Engineering thermotolerant plants: a solution to protecting crop production threatened by global warming

Abstract

The effects of global warming are the environmental and social changes caused directly or indirectly by human emissions of greenhouse gases. Impacts of climate change have already been evidenced by observed glacier retreat, changes in the timing of seasonal events (e.g., earlier flowering of plants) and changes in agricultural productivity. Climate change threatens crop harvests not only by storm-, floodingand drought-caused physical damage, but also by heat stress-induced changes in physiological processes. In China alone, there are six recorded heat damages in the past 50 years, and in particular, heat stress affected 3 million hectares and reduced grain production by 5 million tons in 2003 [1]. Unlike other biotic and abiotic stresses such as pests and drought, which can be managed to certain extent by agricultural practices, air temperature on an open field is much beyond human’s control. Therefore, developing thermotolerant plants is probably the most effective choice to protect crop production under heat stress, although this issue has largely remained elusive. Fortunately, Li et al. [2] have recently identified a major quantitative trait locus (QTL) conferring heat tolerance and cloned the corresponding gene Thermo-tolerance 1 (TT1) from CG14 (Oryza glaberrima), a rice line long adapted in Africa for high temperature. This work is the first breakthrough in genetic mining of QTLs for crop thermotolerance and is of great significance for the increase in crop security threatened by global warming. The authors employed various genetic approaches to underpin the identity of the TT1 gene. Firstly, they constructed nearly isogenic lines (NILs) by introducing the TT1 locus from CG14 to a less thermotolerant Asian cultivated variety Wuyunjing (WYJ; Oryza sativa ssp. japonica) and showed that the NILs, which had the same genetic background but the TT1-carrying fragment, display striking different survival rates upon heat treatment. Secondly, by carrying out high-resolution mapping using 6,721 BC4F2 individuals, they fine-tuned the TT1 locus to a 12.69-kb region, where two ORFs are annotated. Further, they found one amino acid substitution and differential expression between NIL (CG14) and NIL (WYJ) in one ORF, and such differences extend to a broad range of O. glaberrima and O. sativa varieties, with the amino acid H99 and higher expression specific to thermotolerant O. glaberrima. Finally, they confirmed cloning of TT1 by transforming WYJ with TT1 and by knocking-down TT1 in NIL (CG14). TT1 encodes a highly conserved a2 subunit of the 26S proteasome, sharing 82 % similarity with a homologous protein from mice [3] and 70 % similarity with one from yeast [4]. It is known that the proteasome plays an important role in plant stress responses as well as in development [5, 6]. To investigate in which way the plants exploit TT1 to protect themselves against heat, the authors conducted an ubiquitylome study with NIL (CG14) and NIL (WYJ) before and after 30 h of heat treatment using immunoprecipitation coupled with tandem mass spectrometry. Based on analyzing what they obtained, they proposed that TT1 protects plant cells from heat stress by removing cytotoxic proteins denatured by heat stress and balancing a series of protective responses (Fig. 1). The R99H substitution in TT1 may cause a conformational change, which in turn may lead to a change in controlling entrance of unfolded substrates into the catalytic sites, thus making TT1 more efficient in degrading denatured J. Wan (&) National Key Facility for Crop Gene Resources and Genetic Improvement, Institute of Crop Science, Chinese Academy of Agricultural Sciences, Beijing 100081, China e-mail: wanjianmin@caas.cn