species (ROS) scavengers, sugars, plant hormones, and regulatory genes that were previously implicated in other types of stress. The use of cDNA-AFLP enabled the detection of genes representing pollen-specific functions that are missing from the tomato Affymetrix chip, such as those involved in vesicle-mediated transport and a pollen-specific, calcium-dependent protein kinase (CDPK2). For several genes, including LeHSFA2, LeHSP17.4-CII, as well as homologues of LeHSP90 and AtVAMP725, higher basal expression levels were detected in microspores of cv. Hazera 3042 (a heat-tolerant cultivar) compared with microspores of cv. Hazera 3017 (a heat-sensitive cultivar), marking these genes as candidates for taking part in microspore thermotolerance. This work provides a comprehensive analysis of the molecular events underlying the HSR of maturing microspores of a crop plant, tomato.

HsfA2 Controls the Activity of Developmentally and Stress-Regulated Heat Stress Protection Mechanisms in Tomato Male Reproductive Tissues

“…The higher expression of Hsps and other HS response genes has been shown for only a limited number of genes in early stages of tomato pollen development (Frank et al, 2009;Giorno et al, 2010). We extended these findings by monitoring the expression profile of several Hsps in tetrads, postmeiotic microspores, and mature binucleate pollen isolated from wild-type plants.…”

“…A tomato cultivar with higher basal expression of HsfA2 and some Hsps accumulating at early microspore stage exhibited higher pollen thermotolerance compared to another cultivar with lower Hsf-Hsp levels (Frank et al, 2009). This might suggest that the accumulation of chaperones during progression from meiocytes to mitotic microspores enhances protection and priming of HSR for more advanced developmental stages (Chaturvedi et al, 2013;Frank et al, 2009).…”

“…In some cases, homologous or heterologous ectopic expression of HsfA2 co-orthologs resulted in growth and developmental alterations, including growth retardation and dwarfism as well as accelerated plant and callus growth in the absence of HsfA1s in Arabidopsis (Liu and Charng, 2013;Ogawa et al, 2007). In addition, HsfA2 and other HS-induced Hsps that are minimally expressed in vegetative tissues under physiological conditions are strongly expressed at early stages of anther and pollen development (Frank et al, 2009;Giorno et al, 2010). However, despite the increased levels of HsfA2, pollen at meiotic and early mitotic phases exhibit only weak HSR at the transcriptional level, which is assumed to be related with the increased sensitivity of these stages at higher temperatures (Frank et al, 2009;Giorno et al, 2010).…”

“…In addition, HsfA2 and other HS-induced Hsps that are minimally expressed in vegetative tissues under physiological conditions are strongly expressed at early stages of anther and pollen development (Frank et al, 2009;Giorno et al, 2010). However, despite the increased levels of HsfA2, pollen at meiotic and early mitotic phases exhibit only weak HSR at the transcriptional level, which is assumed to be related with the increased sensitivity of these stages at higher temperatures (Frank et al, 2009;Giorno et al, 2010). A tomato cultivar with higher basal expression of HsfA2 and some Hsps accumulating at early microspore stage exhibited higher pollen thermotolerance compared to another cultivar with lower Hsf-Hsp levels (Frank et al, 2009).…”

“…However, despite the increased levels of HsfA2, pollen at meiotic and early mitotic phases exhibit only weak HSR at the transcriptional level, which is assumed to be related with the increased sensitivity of these stages at higher temperatures (Frank et al, 2009;Giorno et al, 2010). A tomato cultivar with higher basal expression of HsfA2 and some Hsps accumulating at early microspore stage exhibited higher pollen thermotolerance compared to another cultivar with lower Hsf-Hsp levels (Frank et al, 2009). This might suggest that the accumulation of chaperones during progression from meiocytes to mitotic microspores enhances protection and priming of HSR for more advanced developmental stages (Chaturvedi et al, 2013;Frank et al, 2009).…”