Ectopic Expression of Distinct PLC Genes Identifies 'Compactness' as Novel Architectural Shoot Strategy to Cope with Drought Stress

Phospholipase C (PLC) has been implicated in several stress responses, including drought. Overexpression (OE) of PLC has been shown to improve drought tolerance in various plant species. Arabidopsis contains nine PLC genes, subdivided into four clades. Earlier, OE of PLC3, -5 or -7 were found to increase Arabidopsis drought tolerance. Here, we confirm this for three other PLCs: PLC2, the only constitutively expressed AtPLC; PLC4, reported to have reduced salt tolerance; and PLC9, of which the encoded enzyme was presumed to be catalytically inactive. To compare each PLC and to discover any other potential phenotype, two independent OE lines of six AtPLC genes, representing all four clades, were simultaneously monitored with the GROWSCREEN FLUORO phenotyping platform, under both control- and mild drought conditions. To investigate which tissues were most relevant to achieve drought survival, we additionally expressed AtPLC5 using 13 different cell- or tissue-specific promoters. While no significant differences in plant size, biomass or photosynthesis were found between PLC lines and wild-type (WT) plants, all PLC-OE lines, as well as those tissue-specific lines that promoted drought survival, exhibited a stronger decrease in convex hull perimeter (= increase in compactness) under water deprivation compared to WT. Increased compactness has not been associated with drought or decreased water loss before, though a hyponastic decrease in compactness in response to increased temperatures has been associated with water loss. We pose that increased compactness leads to decreased water loss and potentially provides a new breeding trait to select for drought tolerance.