Learning from halophytes: physiological basis and strategies to improve abiotic stress tolerance in crops

Abstract

This review argues that learning from halophytes may be a promising way of achieving this goal. The paper is focused around two central questions: what are the key physiological mechanisms conferring salinity tolerance in halophytes that can be introduced into non-halophyte crop species to improve their performance under saline conditions and what specific genes need to be targeted to achieve this goal? The specific traits that are discussed and advocated include: manipulation of trichome shape, size and density to enable their use for external Na(+) sequestration; increasing the efficiency of internal Na(+) sequestration in vacuoles by the orchestrated regulation of tonoplast NHX exchangers and slow and fast vacuolar channels, combined with greater cytosolic K(+) retention; controlling stomata aperture and optimizing water use efficiency by reducing stomatal density; and efficient control of xylem ion loading, enabling rapid shoot osmotic adjustment while preventing prolonged Na(+) transport to the shoot.

Acclimation improves salt stress tolerance in Zea mays plants

“…These findings not only caution against the validity of breeding strategies aimed at reduction of Na + uptake by plants, but also highlight the need to focus on shoot tissue tolerance mechanisms (and, specifically, vacuolar Na + sequestration) as a more promising approach in the production of tolerant plants (eg. Shabala, 2013).…”

Cell-Type-Specific H+-ATPase Activity in Root Tissues Enables K+ Retention and Mediates Acclimation of Barley (Hordeum vulgare) to Salinity Stress

“…To date, attempts to create salttolerant crop germplasm have had limited success (Flowers, 2004;Shabala, 2013), largely due to the high physiological and genetic complexity of this trait. It is estimated that salinity affects transcripts of approximately 8% of all genes (Tester and Davenport, 2003), and fewer than 25% of these salt-regulated genes are salt stress specific (Ma et al, 2006).…”

“…It is estimated that salinity affects transcripts of approximately 8% of all genes (Tester and Davenport, 2003), and fewer than 25% of these salt-regulated genes are salt stress specific (Ma et al, 2006). At the physiological level, numerous subtraits contribute to overall salinity tolerance, most of which are species specific and may require expression in either a particular tissue or cell type (Tester and Davenport, 2003;Shabala, 2013). It is thought that the limited success of transgenic manipulations to increase some of these traits (and, specifically, those related to ion exclusion from the shoot) is due largely to the inability to express important exclusion genes in a cell-specific manner .…”

“…Physiologically, plant adaptive responses to salinity can be grouped into four major categories: (1) dealing with the osmotic component of salt stress; (2) handling toxic Na + and Cl 2 ions; (3) detoxifying reactive oxygen species (ROS) produced in plant tissues under saline conditions; and (4) mediating cytosolic K + homeostasis (Tester and Davenport, 2003;Ji et al, 2013;Shabala, 2013;Shabala and Pottosin, 2014;Flowers et al, 2015;Julkowska and Testerink, 2015;Kurusu et al, 2015). All these responses rely heavily on the regulation of transport activity across cellular membranes and, specifically, those for Na + and K + ions.…”

“…At the same time, superior K + retention and a cell's ability to maintain cytosolic K + homeostasis correlate with salinity tolerance in a broad range of plant species (Anschütz et al, 2014;Shabala and Pottosin, 2014) and are essential for preventing salinityinduced programmed cell death (Shabala, 2009;Demidchik et al, 2010). High cytosolic K + levels also are essential to maintain high vacuolar H + -PPase activity, thus enabling the operation of tonoplast NHX proteins that mediate vacuolar Na + sequestration (Shabala, 2013). Na + and K + also are major inorganic osmolytes that confer over 70% of tissue osmotic adjustment under stress conditions (Shabala and Lew, 2002).…”