Cassava shrunken-2 homolog MeAPL3 determines storage root starch and dry matter content and modulates storage root postharvest physiological deterioration

Getu Beyene,Jackson Gehan,Nigel Taylor,Dimuth Siritunga,Raj Deepika Chauhan

doi:10.1007/s11103-020-00995-z

Abstract

Key messageAmong the five cassava isoforms (MeAPL1–MeAPL5), MeAPL3 is responsible for determining storage root starch content. Degree of storage root postharvest physiological deterioration (PPD) is directly correlated with starch content.AGPase is heterotetramer composed of two small and two large subunits each coded by small gene families in higher plants. Studies in cassava (Manihot esculenta) identified and characterized five isoforms of Manihot esculenta ADP-glucose pyrophosphorylase large subunit (MeAPL1–MeAPL5) and employed virus induced gene silencing (VIGS) to show that MeAPL3 is the key isoform responsible for starch and dry matter accumulation in cassava storage roots. Silencing of MeAPL3 in cassava through stable transgenic lines resulted in plants displaying significant reduction in storage root starch and dry matter content (DMC) and induced a distinct phenotype associated with increased petiole/stem angle, resulting in a droopy leaf phenotype. Plants with reduced starch and DMC also displayed significantly reduced or no postharvest physiological deterioration (PPD) compared to controls and lines with high DMC and starch content. This provides strong evidence for direct relationships between starch/dry matter content and its role in PPD and canopy architecture traits in cassava.

Highlights

Cassava (Manihot esculenta Crantz) is widely grown for its starchy storage roots across the tropics and subtropics, with an estimated annual production of over 292 million metric tonnes (MT) in 2017 (FAOSTAT, accessed 12/17/2019)
Silencing each of these genes individually using virus induced gene silencing (VIGS) showed that starch synthesis and accumulation in cassava storage roots is facilitated very largely by MeAPL3
By generating stable transgenic co-suppression lines of MeAPL3, we showed that dry matter content (DMC) and starch were significantly reduced, thereby substantiating data generated by VIGS

Summary

Introduction

Cassava (Manihot esculenta Crantz) is widely grown for its starchy storage roots across the tropics and subtropics, with an estimated annual production of over 292 million metric tonnes (MT) in 2017 (FAOSTAT, accessed 12/17/2019). Cassava production and utilization is limited by inherent susceptibility of storage roots to rapid postharvest deterioration after removal from the soil (Beeching et al 1998). This commences 24–48 h after harvest and is known as postharvest physiological deterioration (PPD). PPD has been classified as primary and secondary in nature. The former is considered enzymatic, and happens early in the process, while the latter involves mainly microbial deterioration of affected storage root tissues (Booth 1976). Studies have shown association of PPD with increased respiration and water loss (Marriott et al 1978), elevated activities of enzymes involved in wound response, production of phenols and polyphenols

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