1. INTRODUCTION
Water deficit is one of the most important abiotic stress restricting
plant production. As a consequence of climate change, drought events are
projected to increase in intensity, duration, and frequency
(Lesk et al., 2016). Thus, the
improvement of drought-resistance of crops represents an urgent need
that demands the identification of key regulators and pathways as
potential targets for drought-resistance improvement. Reproductive stage
drought stress causes more severe damage and yield loss than any other
stage of development of crop plants
(Dorion et al., 1996;
Sheoran and Saini, 1996). Therefore, an
effective way to improve drought-resistance of crops is to select for
yield and its components during reproductive development under drought
stress. Reduction in grain yield due to drought has mainly been
attributed to male sterility because the male organ is more drought
sensitive than the female organ which remains fertile under stress
condition that causes sterility in the male
(Bingham, 1966;
Saini and Aspinall, 1981). Hence, the
male development under drought has attracted greater research attention
than the female. Commercially, tomato is the most important vegetable
crop. However, it is susceptible to abiotic stresses including drought.
An earlier study examined the effects of drought on tomato utilizing
vegetative tissues (Seng, 2014). However,
for a fruit vegetable crop like tomato, fruit set is the most important
trait for evaluating drought tolerance, thus evaluating drought
tolerance of the reproductive organs especially the male is of enormous
importance.
Anther, the male organ of flowering plants comprises of concentric cell
layers including the epidermis, endothecium, middle layers and tapetum,
the innermost layer surrounding a central locule that contains
sporogenous cells. Sporogenous cells develop into pollen grains, the
male gametophytes within the locule. During pollen development, anther
wall layers play important roles in nutrition, protection, and pollen
release. The tapetum serves as a source of energy for the developing
microspores, secrete enzymes e.g. callase that releases the tetrads from
the callose wall (Izhar and Frankel,
1971) and precursors for exine wall formation. Initially, tapetum
development proceeds normally and at later stage, undergoes program cell
death (PCD) and disintegrates (Parish and
Li, 2010). The timely tapetum-specific PCD and disintegration are
necessary for normal pollen development. Precocious or postponed tapetum
degeneration results in male sterility
(Bhadula and Sawhney, 1988;
Graybosch and Palmer, 1985). The modified
epidermal cells, the stomia modulate pollen release process. Failure of
stomia cells to degenerate leads to male sterility due to anther
indehiscence (Cecchetti et al., 2013).
Abnormalities in anther and pollen development leading to induction of
male sterility as a result of drought have been extensively investigated
in cereal crops (Ji et al., 2010;
Saini and Westgate, 1999). However, there
are limited or no information on the impacts of drought on anther and
pollen development in tomato.
Carbohydrate metabolism and sugar movement from source organs to sink
tissues such as anthers are important processes for pollen development.
Studies with cereals have shown that disturbances in these processes
lead to male sterility associated mainly with abnormal starch
accumulation in the pollen (Koonjul et
al., 2005; Oliver et al., 2005). Sucrose
accumulation, inadequate starch build-up in pollen grains and subsequent
abortion of pollen development are attributed to repression of genes
involved in sucrose and starch metabolism in drought and cold-stress
anthers (Koonjul et al., 2005;
Oliver et al., 2005). Additionally,
drought-stressed rice anthers accumulate more starch granules in
connective tissues than in pollen grains
(Jin et al., 2013;
Lalonde et al., 1997). However, studies
on changes in starch and soluble sugar accumulation and the underlying
molecular mechanisms in developing anthers under drought stress are
scarce in fruit vegetable crops and for tomato in particular, there are
no reports.
Phytohormones are important endogenous chemical messengers that modulate
plant growth and development and responses to adverse stress factors.
Among the phytohormones, abscisic acid (ABA) is the key plant hormone
known to mediate responses to abiotic stresses such as drought and
temperature (Zhang et al., 2006). In
reproductive tissues, there is increasing evidence of a strong
correlation between ABA level increase and pollen sterility during
abiotic stresses. Higher pollen abortion and greater reduction in grain
set with concomitant higher ABA accumulation in anthers of
drought-susceptible than drought-tolerant wheat cultivars have been
reported (Dong et al., 2017;
Ji et al., 2011). Besides, there is proof
of crosstalk between ABA and sugar signaling. The expression ofOSINV4 , a cell wall invertase gene, was repressed by ABA in cold
stress rice anthers (Oliver et al.,
2007). Recent study implicated auxin (IAA) in the control of plant
response to abiotic stress during reproductive development. Reduction in
pollen fertility and grain yield was associated with reduced
accumulation of endogenous IAA in rice spikelets due to drought-induced
repression of YUCCA genes that are involved in IAA biosynthesis
(Sharma et al., 2018). Jasmonic acid (JA)
has been demonstrated to participate in drought stress responses in
vegetative tissues (De Ollas Valverde et
al., 2015; Du et al., 2013) but there
are no reports on JA mediation of drought responses in reproductive
organs. However, induction of stigma exsertion as a result of reduction
in endogenous JA in tomato anthers under high temperature has been
reported (Pan et al., 2019). Although
phytohormones play critical roles in regulating crops responses to
drought stress, the dynamics of endogenous hormone metabolisms and the
relationships with behavioural patterns of anthers and pollen at
different stages of development under drought stress are not well
understood.
In this study, we have examined the effects of drought stress on anther
morpho-physiological and molecular responses in tomato. The major
objectives of the study were to determine: (a) the consequences of
subjecting anthers of varied developmental stages to water deficit on
flowering phenology, flower development anther morphology and fruit
set/yield (b) whether different stages of anther development response
differently to drought stress (c) the effect of drought stress on male
gametophyte fertility (d) the histological and cytological changes in
anthers at different stages under drought stress (e) effects of drought
stress on the levels of endogenous IAA, ABA, and JA; the contents of
sucrose, glucose and fructose; and starch accumulation at different
stages of anther development (f) the underlying molecular mechanisms
determining the physio-morphological changes in the male organ at
different stages of development under stress conditions. Findings in
this study will contribute to the understanding of the behavioural
patterns and defects in the anther, tapetum, and pollen at different
stages of development and the associated physiological and molecular
mechanisms in response to drought stress in tomato.