Abstract
Proline metabolism influences the metabolic and/or signaling pathway in regulating plant stress responses. This study aimed to characterize the physiological significance of glutamate (Glu)-mediated proline metabolism in the drought stress responses, focusing on the hormonal regulatory pathway. The responses of cytosolic Ca2+ signaling, proline metabolism, and redox components to the exogenous application of Glu in well-watered or drought-stressed plants were interpreted in relation to endogenous hormone status and their signaling genes. Drought-enhanced level of abscisic acid (ABA) was concomitant with the accumulation of ROS and proline, as well as loss of reducing potential, which was assessed by measuring NAD(P)H/NAD(P)+ and GSH/GSSG ratios. Glu application to drought-stressed plants increased both salicylic acid (SA) and cytosolic Ca2+ levels, with the highest expression of calcium-dependent protein kinase (CPK5) and salicylic acid synthesis-related ICS1. The SA-enhanced CPK5 expression was closely associated with further enhancement of proline synthesis-related genes (P5CS1, P5CS2, and P5CR) expression and a reset of reducing potential with enhanced expression of redox regulating genes (TRXh5 and GRXC9) in a SA-mediated NPR1- and/or PR1-dependent manner. These results clearly indicate that Glu-activated interplay between SA- and CPK5-signaling as well as Glu-enhanced proline synthesis are crucial in the amelioration of drought stress in Brassica napus.
Highlights
Prolonged water-deficit is considered a major climatic factor limiting plant growth and development
drought alone (Drought) alone treatment tended to decrease total chlorophyll and carotenoid levels, these two photosynthetic pigments were significantly increased by Glu application after 15 d of drought
Values are represented as mean ± SE (n = 3)
Summary
Prolonged water-deficit (e.g., drought) is considered a major climatic factor limiting plant growth and development. An accumulation of reactive oxygen species (ROS) and/or proline is observed as a common stress response [2,3]. Rapid production of ROS (i.e., oxidative burst) is one of the earliest plant responses to stresses caused by a wide range of environmental stresses [4] and pathogen infections [5,6]. Proline accumulation has been found to be a primary stress responsive. Plants 2020, 9, 512 symptom resulting from dehydration in plant tissues such as drought conditions [1,7], high salinity [8], or freezing temperature [9]. The proline pool of plant cells depends on the rate-limiting steps in proline synthesis and degradation, which are catalyzed by ∆1 -pyrroline-5-carboxylate synthase (P5CS)
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