Intracellular Ca2+ accumulation triggered by caffeine provokes resistance against a broad range of biotic stress in rice

Caffeine Produced in Rice Plants Provides Tolerance to Water-Deficit Stress

Exogenous or endogenous caffeine application confers resistance to diverse biotic stresses in plants. In this study, we demonstrate that endogenous caffeine in caffeine-producing rice (CPR) increases tolerance even to abiotic stresses such as water deficit. Caffeine produced by CPR plants influences the cytosolic Ca2+ ion concentration gradient. We focused on examining the expression of Ca2+-dependent protein kinase genes, a subset of the numerous proteins engaged in abiotic stress signaling. Under normal conditions, CPR plants exhibited increased expressions of seven OsCPKs (OsCPK10, OsCPK12, OsCPK21, OsCPK25, OsCPK26, OsCPK30, and OsCPK31) and biochemical modifications, including antioxidant enzyme (superoxide dismutase, catalase, peroxidase, and ascorbate peroxidase) activity and non-enzymatic antioxidant (ascorbic acid) content. CPR plants exhibited more pronounced gene expression changes and biochemical alterations in response to water-deficit stress. CPR plants revealed increased expressions of 16 OsCPKs (OsCPK1, OsCPK2, OsCPK3, OsCPK4, OsCPK5, OsCPK6, OsCPK9, OsCPK10, OsCPK11, OsCPK12, OsCPK14, OsCPK16, OsCPK18, OsCPK22, OsCPK24, and OsCPK25) and 8 genes (OsbZIP72, OsLEA25, OsNHX1, OsRab16d, OsDREB2B, OsNAC45, OsP5CS, and OsRSUS1) encoding factors related to abiotic stress tolerance. The activity of antioxidant enzymes increased, and non-enzymatic antioxidants accumulated. In addition, a decrease in reactive oxygen species, an accumulation of malondialdehyde, and physiological alterations such as the inhibition of chlorophyll degradation and the protection of photosynthetic machinery were observed. Our results suggest that caffeine is a natural chemical that increases the potential ability of rice to cope with water-deficit stress and provides robust resistance by activating a rapid and comprehensive resistance mechanism in the case of water-deficit stress. The discovery, furthermore, presents a new approach for enhancing crop tolerance to abiotic stress, including water deficit, via the utilization of a specific natural agent.

Calcium decoders and their targets: The holy alliance that regulate cellular responses in stress signaling

Calcium (Ca²⁺) signaling is versatile communication network in the cell. Stimuli perceived by cells are transposed through Ca²⁺-signature, and are decoded by plethora of Ca²⁺ sensors present in the cell. Calmodulin, calmodulin-like proteins, Ca²⁺-dependent protein kinases and calcineurin B-like proteins are major classes of proteins that decode the Ca²⁺ signature and serve in the propagation of signals to different parts of cells by targeting downstream proteins. These decoders and their targets work together to elicit responses against diverse stress stimuli. Over a period of time, significant attempts have been made to characterize as well as summarize elements of this signaling machinery. We begin with a structural overview and amalgamate the newly identified Ca²⁺ sensor protein in plants. Their ability to bind Ca²⁺, undergo conformational changes, and how it facilitates binding to a wide variety of targets is further embedded. Subsequently, we summarize the recent progress made on the functional characterization of Ca²⁺ sensing machinery and in particular their target proteins in stress signaling. We have focused on the physiological role of Ca²⁺, the Ca²⁺ sensing machinery, and the mode of regulation on their target proteins during plant stress adaptation. Additionally, we also discuss the role of these decoders and their mode of regulation on the target proteins during abiotic, hormone signaling and biotic stress responses in plants. Finally, here, we have enumerated the limitations and challenges in the Ca²⁺ signaling. This article will greatly enable in understanding the current picture of plant response and adaptation during diverse stimuli through the lens of Ca²⁺ signaling.