Bananas tackling drought and heat – with DREBs and more

To understand the stress tolerance of bananas, we need to dig into their ancestry a bit. The bananas we eat today are delicious, rich in fiber, vitamins and minerals. They are also very good examples of crop domestication. The majority of our supermarket varieties are seedless triploids, which are very different from their diploid ancestors. Most cultivated bananas are descendants of two seeded diploid species – Musa acuminate and Musa balbisiana. Based on the contribution of the parental acuminate (A) and balbisiana (B) sequences in their genomes, the modern triploid varieties are classified into AAA, AAB and ABB groups (Simmonds and Shepherd 1955). These subgroups are quite different in their ability to withstand stress. Grand Nain, one of the AAA cultivars belonging to the Cavendish subgroup, is particularly popular among farmers and consumers and it dominates the banana export market. The superior yield and better taste of this AAA cultivar come at a cost – it is highly susceptible to drought (Ravi et al. 2013). A dash of the balbisiana changes things – the AAB and ABB bananas are more drought resistant but they are somewhat less popular with consumers. Because of its agricultural and economic relevance, understanding and improving the drought resistance mechanisms of banana is of major research interest. Controlled laboratory studies on drought seldom take heat into account, even though drought usually comes hand in hand with searing high temperatures in tropical banana plantations. Jangale et al. (2019) have overcome this shortcoming by introducing both stresses into the equation. They explored the contributing factors behind drought and heat tolerance by comparing the drought-sensitive AAA type Grand Nain variety with the more tolerant AAB type Hill Banana. Their results indicate that heat increases water loss and the severity of the drought response. The cellular consequence of dehydration, such as membrane damage and reactive oxygen species production, was enhanced by heat stress in both varieties but the Hill Banana was able to withstand this combined stress better than Grand Nain. The authors inferred that the smaller but more numerous stomata of Hill Banana were more effective in regulating transpirational water loss over the bigger but fewer stomata of Grand Nain. This simple anatomical feature can be a convenient tool for screening new varieties for growth in arid regions. To assess the molecular effect of combined heat and drought, Jangale et al. (2019) analyzed the expression patterns of DREB (DEHYDRATION RESPONSIVE ELEMENT BINDING) genes. As their name suggests, many of the DREBs are induced by drought and they act as transcriptional regulators of genes involved in the stress response. Modulating the expression levels of DREBs has been explored as a strategy to design stress-tolerant crops (Agarwal et al. 2006, Zandalinas et al. 2018). Jangale et al. (2019) found that drought and heat stress individually enhanced the expression of a number of DREB genes in both the Grand Nain and Hill Banana but this increase was more prominent in the Hill Banana. Combining heat and drought had an additive effect on DREB expression in Grand Nain. However, the effect of the combined stresses was surprising in the Hill Banana; unlike the enhancing effects of individual stresses, the combination of heat and drought strongly suppressed DREB expressions in this variety. This unexpected and somewhat counterintuitive observation underscores why studies that consider combined stresses are more informative. In spite of the suppression of DREB expression, the Hill banana was more tolerant to the combined stress, which would hint toward stress management strategies that are partially independent of DREB induction. A detailed transcriptomic analysis that compares the gene expression profiles of AAA banana varieties with AAB or ABB varieties under combined heat and drought will help us to uncover the molecular factors required for surviving these harsh conditions.