Organogenesis of Dodders, Parasitism, Infestation and Development Phases

Parasitic plants proliferation globally is daunting and a threat to our ecosystems. In this study we explore holoparasites with limitation to dodders (Cuscuta spp. & Cassytha filiformis). An experiment was performed to ascertain anatomical and morphological characteristics of dodder capsule and its stem. We present dodder infestation stages, development phases and close observable internal and external microscopic features. A distinct haustorium trait of dodders is shown by micrographs. The study finds that dodder seeds possess high ecological dispersal character with vast adaptability through morphological analysis. This ramifies their ecological phenotypic plasticity. Externally, dodder stems attack the host phloem through haustoria that suck nutrients from the sap weakening it.


INTRODUCTION
Dodders (Cuscuta spp. & Cassytha filiformis) are among the flowering autotrophic plants that constitute in predominant field weeds that are either parasitic and semi-parasitic flowering plants. Worldwide, parasitic plants represent approximately 4200 species classified in 274 genera that make 1% of flowering plants. From the 274 genera, roughly 11% of species make parasites to cultivated plants (hosts). A study by Nickrent [1] indicates that leading economic implication or damage is to the host plant accelerated by species from four genera, namely: Cassytha, Cuscuta, Striga, Orananche, and Arceuthobium. Different studies [2][3][4] have indicated that dodder is the most important and challenging parasitic group of weed of over 70 varieties and 200 species.
Holoparasite dodder stems are threadlike and coil around the host [5]. These plant parasites are leafless, or in some cases, their leaves are hardly visible with its nature of small scales. When Cuscuta seeds are fully mature, they fall off and hip on the ground in accumulation. At the ground, their germination will depend if there is a suitable host within the vicinity [6,7]. If none, the seeds are rendered dormant until suitable germinating conditions for them to germinate [8]. Dodder entirely attaches its stem to the host for nutrients access and water supply. This assists the parasitic weed to mature. When it reaches the appropriate maturation phase, it forms inflorescences accompanied by abounding actinomorphic and hermaphrodite flowers. These flowers are tiny, hermaphroditic, reddish and whitish or yellow.
According to Runyon et al. [9,10], for dodder seedlings to attach or identify the host plant, they use plant volatiles that act as chemo-attractants that serve as a guide for the growth of the seedling. This haustoria characteristic is important to them to survive. It escalates their survival chances to establish a connection with the host plant successfully. Additionally, scholars [11,12], Cuscuta stem is impacted by the light that actively facilitates its seeds to germinate. They argue that its seedlings grow towards the light, thus assisting them in finding the appropriate host.
Immediately dodder seed germinates and identifies the host; it undergoes three distinctive phases -the adhesive phase, the intrusive phase, and the conductive phase. During the adhesive phase, a holdfast (specialized adhesive organ) develops at the Cuscuta stem that is in contact with the host plant [13]. Formation of the holdfast results from elongation of cortical layers of dodders and in the epidermal cells. This is characterized by the existence of secretory cells that churn or secret adhesive compounds. At the intrusive phase, Cuscuta develops hook-like structures through haustorium. The haustorium behavior immediately gets into the host's vascular tissues; the haustoria cells differentiate, forming conductive cells that transform into the conductive phase. In the conductive phase as the final third phase, Cuscuta starts to exchange information molecules with the host where nutrients and water absorption is done through sucking [14].
This study presents the primary and secondary review findings by identifying dodder development phases from the onset. The anatomical and morphological capsule/berry (dodder seed) are shown to include dodder haustorium development stages.

METHODOLOGY
Mature dodder seed capsules were collected from Mombasa County on latitude & longitude (4° 2' 12.7608'' S and 39° 40' 10.4556'' E) at infested areas and taken to Kenyatta University on location coordinates (1°10'50.0"S, 36°55'41.0"E) laboratory for anatomical observation. Seed berries were packed in plastic sample collecting bottles, and experiments were done within 12 hours to avoid analyte loss. Mombasa County is a low altitude ( 50 m/160 ft elevation) setting with higher temperatures, unlike Nairobi of higher altitude (1,795 m/5,889 ft elevation) and low temperatures.

