Epigenetic Regulation of Heat Stress in Plant Male Reproduction

The roles of non-coding RNAs in male reproductive development and abiotic stress responses during this unique process in flowering plants

Successful male reproductive development is the guarantee for sexual reproduction of flowering plants. Male reproductive development is a complicated and multi-stage process that integrates physiological processes and adaptation and tolerance to a myriad of environmental stresses. This well-coordinated process is governed by genetic and epigenetic machineries. Non-coding RNAs (ncRNAs) play pleiotropic roles in the plant growth and development. The identification, characterization and functional analysis of ncRNAs and their target genes have opened a new avenue for comprehensively revealing the regulatory network of male reproductive development and its response to environmental stresses in plants. This review briefly addresses the types, origin, biogenesis and mechanisms of ncRNAs in plants, highlights important updates on the roles of ncRNAs in regulating male reproductive development and emphasizes the contribution of ncRNAs, especially miRNAs and lncRNAs, in responses to abiotic stresses during this unique process in flowering plants.

The role of microRNAs in responses to drought and heat stress in peanut (Arachis hypogaea)

MicroRNAs (miRNAs) are 21-24 nt small RNAs (sRNAs) that negatively regulate protein-coding genes and/or trigger phased small-interfering RNA (phasiRNA) production. Two thousand nine hundred miRNA families, of which ∼40 are deeply conserved, have been identified in ∼80 different plant species genomes. miRNA functions in response to abiotic stresses is less understood than their roles in development. Only seven peanut MIRNA families are documented in miRBase, yet a reference genome assembly is now published and over 480 plant-like MIRNA loci were predicted in the diploid peanut progenitor Arachis duranensis genome. We explored by computational analysis of a leaf sRNA library and publicly available sRNA, degradome, and transcriptome datasets the miRNA and phasiRNA space associated with drought and heat stresses in peanut. We characterized 33 novel candidate and 33 ancient conserved families of MIRNAs and present degradome evidence for their cleavage activities on mRNA targets, including several noncanonical targets and novel phasiRNA-producing noncoding and mRNA loci with validated novel targets such as miR1509 targeting serine/threonine-protein phosphatase7 and miRc20 and ahy-miR3514 targeting penta-tricopeptide repeats (PPRs), in contradistinction to other claims of miR1509/173/7122 superfamily miRNAs indirectly targeting PPRs via TAS-like noncoding RNA loci. We characterized the inverse correlations of significantly differentially expressed drought- and heat-regulated miRNAs, assayed by sRNA blots or transcriptome datasets, with target mRNA expressions in the same datasets. Meta-analysis of an expression atlas and over representation of miRNA target genes in co-expression networks suggest that miRNAs have functions in unique aspects of peanut gynophore development. Genome-wide MIRNA annotation of the published allopolyploid peanut genome can facilitate molecular breeding of value-added traits.

High Temperature Tolerance in a Novel, High-Quality Phaseolus vulgaris Breeding Line Is Due to Maintenance of Pollen Viability and Successful Germination on the Stigma

The common bean (Phaseolus vulgaris L.) is an important nutritional source globally but is sensitive to high temperatures and thus particularly vulnerable to climate change. Derived from a breeding program at CIAT (Colombia), a heat-tolerant breeding line, named heat-tolerant Andean-type 4 (HTA4), was developed by a series of crosses of parents with a small-bean tepary genotype (Phaseolus acutifolius L.) in their pedigree, which might be the donor of heat stress (HS) tolerance. Importantly, in HTA4, the large, commercially desirable Andean-type beans was restored. To assess underlying tolerance mechanisms, HTA4, together with a heat-sensitive Colombian variety (Calima), was exposed to HS (31 °C/24 °C HS vs. 26 °C/19 °C day/night) under controlled environment conditions. Vegetative growth and photosynthetic performance were not negatively impacted by HS in either genotype, although senescence was delayed in Calima. HS during the reproductive stage caused an increase in pod number in Calima but with few fully developed seeds and many pods aborted and/or abscised. In contrast, HTA4 maintained a similar filled pod number under HS and a higher seed weight per plant. Pollen showed high sterility in Calima, with many non-viable pollen grains (24.9% viability compared to 98.4% in control) with a thicker exine and fewer starch granules under HS. Calima pollen failed to adhere to the stigma and germinate under HS. In HTA4, pollen viability was significantly higher than in Calima (71.1% viability compared to 95.4% under control), and pollen successfully germinated and formed pollen tubes in the style under HS. It is concluded that HTA4 is heat tolerant and maintains a high level of reproductive output due to its ability to produce healthy pollen that is able to adhere to the stigma.

