Sadhukhan, Ayan

Main conclusion: Plants develop both short-term and transgenerational memory of drought stress through epigenetic regulation of transcription for a better response to subsequent exposure. Abstract: Recurrent spells of droughts are more common than a single drought, with intermittent moist recovery intervals. While the detrimental effects of the first drought on plant structure and physiology are unavoidable, if survived, plants can memorize the first drought to present a more robust response to the following droughts. This includes a partial stomatal opening in the watered recovery interval, higher levels of osmoprotectants and ABA, and attenuation of photosynthesis in the subsequent exposure. Short-term drought memory is regulated by ABA and other phytohormone signaling with transcriptional memory behavior in various genes. High levels of methylated histones are deposited at the drought-tolerance genes. During the recovery interval, the RNA polymerase is stalled to be activated by a pause-breaking factor in the subsequent drought. Drought leads to DNA demethylation near drought-response genes, with genetic control of the process. Progenies of the drought-exposed plants can better adapt to drought owing to the inheritance of particular methylation patterns. However, a prolonged watered recovery interval leads to loss of drought memory, mediated by certain demethylases and chromatin accessibility factors. Small RNAs act as critical regulators of drought memory by altering transcript levels of drought-responsive target genes. Further studies in the future will throw more light on the genetic control of drought memory and the interplay of genetic and epigenetic factors in its inheritance. Plants from extreme environments can give queues to understanding robust memory responses at the ecosystem level.

Cowpea is sensitive to drought and heat stress, particularly at the reproductive stages of development. Both stresses limit growth and yield, and their effect is more devastating when occurring concurrently. Dehydration-responsive element-binding protein 2A (DREB2A) is an important signaling hub integrating information about two different abiotic stresses, drought and heat. We identified VuDREB2A as a canonical DREB ortholog in cowpea, activating downstream stress-responsive genes by binding to DREs in their promoter. Post-translational modification of a negative regulatory domain (NRD) within the VuDREB2A protein prevents its degradation. Targeted deletion of the NRD produces a stable and constitutively active form VuDREB2A-CA. However, there is very little evidence of its practical utility under field conditions. This study overexpressed the VuDREB2A-CA in a popular cowpea variety and conducted drought- and heat-tolerance experiments across various stress regimes. Transgenic cowpea exhibited significant tolerance with consistently higher yield when exposed to over 30-d drought stress and 3-d exposure to high temperature (28 °C˗52 °C) without any pleiotropic alterations. The transgenic lines showed higher photosynthetic efficiency, osmotic adjustment, antioxidant defense, thermotolerance, and significantly higher survival and increased biomass than the wild type. Late embryogenesis abundant 5, heat shock protein 70, dehydrin, mitogen-activated protein kinase 2/4, isoflavonoid reductase, and myoinositol phosphate synthase were upregulated in transgenic lines under drought and heat stress. Through transcriptome analysis of the transgenic lines, we found significant up-regulation of various stress-responsive cowpea genes, having DRE in their promoter. Our results suggest that overexpression of VuDREB2A could improve cowpea production under drought and high temperatures.

Aluminum (Al) toxicity in acid soil is one of the most severe environmental stress factors that limit world food production. Al ion (Al 3+) is highly reactive with negatively charged ligands of the cell and disturbs cell division and expansion at sub-micromolar levels. Since the harmful effects are caused by both cyto- and genotoxicities of Al, plant cells show various dose- and time-dependent responses. On the other hand, plants have evolved different Al tolerance strategies that protect sensitive cells from Al 3+ . These include restructuring of the cell wall to prevent deposition of Al 3+ and organic acid release by plasma membrane transporters to chelate out and exclude Al 3+ in the rhizosphere, regulated by the transcription factor STOP1 or its orthologs, and internal detoxification mechanisms including root-to-shoot translocation, antioxidant defense, and Al sequestration in vacuoles to prevent cytotoxicity. In recent years, genomics approaches, particularly genome-wide association studies and transcriptome analyses, have provided genetic evidence of well-known genes’ involvement in Al tolerance and uncovered new Al tolerance genes in plants. This chapter summarizes the findings from recent genomic studies that add to our understanding of the complex plant responses to Al toxicity.