Anatomy of Dodder Capsule
A cross-sectional cut of two of the collected samples (a, b) of succulent and dry dodder capsules was done respectively at Kenyatta University laboratory to observe the seed anatomical nature (Fig. 1). From the observation, dodder is a dicotyledonous plant parasite. The seed had duo cotyledons that primarily serve as the source of nutrient supply to the embryo's early development. The sticky, whitish sap of the capsule dries, turning brownish immediately the seed matures transforming as a berry skin that covers the Testa. The cotyledon turns from white to glittering yellow color. Additionally, as the seed capsule dries, the size of cotyledons shrinks. Another noticeable feature at the cut dodder berry was its single seed trait per seed berry covered with mesocarp that dries as it matures and shrinks.
Dodder seed observable features that enhance its dispersal pathway towards its phenotypic plasticity were: a) The sticky seed juice -some animals are attracted to the juicy mature seeds, which they end up transporting to other areas as a stool.  Fig. 1(a) 3 to glittering yellow color. Additionally, as the seed capsule dries, the size of cotyledons ceable feature at the cut dodder berry was its single seed trait per seed berry covered with mesocarp that dries as it Dodder seed observable features that enhance its dispersal pathway towards its phenotypic some animals are attracted to the juicy mature seeds, which to other areas as a stool. The fallen matures seed float on water, thus transported to other far distant . This was observed too by Kidunda et al. [15] on assessing the existence and spread of dodders (C. ) on Cashew Trees in Tanzania. Their findings conclude that dodder dispersal mechanisms were; wind, birds, water, machines, equipment, and animals that work at already dodder infested areas.
the dried hard berries develop into a hard seed coat that can withstand harsh environmental conditions until they find a potential host that triggers their development life cycle. c) Color change to browncamouflage while in or on the soil. The ability to identify the seeds is very limiting only with the use of specialized equipment like magnifying glasses. d) Seed reserved endospermof the seed reproductive life internal nutritional reserves. Dodders have endosperms that can remain actively dormant until favorable conditions for the seed to germinate are felt. This is triggered by the micropyle when a suitable environmental condition emerges. A sim claim is proposed by Ghantous Sandler [16][17][18] in their study on dodder seed dormancy. They argue that seed scarification of capsuled dodder berries is a must for them to germinate. This is normally done to weaken hard coats of dodder seeds either mechanically (scratching or soaking in water), thermal (exposed to heat like fire or burning), and chemically (corroding and dissolving components of the seed coat) (b)

Fig. 1. Anatomy of dodder capsule
succulent dodder capsule with the outer ring representing the Mesocarp, the Testa comes as the immediate dark brown ring and the interior is dominated by the whitish Endosperm. Fig. 1(b). is and dry dodder capsule with a berry skin at the outer layer, the Micropyle that opens for germination and the inner Rad by the dicotyledons ; Article no.AJGR.63333 until they find a potential host that triggers their development life cycle.
-this acts as camouflage while in or on the soil. The ability to identify the seeds is very limiting only with the use of specialized equipment -the longevity depends on its internal nutritional reserves. Dodders have endosperms that can remain actively dormant until favorable conditions for the seed to germinate are felt. This is triggered by the micropyle when a suitable environmental condition emerges. A similar Ghantous and in their study on dodder seed dormancy. They argue that seed scarification of capsuled dodder berries is a must for them to germinate. This is normally done to weaken hard coats of dodder seeds either mechanically (scratching or soaking in water), thermally (exposed to heat like fire or burning), and chemically (corroding and dissolving components of the seed coat) Other studies [20][21][22] show dodders start foraging on compatible ornamental host plant (Pelargonium zonale) in stages (Fig. 3)

Fig. 3. Dodder infestation stages
formed (apical shoot) and other additional side shoots (the arrowheads in E). The presented cross-sections shown are in respect of (the parasite shoot axis at F and host shot axis in G and H)-the scale bars = 500 µm.

CONCLUSION
Dodders are opportunistic foraging plant parasite weeds that are obligate. Their specialized haustoria trait propel their development that is notable in different phases and stages. Dodder seed capsules possess ecological phenotypic plasticity that is significant to their survival, through seed dormancy and dispersal abilities. Their stems are well adapted to spin around host with specialized hooks that suck sap to serve their nutritional demands from the host. These adaptations and biological character of dodder weed make it notorious and highly invasive.

ACKNOWLEDGEMENT
We acknowledge the support accorded by Kenyatta University's Environmental Science research team.