Abstract
Drought is a major threat to plant growth and crop productivity. Reduced level of the gibberellin would result in increased drought tolerance, but the underlying mechanism is still unclear. In Brassica napus, there are four BnaRGA genes that code for DELLA proteins, negative regulators of GA signaling. Among them, expression of BnaA6.RGA was greatly induced by drought and abscisic acid (ABA). Previously, we created the gain-of-function mutant of BnaA6.RGA, bnaa6.rga-D, and the loss-of-function quadruple mutant, bnarga by CRISPR/Cas9, respectively. Here we show that bnaa6.rga-D displayed enhanced drought tolerance, and its stomatal closure was hypersensitive to ABA treatment. By contrast, bnarga displayed reduced drought tolerance and was less sensitive to ABA treatment, but there is no difference in drought tolerance between single BnaRGA mutant and WT, suggesting a functional redundancy between the BnaRGA genes in this process. Furthermore, we found that BnaRGAs were able to interact physically with BnaA10.ABF2, an essential transcription factor in ABA signaling. The BnaA10.ABF2-BnaA6.RGA protein complex greatly increased the expression level of the drought responsive gene BnaC9.RAB18. Taken together, this work highlighted the fundamental roles of DELLA proteins in drought tolerance in B. napus, and provide desirable germplasm for further breeding of drought tolerance in rapeseed.
Highlights
Drought is a major stress that causes decreases in crop yield
Sequence analyses identified all the abscisic acid (ABA) biosynthesis and signaling components in B. napus, indicating that this pathway is conserved in this species (Zhu et al, 2016)
We hypothesized that the negative regulators of GA signaling, the DELLA proteins, may play important roles in the drought tolerance of B. napus
Summary
Drought is a major stress that causes decreases in crop yield. Genetic engineering by regulating drought responsive genes are the effective approach for enhancing crop drought tolerance, which increases agricultural productivity to meet the food demand of expanding population (Zhu, 2016). These activated SnRK2s can phosphorylate downstream transcription factors to increase drought tolerance (Ma et al, 2009; Park et al, 2009) Among these transcription factors, the bZIP group of ABA response element (ABRE)-binding factors (ABFs) play important roles in ABA signaling transduction (Zhu, 2002). In the crosstalk with ABA signaling, DELLA proteins interact with other transcription factors, such as ABI3 and ABI5, to promote the expression levels of ABA-responsive genes that inhibit seed germination (Lim et al, 2013). We demonstrated that BnaA6.RGA acts as a positive regulator of drought tolerance by promoting stomatal closure through increased ABA sensitivity and subsequently by reducing water loss in response to a water deficit. Our findings provide novel insights into the crosstalk between GA and ABA signaling pathways, and provide a useful germplasm for improving rapeseed drought tolerance
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