Received: 18 July, 2011. Accepted: 10 December, 2011. Invited Review Functional Plant Science and Biotechnology ©2012 Global Science Books Protease Inhibitors and Stress Tolerance Tantravahi Srinivasan 1* • P. B. Kirti 2 1 Department of Biotechnology, GIS, GITAM University, Visakhapatnam, Andhra Pradesh, 530 045, India 2 Department of Plant Sciences, University of Hyderabad, Prof. C.R. Rao Road, Hyderabad, Andhra Pradesh, 500 045, India Corresponding author: * saveplants@gmail.com, srinivasan@gitam.edu ABSTRACT Plant protease inhibitors (PIs) are extensively studied for their role in defense against pests and pathogens due to their ability to inhibit specific proteases of the intruder. All classes of PIs were found to play important roles in innate host defense mechanisms and are used to develop transgenic plants resistant to pests and pathogens. Recent reports suggest that they have a role in modulating abiotic stress tolerance also. The induction of PIs was observed in response to not only biotic but also abiotic cues suggesting their possible role in abiotic stress responses and in modulating tolerance to various stresses. During biotic stress, pathogen invasion or damage due to herbivory, induces SA/JA-mediated signalling pathways to combat the attack. Abiotic stress induces injury and different conditions like osmotic stress, oxidative stress etc. as long as the causative factor exists in the milieu. Unlike in the biotic stresses, the causative agent in abiotic stress cannot be countered by the plant. Hence, the abiotic stress has to be tolerated by the plant nullifying all its related damages mostly by adjustment through ABA-dependent, and sometimes through an independent pathway. The overexpression of PIs has been shown to enhance abiotic stress tolerance in the transgenic plants suggesting that their role is multidimensional. In this review, we focus on the possible role of PIs in plant growth, and biotic and abiotic stress tolerance. _____________________________________________________________________________________________________________ Keywords: abiotic stress, biotic stress, cross-talk, protease inhibitor Abbreviations: ABA, abscisic acid; ABRE, ABA-responsive; CDPK, Ca 2+ -dependent protein kinases; COI1, CORONATINE- INSENSITIVE1; CpTI, cowpea protease inhibitor; EIN4, ETHYLENE INSENSITIVE-4; ERF1, Ethylene Response Factor1; ERS1, ETHYLENE RESPONSE-1; ERS2, ETHYLENE RESPONSE SENSOR-2; ET, ethylene; ETR1, ETHYLENE RESPONSE-1; ETR2, ETHYLENE RESPONSE-2; H 2 O 2, hydrogen peroxide; JA, jasmonic acid; JA-Ile, jasmonoyl isoleucine; JAZ, JASMONATE ZIM- motif; LA, linolenic acid; MAPK, mitogen-activated protein kinase; OPC-8, 3-oxo-2-(2[Z]-pentenyl)-cyclopentane-1-octanoic acid; OPDA, 12-oxo-phytodienoic acid; PI, protease inhibitor; ROS, reactive oxygen species; SA, salicylic acid CONTENTS INTRODUCTION........................................................................................................................................................................................ 59 BIOTIC STRESS AND INDUCTION OF PROTEASE INHIBITORS....................................................................................................... 60 BIOTIC STRESS RESISTANCE BY OVEREXPRESSION OF PI ............................................................................................................ 61 MODE OF ACTION .................................................................................................................................................................................... 61 ROLE OF PROTEASE INHIBITORS IN ABIOTIC STRESS AND ITS TOLERANCE ........................................................................... 61 ABIOTIC STRESS TOLERANCE BY OVER-EXPRESSION OF PI ........................................................................................................ 63 FUTURE PROSPECTS ............................................................................................................................................................................... 63 REFERENCES............................................................................................................................................................................................. 64 _____________________________________________________________________________________________________________ INTRODUCTION Plants have sophisticated mechanisms to protect themselves from changing environmental factors, which adversely in- fluence their growth, survival and reproduction. These include both biotic as well as abiotic factors. The stresses in general occur in multitude and plants induce different path- ways under the control of various stress hormones to sur- vive unfavourable situations. Hormones like jasmonic acid (JA), salicylic acid (SA), ethylene (ET) are known to be particularly induced under biotic stress conditions, whereas abscisic acid (ABA) is induced under abiotic stress con- ditions (Fujita et al. 2006; Cao et al. 2011). The later stu- dies have shown that all the hormones are involved in stress responses and work synergistically or antagonistically with each other (Fujita et al. 2006). The growth hormones viz., cytokinins, auxins, gibberellic acid and brassinosteroids have also been recognized as important players in plant immunity (Mauch-Mani and Mauch 2005; Grant and Jones 2009). As all the stress hormones are induced under biotic and abiotic stress conditions, their downstream pathways are also induced with possible overlapping and crosstalk (Fujita et al. 2006; Abuqamar et al. 2009). The overlapping and crosstalk between different pathways would be modu- lated by common players for different stresses, which can influence multiple pathways. The basic helix-loop-helix (HLH) transcription factor MYC2 is shown to be involved in the ABA-mediated drought stress signaling pathway (Abe et al. 2003). It was later identified that MYC2 up- regulates the expression of genes that are involved in JA mediated wounding response and negatively regulates the expression of JA/ET-mediated pathogen defense genes (Anderson et al. 2004; Lorenzo et al. 2004). RD26, a de- hydration-responsive NAC transcription factor is involved in regulation of both biotic and abiotic signaling. Its expres- sion is induced by JA, ABA, hydrogen peroxide (H 2 O 2 ) and pathogen infections, as well as by drought and high salinity (Fujita et al. 2004; Zimmermann et al. 2004). It protects the plants by inducing the gene products that are involved in the detoxification of reactive oxygen species (ROS), defense, ®