Importance of ABA homeostasis under terminal drought stress in regulating grain filling events

Abstract

R ecent studies suggest that abscisic acid (ABA) at its basal level plays an important role during seed set and grain filling events. Under drought stress ABA levels were found to be significantly enhanced in the developing seed. Until now we lacked an understanding of (1) ABA homeostasis in developing seeds under terminal drought and (2) the interactive role of ABA in regulating the starch biosynthesis pathway in developing grains under terminal drought. We have recently reported the possible regulation of ABA homeostasis in source (flag leaf) and sink (developing grains) tissues under post-anthesis drought stress in barley and concluded that significantly enhanced ABA levels in developing grains are due to strong activation of the ABA deconjugation pathway and fine regulation of the ABA biosynthesisdegradation pathway.1 Additionally, we provided evidence for the role of ABA in differential regulation of starch biosynthesis genes and a significant upregulation of starch degradation beta amylase genes under drought, i.e., ABA not only influences the rate of starch accumulation but also starch quality.ABA produced under normal conditions regulate various physiological processes of plant development like flowering, seed development, synthesis of storage proteins and lipids, embryo morphogenesis, dormancy, stomatal opening and trigger senescence and remobilization.2-5 Under water stress, ABA regulates flower development directly via a fernesol kinase. 6 ABA is not only correlated with regulation of primary carbon metabolism, 7 but

Transcription Factor ATAF1 in Arabidopsis Promotes Senescence by Direct Regulation of Key Chloroplast Maintenance and Senescence Transcriptional Cascades

“…The synchronized expression of genes involved in ABA signaling as well as ABA metabolism and transport help to maintain ABA homeostasis in crops such as rice, wheat, and barley (Hordeum vulgare; Govind et al, 2011;Ji et al, 2011). The expression of genes involved in ABA signaling, ABA metabolism/import, and ABA responses was up-regulated in ATAF1 overexpressors.…”

Drought Stress Tolerance Mechanisms in Barley and Its Relevance to Cereals

“…Enhanced seed filling under mild drying was attributed to accumulation of ABA which enhanced sink strength and remobilization of stem reserves. Similarly, a role of ABA in seed filling under terminal drought was also shown by Seiler et al (2011) and Govind et al (2011). But terminal drought stress had no effect on the germination of barley seeds, but reduced germination was noticed after the accelerated aging test (Samarah and Alqudah 2011).…”

“…However, so far we have not clearly understood the holistic mechanisms for improved seed yield per se under terminal drought, which will be the major topic of the current review. Drought stress responses have been well studied in vegetative tissues under short-term stress response, but little is known about the situation under long-term stress and its relevance under terminal drought (Govind et al 2011;Seiler et al 2011). Due to lack of sufficient knowledge from barley, we have also consulted the knowledge revealed in other cereal species to decipher and summarize the drought tolerance mechanisms.…”

Signal transduction during wheat grain development

“…Therefore, exogenous applied ABA to leaves increases carbohydrate accumulation and redistribution to grains, resulting in an increase in yield (Travaglia et al 2007). It has been also demonstrated that grain ABA homeostasis regulates starch accumulation rate and grain filling in barley under drought (Govind et al 2011), is involved in regulating the starch biosynthesis as a signaling , and probably exerts control over starch biosynthetic genes in the endosperm via SNF1 kinase and several TFs in barley (Sreenivasulu et al 2006). Novel ABA-and dehydration-responsive cis-elements have been found in the promoters of key genes of starch biosynthesis (HvSUS1, HvAGP-L1) and degradation (HvBAM1); spraying of fluridone (an ABA biosynthesis inhibitor) to drought-stressed plants results in severely impaired starch content and thousand grain weight of mature seeds ).…”

“…Generally grain ABA content was low at the early grain-filling stage (1.0 lmol kg -1 DM in wheat), reaching a maximum when the grain-filling rate was highest, namely, the ABA peak values were significantly correlated with the maximum grain-filling rates (Hess et al 2002;Govind et al 2011). It seems that ABA has little effect on early embryo development; wheat embryos develop normally in ABA-deficient kernels and become dormant if exposed to ABA after embryo differentiation has occurred (Rasmussen et al 1997;Suzuki et al 2000).…”