’ s Choice Series on the Next Generation of Biotech Crops Bacterial RNA Chaperones Confer Abiotic Stress Tolerance in Plants and Improved Grain Yield in Maize under Water-Limited Conditions [ W ]

Limited available water is the single most important factor that reduces global crop yields, with far reaching socioeconomic implications. In North America alone, it is estimated that 40% of yearly maize (Zea mays) crop losses are due to suboptimal water availability (Boyer, 1982). Agriculture currently accounts for 70% of the fresh water used by humans. This rate of water use can exceed local regeneration rates, often relying on underground aquifers that are rapidly being depleted (Morison et al., 2008). The impending scarcity of water available for agriculture will surely increase overall costs of crop production and drive the need for crops that use water more efficiently. While tremendous progress has been made through breeding and through cultural practices that improve maize yields in water-limited environments, the potential for additional large improvements still exists, and positive impacts on yield and increased yield stability across a broad range of water availability is of great value to farmers, consumers, and the environment. Maize plants are sensitive to water-deficit stress throughout the growing season. Stresses that occur during the flowering stage, either just before floral initiation or immediately after pollination, result in the most significant reductions in end-of-season grain yields (Claassen and Shaw, 1970; Boyer and Westgate, 2004). Water-deficit stress during the vegetative growth phases typically leads to reductions in overall productivity, resulting in grain loss through reductions in kernel numbers. Late-stage drought stress, during the grain-filling period, can frequently lead to reductions in yield by reducing kernel size as well as increasing rates of kernel abortion, depending upon the severity of the stress. Improved plant performance under severe waterlimited growth chamber and greenhouse conditions has been achieved through multiple transgenic approaches, including the use of osmotic protectants such as the sugar alcohols trehalose (Romero et al., 1997), mannitol (Tarczynski et al., 1993), galactinol (Taji et al., 2002), and ononitol (Sheveleva et al., 1997). Accumulation of zwitterionic compounds such as Pro (Kishor et al., 1995) and Gly-betaine (Rathinasabapathi et al., 1994), or of protein protectants such as HVA1 (Xu et al., 1996), have all demonstrated an ability to confer tolerance to a variety of abiotic stresses. To date, none of the above approaches has been shown to provide durable tolerance in an agriculture production setting. Expression of a maize CAAT box transcription factor, ZmNF-YB2, has been shown to confer drought tolerance and enhanced photosynthetic capacity under drought stress with improvements in grain yield observed across several growing seasons in maize (Nelson et al., 2007). These studies clearly demonstrate that plants are amenable to improved stress tolerance through multiple mechanisms of action. Biotechnology approaches facilitate our ability to survey and capitalize on the extensive genetic diversity that exists in nature and to improve on conserved pathways important for adaptation to environmental stress. Adaptation requires rapid recovery in growth and maintenance of cellular function following stress. We have looked to model systems such as bacteria and plants for insights into adaptive stress response pathways and commonalities among stress response mechanisms broadly in search of candidates for crop improvement. In this article, we demonstrate that expression of related cold shock proteins (CSPs) from bacteria, CspA from Escherichia coli and CspB from 1 Present address: 1920 Fifth Street, Davis, CA 95616. 2 Present address: 700 Chesterfield Parkway, Chesterfield, MO 63017. * Corresponding author; e-mail jacqueline.e. heard@monsanto.com. The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Jacqueline E. Heard (jacqueline.e.heard@monsanto.com). [W] The online version of this article contains Web-only data. www.plantphysiol. org/cgi/doi/10.1104/pp.108. 118828