Long non-coding RNA transcriptome landscape of anthers at different developmental stages in response to drought stress in tomato.

Pollen development is particularly susceptible to drought stress. Long non-coding RNAs (lncRNAs) are known to play a role in plant development and responses to drought stress. However, the expression profile and putative function of lncRNAs in drought-induced male sterility remain largely unknown. In this study, we investigated the lncRNA transcriptome landscape of tomato anthers at early and late stages-tetrad-vacuolated microspore (TED-VUM) and binucleate-mature pollen (BIN-MP) anthers, respectively-in response to drought stress using RNA-sequencing. In total, we identified 67,770 lncRNAs, of which 3053 lncRNAs were drought responsive. Interestingly, there were more differentially expressed (DE) lncRNAs in TED-VUM (2879) than in BIN-MP (174) anthers, which was consistent with the TED-VUM anthers being more drought sensitive. Functional enrichment analysis revealed that the target genes of DE lncRNAs were significantly enriched in diverse metabolic processes, including in carbohydrate metabolism and hormone synthesis. Co-expression analysis also identified 1407 lncRNAs that strongly co-expressed with 8 target genes that are involved in hormone (abscisic acid and jasmonic acid) and carbohydrate (sucrose and starch) metabolisms and tapetum development, highlighting the potential of lncRNA-target-gene modulation of anther development under drought stress. Our results serve as a baseline for future investigations of the potential function of lncRNAs in plant reproductive development under drought stress.

Morpho-Physiological and Transcriptome Changes in Tomato Anthers of Different Developmental Stages under Drought Stress.

Drought limits the growth and productivity of plants. Reproductive development is sensitive to drought but the underlying physiological and molecular mechanisms remain unclear in tomatoes. Here, we investigated the effect of drought on tomato floral development using morpho-physiological and transcriptome analyses. Drought-induced male sterility through abnormal anther development includes pollen abortion, inadequate pollen starch accumulation and anther indehiscence which caused floral bud and opened flower abortions and reduced fruit set/yield. Under drought stress (DS), pollen mother cell to meiotic (PMC-MEI) anthers survived whereas tetrad to vacuolated uninucleate microspore (TED-VUM) anthers aborted. PMC-MEI anthers had lower ABA increase, reduced IAA and elevated sugar contents under DS relative to well-watered tomato plants. However, TED-VUM anthers had higher ABA increase and IAA levels, and lower accumulation of soluble sugars, indicating abnormal carbohydrate and hormone metabolisms when exposed to drought-stress conditions. Moreover, RNA-Seq analysis identified altogether >15,000 differentially expressed genes that were assigned to multiple pathways, suggesting that tomato anthers utilize complicated mechanisms to cope with drought. In particular, we found that tapetum development and ABA homeostasis genes were drought-induced while sugar utilization and IAA metabolic genes were drought-repressed in PMC-MEI anthers. Our results suggest an important role of phytohormones metabolisms in anther development under DS and provide novel insight into the molecular mechanism underlying drought resistance in tomatoes.

PIF4 negatively modulates cold tolerance in tomato anthers via temperature-dependent regulation of tapetal cell death.

Extreme temperature conditions seriously impair male reproductive development in plants; however, the molecular mechanisms underlying the response of anthers to extreme temperatures remain poorly described. The transcription factor phytochrome-interacting factor4 (PIF4) acts as a hub that integrates multiple signaling pathways to regulate thermosensory growth and architectural adaptation in plants. Here, we report that SlPIF4 in tomato (Solanum lycopersicum) plays a pivotal role in regulating cold tolerance in anthers. CRISPR (clustered regularly interspaced short palindromic repeats)-associated nuclease Cas9-generated SlPIF4 knockout mutants showed enhanced cold tolerance in pollen due to reduced temperature sensitivity of the tapetum, while overexpressing SlPIF4 conferred pollen abortion by delaying tapetal programmed cell death (PCD). SlPIF4 directly interacts with SlDYT1, a direct upstream regulator of SlTDF1, both of which (SlDYT1 and SlTDF1) play important roles in regulating tapetum development and tapetal PCD. Moderately low temperature (MLT) promotes the transcriptional activation of SlTDF1 by the SlPIF4-SlDYT1 complex, resulting in pollen abortion, while knocking out SlPIF4 blocked the MLT-induced activation of SlTDF1. Furthermore, SlPIF4 directly binds to the canonical E-box sequence in the SlDYT1 promoter. Collectively, these findings suggest that SlPIF4 negatively regulates cold tolerance in anthers by directly interacting with the tapetal regulatory module in a temperature-dependent manner. Our results shed light on the molecular mechanisms underlying the adaptation of anthers to low temperatures.

