Overexpression of GSK3-like Kinase 5 (OsGSK5) in rice (Oryza sativa) enhances salinity tolerance in part via preferential carbon allocation to root starch

Abstract

Rice (Oryza sativa L.) is very sensitive to soil salinity. To identify endogenous mechanisms that may help rice to better survive salt stress, we studied a rice GSK3-like isoform (OsGSK5), an orthologue of a Medicago GSK3 previously shown to enhance salinity tolerance in Arabidopsis by altering carbohydrate metabolism. We wanted to determine whether OsGSK5 functions similarly in rice. OsGSK5 was cloned and sequence, expression, evolutionary and functional analyses were conducted. OsGSK5 was expressed highest in rice seedling roots and was both salt and sugar starvation inducible in this tissue. A short-term salt-shock (150 mM) activated OsGSK5, whereas moderate (50 mM) salinity over the same period repressed the transcript. OsGSK5 response to salinity was due to an ionic effect since it was unaffected by polyethylene glycol. We engineered a rice line with 3.5-fold higher OsGSK5 transcript, which better tolerated cultivation on saline soils (EC = 8 and 10 dS m–2). This line produced more panicles and leaves, and a higher shoot biomass under high salt stress than the control genotypes. Whole-plant 14C-tracing and correlative analysis of OsGSK5 transcript with eco-physiological assessments pointed to the accelerated allocation of carbon to the root and its deposition as starch, as part of the tolerance mechanism.

A pivotal role for starch in the reconfiguration of 14C-partitioning and allocation in Arabidopsis thaliana under short-term abiotic stress

“…The sugars so produced may act as Reactive Oxygen Species scavengers, osmoprotectants and be an immediate source of carbon and energy to mitigate against stress 66 – 70 . Sugar conversion to starch in leaves may prevent feedback inhibition of photosynthesis, and higher starch in the roots could help gravitational response under stress, and enhance biomass for better foraging 17 , 38 .…”

“…A single leaf 14 CO 2 -labeling device for Arabidopsis was modified based on the rice device 17 that was created following the prototype 28 . The 14 CO 2 feeding was carried out on the mature leaf of each twenty-leaf-old plant in the ‘leaf chamber’ in the middle of the photoperiod for 5 min.…”

“…Available data suggests that stress generally accelerates allocation to the sinks as an adaptive response 35 . Salinity increased flux from source to developing fruits in tomato 36 and to the roots in transgenic rice seedlings 17 . Water-stress elicited a similar distribution pattern in (a) Arabidopsis, with higher 14 C allocated to the roots 30 , (b) in beans, where 14 C flux to the pods increased 8 , and (c) in rice, where it stimulated 14 C mobilization from the stem and allocation to the grain 37 .…”

“…Drought is associated with reduced starch or sugar levels in source tissues 7 – 11 . Salinity stress can induce higher starch accumulation in the source or sink of some species 12 – 17 , but trigger starch reduction in others 18 , 19 . Similarly, chilling stress is associated with accelerated source-starch accumulation 20 – 22 or degradation 23 – 25 .…”

“…Feeding plants with 14 CO 2 is useful for tracking carbon movement, and can inform on changes in carbon allocation due to stress 17 , 28 – 34 . Available data suggests that stress generally accelerates allocation to the sinks as an adaptive response 35 .…”