High-temperature stress in crops: male sterility, yield loss and potential remedy approaches

Global food security is one of the utmost essential challenges in the 21st century in providing enough food for the growing population while coping with the already stressed environment. High temperature (HT) is one of the main factors affecting plant growth, development and reproduction and causes male sterility in plants. In male reproductive tissues, metabolic changes induced by HT involve carbohydrates, lipids, hormones, epigenetics and reactive oxygen species, leading to male sterility and ultimately reducing yield. Understanding the mechanism and genes involved in these pathways during the HT stress response will provide a new path to improve crops by using molecular breeding and biotechnological approaches. Moreover, this review provides insight into male sterility and integrates this with suggested strategies to enhance crop tolerance under HT stress conditions at the reproductive stage.

Epigenetic stress memory: A new approach to study cold and heat stress responses in plants

Understanding plant stress memory under extreme temperatures such as cold and heat could contribute to plant development. Plants employ different types of stress memories, such as somatic, intergenerational and transgenerational, regulated by epigenetic changes such as DNA and histone modifications and microRNAs (miRNA), playing a key role in gene regulation from early development to maturity. In most cases, cold and heat stresses result in short-term epigenetic modifications that can return to baseline modification levels after stress cessation. Nevertheless, some of the modifications may be stable and passed on as stress memory, potentially allowing them to be inherited across generations, whereas some of the modifications are reactivated during sexual reproduction or embryogenesis. Several stress-related genes are involved in stress memory inheritance by turning on and off transcription profiles and epigenetic changes. Vernalization is the best example of somatic stress memory. Changes in the chromatin structure of the Flowering Locus C (FLC) gene, a MADS-box transcription factor (TF), maintain cold stress memory during mitosis. FLC expression suppresses flowering at high levels during winter; and during vernalization, B3 TFs, cold memory cis-acting element and polycomb repressive complex 1 and 2 (PRC1 and 2) silence FLC activation. In contrast, the repression of SQUAMOSA promoter-binding protein-like (SPL) TF and the activation of Heat Shock TF (HSFA2) are required for heat stress memory. However, it is still unclear how stress memory is inherited by offspring, and the integrated view of the regulatory mechanisms of stress memory and mitotic and meiotic heritable changes in plants is still scarce. Thus, in this review, we focus on the epigenetic regulation of stress memory and discuss the application of new technologies in developing epigenetic modifications to improve stress memory.

Dynamics of Fertility-Related Traits in Tomato Landraces under Mild and Severe Heat Stress

Studies on the reproductive dynamics under heat stress are crucial to breed more tolerant cultivars. In tomato, cultivars, breeding lines, and wild species have been evaluated for their response to heat stress. Here, we addressed the study to a panel of selected landraces representing traditional genotypes that usually show high adaptation to local environments. In two experiments, spaced by 12 years, we set-up an identical experimental design with plants transplanted at two different dates to expose the second field to thermic stress with natural fluctuations. Such a strategy resulted in both a mild and severe stress in the two years. The landraces showed wide variation for both vegetative and reproductive traits; all traits were affected by heat, mostly with a significant Genotype*Environment interaction. A high broad-sense heritability was estimated for plant height, stigma position, pollen viability, and fruit weight. Low heritability estimates were found for the number of flowers, fruit set, and yield. Despite the interaction, traits recorded under control and heat conditions were positively correlated. Multivariate analysis located the genotypes in a topography that was stable under all conditions, except under the harshest temperatures. The study revealed that landraces present a wide variability for the response of reproductive traits to thermic challenges and that such a variation could be useful to dissect the traits with higher heritability and identify quantitative trait loci for breeding more resilient varieties.