The role of chloroplasts in responding to drought stress in plants

Abstract

Despite the discovery of chloroplast-to-nucleus retrograde signalling over 30 years ago, much is still unknown about the nature of the signals and pathways. The prevailing hypothesis involves heme synthesised via the activity of ferrochelatase 1 (FC1) as part of the tetrapyrrole biosynthetic pathway acting as a positive signal promoting nuclear gene expression. To gain a better understanding of this heme-based signal, this study manipulates heme levels through the overexpression of the heme degrading enzyme heme oxygenase 1 (HO1) and the heme synthesising enzymes ferrochelatase 1 and 2.

Additionally, due to the growing body of evidence implicating ferrochelatase activity in responding to drought stress, the transgenic lines overexpressing either FC1 or FC2 are analysed in terms of their ability to respond to and tolerate drought.

Here it was shown that overexpressing HO1 targeted to the cytosol results in a significant reduction in Pchlide and chlorophyll accumulation, additionally this caused a pale phenotype in two of the lines. This implies that the heme status of the cytosol is communicated to the chloroplast. Additionally, chloroplast targeted HO1 overexpression resulted in slight reductions to Pchlide accumulation. Nuclear gene expression after treatment with norflurazon (NF) displayed varying degrees of maintenance across the chloroplast and cytosol-targeted overexpressors. Gene expression data were inconclusive, however two plastid-targeted overexpressors and two cytosol-targeted lines showed potential gun-like phenotypes with maintenance of CA1 expression after NF, however this did not consistently correlate with HO1 expression. No significant results were observed in the mitochondria targeted HO1 overexpressors, however stronger overexpressing lines are needed to characterise this phenotype accurately. Further characterisation of the HO1 overexpressing lines is required to accurately determine the impacts of this manipulation on a plants’ function.

Drought experiments showed that overexpressing FC1 in the plastid consistently improved drought tolerance in terms of survival and expression of drought responsive genes. This was also seen to varying degrees with FC1 overexpression targeted to the mitochondria, potentially implying that the benefit is not due to retrograde signalling. No discernible differences were observed between the overexpressing lines and the wild type in terms of water loss when measured over the course of 90 minutes. However, when this was increased to 5 hours, chloroplast targeted FC1 overexpressing lines, including one previously shown not to have a retrograde signalling phenotype, displayed reduced water loss. The results add to the body of evidence implicating FC activity in drought tolerance and potentially implies that this is not due to retrograde signalling, but further analysis is required.

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Identifiers

ORCID for Matthew Alan Woodard: orcid.org/0000-0001-5330-3070

ORCID for Matthew Terry: orcid.org/0000-0001-5002-2708

ORCID for Paul Skipp: orcid.org/0000-0002-2995-2959

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