The Arabidopsis SMALL AUXIN UP RNA32 Protein Regulates ABA-Mediated Responses to Drought Stress.

SMALL AUXIN UP-REGULATED RNAs (SAURs) are recognized as auxin-responsive genes involved in the regulation of abiotic stress adaptive growth. Among the growth-limiting factors, water-deficit condition significantly affects plant growth and development. The putative function of SAUR family member AtSAUR32 has the potential to diminish the negative impact of drought stress, but the exact function and mode of action remain unclear in Arabidopsis. In the current study, AtSAUR32 gene was cloned and functionally analyzed. AtSAUR32 localized to the plasma membrane and nucleus was dominantly expressed in roots and highly induced by abscisic acid and drought treatment at certain time points. The stomatal closure and seed germination of saur32 were less sensitive to ABA relative to AtSAUR32-overexpressed line (OE32-5) and wild type (WT). Moreover, the saur32 mutant under drought stress showed increased ion leakage while quantum yield of photosystem II (ΦPSII) and endogenous ABA accumulation were reduced, along with the expression pattern of ABA/stress-responsive genes compared with WT and the OE32-5 transgenic line. Additionally, yeast two-hybrid (Y2H) and bimolecular fluorescence complementation (BiFC) assays showed that AtSAUR32 interacted with clade-A PP2C proteins (AtHAI1 and AtAIP1) to regulate ABA sensitivity in Arabidopsis. Taken together, these results indicate that AtSAUR32 plays an important role in drought stress adaptation via mediating ABA signal transduction.

Tomato stigma exsertion induced by high temperature is associated with the jasmonate signalling pathway.

High temperature (HT) is becoming an increasingly serious factor in limiting crop production with global climate change. During hot seasons, owing to prevailing HT, cultivated tomatoes are prone to exhibiting stigma exsertion, which hampers pollination and causes fruit set failure. However, the underlying regulatory mechanisms of the HT-induced stigma exsertion remain largely unknown. Here, we demonstrate that stigma exsertion induced by HT in cultivated tomato is caused by more seriously shortened stamens than pistils, which is different from the stigma exsertion observed in wild tomato species. Under the HT condition, the different responses of pectin, sugar, expansin, and cyclin cause cell wall remodelling and differentially localized cell division and selective cell enlargement, which further determine the lengths of stamens and pistils. In addition, auxin and jasmonate (JA) are implicated in regulating cell division and cell expansion in stamens and pistils, and exogenous JA instead of auxin treatment can effectively rescue tomato stigma exsertion through regulating the JA/COI1 signalling pathway. Our findings provide a better understanding of stigma exsertions under the HT condition in tomato and uncover a new function of JA in improving plant abiotic stress tolerance.

Tomato AUXIN RESPONSE FACTOR 5 regulates fruit set and development via the mediation of auxin and gibberellin signaling.

Auxin response factors (ARFs) encode transcriptional factors that function in the regulation of plant development processes. A tomato ARF gene, SlARF5, was observed to be expressed at high levels in emasculated ovaries but maintained low expression levels in pollinated ovaries. The amiRNA SlARF5 lines exhibited ovary growth and formed seedless fruits following emasculation. These parthenocarpic fruits developed fewer locular tissues, and the fruit size and weight were decreased in transgenic lines compared to those of wild-type fruits. Gene expression analysis demonstrated that several genes involved in the auxin-signaling pathway were downregulated, whereas some genes involved in the gibberellin-signaling pathway were enhanced by the decreased SlARF5 mRNA levels in transgenic plants, indicating that SlARF5 may play an important role in regulating both the auxin- and gibberellin-signaling pathways during fruit set and development.