Allelic polymorphism of GIGANTEA is responsible for naturally occurring variation in circadian period in Brassica rapa

Abstract

GIGANTEA (GI) was originally identified by a late-flowering mutant in Arabidopsis, but subsequently has been shown to act in circadian period determination, light inhibition of hypocotyl elongation, and responses to multiple abiotic stresses, including tolerance to high salt and cold (freezing) temperature. Genetic mapping and analysis of families of heterogeneous inbred lines showed that natural variation in GI is responsible for a major quantitative trait locus in circadian period in Brassica rapa. We confirmed this conclusion by transgenic rescue of an Arabidopsis gi-201 loss of function mutant. The two B. rapa GI alleles each fully rescued the delayed flowering of Arabidopsis gi-201 but showed differential rescue of perturbations in red light inhibition of hypocotyl elongation and altered cold and salt tolerance. The B. rapa R500 GI allele, which failed to rescue the hypocotyl and abiotic stress phenotypes, disrupted circadian period determination in Arabidopsis. Analysis of chimeric B. rapa GI alleles identified the causal nucleotide polymorphism, which results in an amino acid substitution (S264A) between the two GI proteins. This polymorphism underlies variation in circadian period, cold and salt tolerance, and red light inhibition of hypocotyl elongation. Loss-of-function mutations of B. rapa GI confer delayed flowering, perturbed circadian rhythms in leaf movement, and increased freezing and increased salt tolerance, consistent with effects of similar mutations in Arabidopsis. Collectively, these data suggest that allelic variation of GI-and possibly of clock genes in general-offers an attractive target for molecular breeding for enhanced stress tolerance and potentially for improved crop yield.abiotic stress tolerance | circadian clock | hypocotyl elongation | flowering time | natural variation T he last half-century has seen dramatic increases in agricultural productivity. Despite the approximate doubling in world population since 1964, the proportion with insufficient food has dropped by ∼75%, although ∼1 billion remain underfed, and twice that many suffer from micronutrient deficiencies (1). Predicted growth in population and in per capita consumption will require an estimated doubling of crop production by 2050 (2). However, yield trends for maize, rice, wheat, and soybean-four major crops that currently produce nearly two-thirds of global agricultural calories-are insufficient to achieve this doubling (3). Therefore, there is a pressing need not simply to sustain, but actually to accelerate yield improvement.One strategy to increase yield is to identify genetic variation in plant regulatory networks that limit yield to define targets for programs of marker-assisted (molecular) breeding. The circadian clock has been implicated as a target for increasing yield (4-6). Plant circadian clocks comprise multiple interlocked feedback loops (7-9). There is natural variation in clock function in both weedy and cultivated species (10-15), although few of the genes responsible for these quantitative trait loc...

Reduction of GIGANTEA expression in transgenic Brassica rapa enhances salt tolerance

“…As a positive control, we showed that a loss-of-function gi-1 mutant obtained by TILLING in B. rapa R-o-18 increased salt tolerance, as determined by photosynthetic efficiency (Fv/Fm) (Fig. 6b), consistent with the results of Xie et al (2015). In response to salt stress, fresh weight and photosynthetic efficiency (Fv/Fm) were significantly greater in the GK1-11 and GK1-12 lines than in the DH03, GK4-4 and GK4-6 lines.…”

“…These oscillation patterns are similar to those of GI expression in Arabidopsis (Fowler et al 1999;Park et al 1999) and Brachypodium distachyon (Hong et al 2010), which peaked at ZT12 and ZT8 under LD and SD conditions, respectively. This pattern was also similar to that of BrGI transcripts in the R500 and IMB211 B. rapa accessions, which peaked at ZT8 under a 12/12 h photoperiod (Xie et al 2015). To determine if the rhythmic cycling of the BrGI transcript was under the control of the circadian clock, B. rapa plants entrained under either LD or SD conditions were transferred to continuous light conditions.…”

“…However, under saline conditions, SOS2, released by proteolytic degradation of GI, can interact with the SOS3 sodium ion sensor, which activates SOS1 to establish salt tolerance (Kim et al 2013b). GI overexpression increases salt sensitivity, whereas loss of GI function enhances salt tolerance (Kim et al 2013b;Xie et al 2015). These and other studies indicate that a circadian clock synchronized with the environment enhances fitness through elevated stress resistance as well as via enhanced photosynthetic capacity and carbohydrate metabolism (Dodd et al 2005;Graf and Smith 2011;Sanchez et al 2011).…”

“…GI was first characterized with respect to its functions in the elicitation of photoperioddependent flowering and in circadian clock maintenance (Koornneef et al 1991;Fowler et al 1999;Park et al 1999;Mizoguchi et al 2005). More recently, it has been discovered that GI also plays important roles in the response to multiple abiotic stresses, including tolerance to high salt and low (freezing) temperature (Cao et al 2005;Kim et al 2013b;Riboni et al 2013;Xie et al 2015).…”

“…In Arabidopsis, GI contributes to the flowering and abiotic stress response pathways (Fowler et al 1999;Edwards et al 2011;Xie et al 2015). We generated eight BrGI-RNAi constructs (Fig.…”