Raffinose accumulation and preferential allocation of carbon (¹⁴C) to developing leaves impart salinity tolerance in sugar beet

Abstract

Sugar beet is a salt-tolerant crop that can be explored for crop production in degraded saline soils. Seeds of multigerm genotypes LKC-2006 (susceptible) and LKC-HB (tolerant) were grown in 150 mM NaCl, from germination to 60 days after sowing, to decipher the mechanism of salinity tolerance at the vegetative stage.The biomass of the root and leaf were maintained in the tolerant genotype, LKC-HB, under saline conditions. Na + /K + ratios were similar in roots and leaves of LKC-HB, with lower values under salinity compared to LKC 2006. Infrared temperatures were 0.96 C lower in LKC-HB than in LKC-2006, which helped regulate the leaf water status under stressed conditions. Pulse-chase experiment showed that 14 C photosynthate was preferentially allocated towards the development of new leaves in the tolerant genotype. The sugar profile of leaves and roots showed accumulation of raffinose in leaves of LKC-HB, indicating a plausible role in imparting salinity tolerance by serving as an osmolyte or scavenger. The molecular analysis of the genes responsible for raffinose synthesis revealed an 18-fold increase in the expression of BvRS2 in the tolerant genotype, suggesting its involvement in raffinose synthesis. Our study accentuated that raffinose accumulation in leaves is vital for inducing salinity tolerance and maintenance of shoot dry weight in sugar beet.

Salt and Drought Stress Responses in Cultivated Beets (Beta vulgaris L.) and Wild Beet (Beta maritima L.)

“…In another study, at 30 days after salinity, the salt-susceptible genotype (LKC-2006) enhanced sucrose content in roots compared to the control groups and reduced with increasing stress duration. In contrast to the LKC-2006, higher sucrose accumulation was observed in the roots of salttolerant sugar beet genotype (LKC-HB) after 60 days of salt stress [61]. Moreover, in the sugar beet cultivar (O68), the soluble sugars remarkably enhanced in both leaves and roots at 300 mM NaCl, but the elevation was higher in leaves than in roots [55].…”

“…Capability of germination and seedling growth [17,22,25,30,38] Decline in the root weight and root length Higher root/shoot ratio Decline in the root weight and root length Higher root growth [8,34,38,61] Low water content and small leaf area The initial eight-weeks of growth associated with seedling vigor in sugar beet is considered to be the most crucial stage [79], which determines field emergence and stand establishment. Salt and drought stress negatively affect germination and seedling development in sugar beet [4,80] by affecting ionic balance, resulting in hyperosmotic stress and oxidative damage [25,70,81].…”

“…A sugar beet cultivar, O68, under salt stress, was also found to have better seedling growth than unstressed plants [85]. According to Naguib et al (2021), salt tolerance strategies at seedling stage of sugar beet include partitioning of photosynthate to new developing leaves, equal distribution of Na + /K + in leaves and roots, and raffinose accumulation in leaves [61]. However, changes in Na + concentrations and Na + /K + ratio are found at different developmental stages.…”

“…Compatible solutes or osmoprotectants are small, non-toxic molecules that protect cells against stress and function in reactive oxygen species (ROS) scavenging, maintaining membranes and protein structures [91]. Accordingly, in beets, increased levels of compatible solutes, including glycine betaine, proline, glucose, fructose [49,68], raffinose [61,72], and sucrose [71,77], during salt or water stress, maintain photosynthesis and stomatal conductance [70,92]. Under water stress, accumulation of osmoprotectants reduces the osmotic potential in sugar beet cells [40,71], facilitating water influx [18].…”

“…Soluble sugars (sucrose, fructose, and glucose), which take part in maintaining cellular osmoticum, may help to reduce leaf temperatures under salt and drought stress in beet genotypes [61,100]. Sugar beet is a rich source of sucrose and raffinose.…”