Abstract
Pearl millet [Pennisetum glaucum (L) R. Br.] is an important cereal crop of the semiarid tropics, which can withstand prolonged drought and heat stress. Considering an active involvement of the aquaporin (AQP) genes in water transport and desiccation tolerance besides several basic functions, their potential role in abiotic stress tolerance was systematically characterized and functionally validated. A total of 34 AQP genes from P. glaucum were identified and categorized into four subfamilies, viz., plasma membrane intrinsic proteins (PIPs), tonoplast intrinsic proteins (TIPs), nodulin-26-like intrinsic proteins (NIPs), and small basic intrinsic proteins (SIPs). Sequence analysis revealed that PgAQPs have conserved characters of AQP genes with a closer relationship to sorghum. The PgAQPs were expressed differentially under high vapor pressure deficit (VPD) and progressive drought stresses where the PgPIP2;6 gene showed significant expression under high VPD and drought stress. Transgenic tobacco plants were developed by heterologous expression of the PgPIP2;6 gene and functionally characterized under different abiotic stresses to further unravel their role. Transgenic tobacco plants in the T2 generations displayed restricted transpiration and low root exudation rates in low- and high-VPD conditions. Under progressive drought stress, wild-type (WT) plants showed a quick or faster decline of soil moisture than transgenics. While under heat stress, PgPIP2;6 transgenics showed better adaptation to heat (40°C) with high canopy temperature depression (CTD) and low transpiration; under low-temperature stress, they displayed lower transpiration than their non-transgenic counterparts. Cumulatively, lower transpiration rate (Tr), low root exudation rate, declined transpiration, elevated CTD, and lower transpiration indicate that PgPIP2;6 plays a role under abiotic stress tolerance. Since the PgPIP2;6 transgenic plants exhibited better adaptation against major abiotic stresses such as drought, high VPD, heat, and cold stresses by virtue of enhanced transpiration efficiency, it has the potential to engineer abiotic stress tolerance for sustained growth and productivity of crops.
Highlights
Localization of PgAQPs indicated that, while they were abundant in the plasma membranes with an average of six transmembrane helices, only NIP3 subfamily members were found in the vacuole (Table 1)
Maintaining water budget levels under different abiotic stress conditions is crucial for the plants to survive and to employ several adaptive mechanisms to cope with the environmental stresses (Mahdieh et al, 2008)
The AQP gene family has been characterized in diverse taxa, such as Oryza sativa, Zea mays, Triticum aestevium, Hordeum vulgare, Sorghum bicolor, Panicum virgatum, and Brachypodium distachyon (Johanson et al, 2001; Forrest and Bhave, 2008; Nguyen et al, 2013; Hove et al, 2015; Azad et al, 2016)
Summary
Pearl millet possesses abiotic stress tolerance due to efficient water conservation mechanisms (Kholová et al, 2010a; Reddy et al, 2017, 2021), which is considered as an efficient way of enhancing crop yield under terminal drought stress (Kholová et al, 2010a; Zaman-Allah et al, 2011b; Belko et al, 2012). The rapid response of the transpiration rate (Tr) to increased vapor pressure deficit (VPD) suggests a possible role of hydraulic signals mediating the Tr response upon rising VPD. These hydraulic signals are regulated by aquaporins (AQPs), the specialized membrane transporters that play a pivotal role in conducting water at the cellular level (Reddy et al, 2017; Shekoofa and Sinclair, 2018; Tharanya et al, 2018)
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